Selective Coordination

Selective Coordination
Selective Coordination
Introduction
What Is Selective Coordination?
Today, more than ever, one of the most important parts of any facility is the
electrical distribution system. Nothing will stop all activity, paralyze
production, inconvenience and disconcert people, and possibly cause a
panic, more than a major power failure. Selective coordination is critical for
the reliability of the electrical distribution system and must be analyzed.
Selective coordination of overcurrent protective devices is required by the
NEC® for a few building systems for a limited number of circuits that supply
power to vital loads. These requirements will be discussed in a later section.
For circuits supplying power to all other loads, selective coordination is a
very desirable design consideration, but not mandatory. It is important to
deal with selective coordination in the design phase. After switchboards,
distribution panels, motor control centers, lighting panelboards, etc. are
installed, there typically is little that can be done to retroactively "fix" a
system that is not selectively coordinated.
While it's very important, it is not enough to select protective devices based
solely on their ability to carry the system load current and interrupt the
maximum fault current at their respective points of application. It is
important to note that the type of overcurrent protective devices and ratings
(or settings) selected determine if a system is selectively coordinated. A
properly engineered and installed system will allow only the nearest
upstream overcurrent protective device to open for both overloads and all
types of short-circuits, leaving the remainder of the system undisturbed and
preserving continuity of service. Isolation of a faulted circuit from the
remainder of the installation is critical in today’s modern electrical systems.
Power blackouts cannot be tolerated.
Article 100 of the NEC® defines this as:
Coordination (Selective). Localization of an overcurrent condition to restrict
outages to the circuit or equipment affected, accomplished by the choice
of overcurrent protective devices and their ratings or settings.
The two one-line diagrams in Figure 1 illustrate the concept of selective
coordination. The system represented by the one-line diagram to the left is
a system without selective coordination. A fault on the loadside of one
overcurrent protective device unnecessarily opens other upstream
overcurrent protective device(s). The result is unnecessary power loss to
loads that should not be affected by the fault. This is commonly known as a
"cascading effect" or lack of coordination. The system represented by the
one-line diagram to the right is a system with selective coordination. For the
full range of overload or fault currents possible for this system, only the
nearest upstream overcurrent protective device opens. All the other upstream
overcurrent protective devices do not open. Therefore, only the circuit with
the fault is removed and the remainder of the power system is unaffected.
The power for other loads in the system continue uninterrupted. The
overcurrent could occur on a feeder circuit, too, and a selectively coordinated
circuit would only have the immediate upstream feeder overcurrent
protective device open.
Selective coordination is an easy concept to understand. However, quite
often in the design or equipment selection phase, it is ignored or overlooked.
And when it is evaluated, many people misinterpret the information thinking
that selective coordination has been achieved, when in fact, it has not. The
following sections explain how to evaluate whether overcurrent protective
devices provide selective coordination for the full range of overcurrents.
Methods of Performing a Selective Coordination Study
Currently three methods are most often used to perform a coordination
study:
1. For fuse systems, 600V or less, use the published selectivity ratios which are
presented in the next section for Cooper Bussmann® fuses. The ratios apply for
all overcurrent conditions including overloads and short-circuit currents. Using
the fuse selectivity ratio method is easy and quick. There is no need to use
time-current curves.
2. Computer programs allow the designer to select time-current curves
published by manufacturers and place curves of all OCPDs of a circuit on one
graph. However, simply plotting the curves does not prove selective
coordination. The curves must be analyzed and interpreted properly in relation to
the available fault currents at various points in the system.
3. Overlays of time-current curves, with the manufacturers’ published data are
hand traced on log-log paper. Proper analysis and interpretation is important in
this case, also.
Note: Some circuit breaker manufacturers provide tested coordination tables that may be used in
place of or in addition to method 2 or 3 above.
Coordination Analysis
The next several pages cover selective coordination from various
perspectives. The major areas include:
• Fuses
• Circuit breakers
• Systems with fuse and circuit breaker mixture
• Mandatory selective coordination requirements
• Why selective coordination is mandatory
• Selective coordination system considerations
• Ensuring compliance
• Requirements inspection check list
• Fuse and circuit breaker choice considerations table
• Objections and misunderstandings
• Ground fault protection relays
Selective Coordination: Avoids Blackouts
Without Selective Coordination
With Selective Coordination
OPENS
OPENS
NOT AFFECTED
UNNECESSARY
POWER LOSS
NOT AFFECTED
Fault
Fault
Figure 1
©2008 Cooper Bussmann
1
Selective Coordination
600
400
300
200
400A
100
Point E
80
Point C
60
40
100A
30
Available
Fault
Current
Level
1000A
20
TIME IN SECONDS
Point G
10
8
Figure 3a.
6
4
3
2
Point B
Point D
1
.8
.6
.4
.3
.2
Point F
.1
.08
Minimum Melt
Total Clearing
.06
.04
.03
H
20,000
6000
4000
3000
2000
800
1000
400
600
CURRENT IN AMPERES
8000
10,000
Point A 1000A
300
.01
200
.02
100
Figure 2 illustrates the time-current characteristic curves for two amp
ratings of time-delay, dual-element fuses in series, as depicted in the oneline diagram. The horizontal axis of the graph represents the RMS
symmetrical current in amps. The vertical axis represents the time, in
seconds. Each fuse is represented by a band: the minimum melt
characteristic (solid line) and the total clear characteristics (hash line).
The band between the two lines represents the tolerance of that fuse
under specific test conditions. For a given overcurrent, a specific fuse,
under the same circumstances, will open at a time within the fuse’s timecurrent band.
Fuses have an inverse time-current characteristic, which means the greater
the overcurrent, the faster they interrupt. Look at the 100A fuse curve: for an
overcurrent of 200A, the fuse will interrupt in approximately 200 seconds
and for an overcurrent of 2000A, the fuse will open in approximately 0.15
second.
In some cases, to assess coordination between two or more fuses, the fuse
time-current curves are compared. This method is limited to only the
overcurrent range for which the fuse curves are visible on the graph.
For example: Assume an overcurrent level of 1000A RMS symmetrical on the
loadside of the 100A fuse. To determine the time it would take this
overcurrent to open the two fuses, first find 1000A on the horizontal axis
(Point A), follow the dotted line vertically to the intersection of the total clear
curve of the 100A fuse (Point B) and the minimum melt curve of the 400A
fuse (Point C). Then, horizontally from both intersection points, follow the
dotted lines to Points D and E. At 1.75 seconds, Point D represents the
maximum time the 100A fuse will take to open the 1000A overcurrent. At 90
seconds, Point E represents the minimum time at which the 400A fuse could
open this overcurrent. These two fuses are coordinated for a 1000A
overcurrent.
For overcurrents up to approximately 11,000A (Point H), since no overlap of
curves exists, it can be determined that the two fuses are selectively
coordinated. The 100 amp fuse will open before the 400 amp fuse can melt.
However, notice above approximately 11,000A, selective coordination cannot
be determined by the time-current curves. The operating characteristics for
both fuses are less than 0.01 second. For operating times less than 0.01
second, a fuse is operating in or near its current-limiting range and another
method must be used to assess whether two fuses selectively coordinate.
Cooper Bussmann publishes selectivity ratios for their fuses that make it
simple to assess whether fuses selectively coordinate. If you use the
selectivity ratios, plotting fuse curves is unnecessary.
100A
Fuses
400A
Fuse Curves
Figure 2
2
©2008 Cooper Bussmann
Selective Coordination
Fuse Selectivity Ratio Guide
Selective Coordination with Fuses
To determine fuse selectivity is simple physics. Selectivity between two
fuses operating under short-circuit conditions exists when the total clearing
energy of the loadside fuse is less than the melting energy of the lineside
fuse. The following explains this process.
Figure 3 illustrates the principle of selective coordination when fuses are
properly applied. Where high values of fault current are available, the
sub-cycle region (less than 0.01 second) becomes the most critical region
for selective operation of current-limiting fuses. The available short-circuit
current that could flow is depicted by the dotted line. If no protective device
were present, or if mechanical type overcurrent devices with opening times
of one-half cycle or longer were present, the full available short-circuit
current energy could be delivered to the system. When a fuse is in its
current-limiting range, the fuse will clear the fault in approximately one-half
cycle or less, and can greatly reduce the effective let-through current.
Note that Tm is the melting time of the fuse and Tc is the total clearing time
of the fuse. The area under the current curves over a time period is
indicative of the energy let-through. The amount of thermal energy delivered
is directly proportional to the square of the current multiplied by clearing
time (I2t). The amount of energy being released in the circuit while the fuse
element is melting (or vaporizing) is called the melting energy and energy
produced during the entire interruption process (melting plus arcing) is called
total clearing. To achieve a selectively coordinated system the Tc and clearing
I2t of the downstream fuse must be less than the Tm and melting I2t of the
upstream fuse.
Requirements for selective coordination: total clearing energy of load side fuse is
less than melting energy of line side fuse.
Figure 3
©2008 Cooper Bussmann
3
Selective Coordination
Fuse Selectivity Ratio Guide
Simply adhering to fuse selectivity ratios makes it easy to design and install
fusible systems that are selectively coordinated. See the Cooper Bussmann
Selectivity Ratio Guide. The top horizontal axis shows loadside fuses and the
left vertical axis shows lineside fuses. These selectivity ratios are for all
levels of overcurrents up to the fuse interrupting ratings or 200,000A,
whichever is lower. The ratios are valid even for fuse opening times less than
0.01 second. The installer just needs to install the proper fuse type and amp
rating. It is not necessary to plot time-current curves or do a short-circuit
current analysis (if the available short-circuit current is less than 200,000A
or the interrupting rating of the fuses, whichever is less). All that is
necessary is to make sure the fuse types and amp rating ratios for the
mains, feeders and branch circuits meet or exceed the applicable selectivity
ratios. If the ratios are not satisfied, then the designer should investigate
another fuse type or design change.
Notice the Low-Peak® fuses (LPJ_SP, LPN-RK_SP, LPS-RK_SP, and
KRP-C_SP) as well as the CUBEFuse® (TCF) only require a 2:1 amp rating
ratio to achieve selective coordination. This simplifies the design process and
flexibility.
Selectivity Ratio Guide (Lineside to Loadside)1
Circuit
Current Rating
Type
Lineside Fuse
Trade Name
Class
Cooper Bussmann
Symbol
601 to
6000A
601 to
4000A
TimeDelay
TimeDelay
0
to
DualEle-
Low-Peak®
(L)
Limitron®
(L)
Low-Peak
(RK1)
(J)
601-6000A
TimeDelay
Low-Peak
(L)
KRP-C_SP
KRP-C_SP
KLU
LPN-RK_SP
LPS-RK_SP
LPJ-SP
TCF1
FRN-R
FRS-R
KTU
Loadside Fuse
601-4000A
TimeDelay
Limitron
(L)
KLU
0-600A
Dual-Element
Time-Delay
Low-Peak
Low-Peak Fusetron
(RK1)
(J)
(RK5)
LPN-RK_SP
LPJ-SP
FRN-R
LPS-RK_SP
TCF2
FRS-R
601-6000A
FastActing
Limitron
(L)
KTU
0-600A
FastActing
Limitron
(RK1)
KTN-R
KTS-R
0-1200A
FastActing
T-Tron
(T)
JJN
JJS
0-600A
FastActing
Limitron
(J)
JKS
0-60A
TimeDelay
SC
(G)
SC
4
ment
(CC)
LP-CC
FNQ-R
KTK-R
2.5:1
2:1
2:1
2:1
2:1
4:1
2:1
2:1
2:1
2:1
2:1
2:1
–
–
2:1
2:1
8:1
–
3:1
3:1
3:1
4:1
2:1
Fusetron®
–
–
1.5:1
1.5:1
2:1
–
1.5:1
1.5:1
1.5:1
1.5:1
(RK5)
601 to
Limitron
2:1
2.5:1
2:1
2:1
6:1
2:1
2:1
2:1
2:1
2:1
6000A
(L)
0 to
Fast- Limitron
KTN-R
–
–
3:1
3:1
8:1
–
3:1
3:1
3:1
4:1
600A
Acting (RK1)
KTS-R
0 to
T-Tron®
JJN
–
–
3:1
3:1
8:1
–
3:1
3:1
3:1
4:1
1200A
(T)
JJS
0 to
Limitron
JKS
–
–
2:1
2:1
8:1
–
3:1
3:1
3:1
4:1
600A
(J)
0 to
Time- SC
SC
–
–
3:1
3:1
4:1
–
2:1
2:1
2:1
2:1
60A
Delay (G)
1. Where applicable, ratios are valid for indicating and non-indicating versions of the same fuse. At some values of fault current, specified ratios may be lowered to permit closer fuse sizing.
Consult with Cooper Bussmann. Ratios given in this Table apply only to Cooper Bussmann® fuses. When fuses are within the same case size, consult Cooper Bussmann.
2. TCF (CUBEFuse®) is 1 to 100A Class J performance; dimensions and construction are unique, finger-safe IP20 design.
NOTE: All the fuses in this table have interrupting ratings of 200kA or greater, except the SC fuses have 100kA IR.
600A
0-30A
2:1
2:1
©2008 Cooper Bussmann
Selective Coordination
Fuse Selectivity Ratio Guide
Example of Fuse Selective Coordination
The following example illustrates the simple process to achieve selective
coordination with a fusible system. Review the oneline diagram of the
fusible system in the Figure 4. All the fuses are Low-Peak® fuses. The
Selectivity Ratio Guide provides the minimum ampacity ratio that must be
observed between a lineside fuse and a loadside fuse in order to achieve
selective coordination between the two fuses. If the entire electrical system
maintains at least these minimum fuse ampacity ratios for each circuit path,
the entire electrical system will be selectively coordinated for all levels of
overcurrent. Note, time-current curves do not need to be plotted.
One-Line For Fuse
System Coordination
Analysis
Check the LPJ-400SP fuse coordination with the KRP-C-1200SP fuse.
Use the same steps as in the previous paragraph. The ampacity ratio of the
two fuses in this circuit path is 1200:400, which yields an ampacity ratio of
3:1. The Selectivity Ratio Guide shows that the ampacity ratio must be
maintained at 2:1 or more to achieve selective coordination for these specific
fuses. Since the fuses used have a 3:1 ratio, and all that is needed is to
maintain a 2:1 ratio, these two fuses are selectively coordinated for any
overcurrent condition up to 200,000A. The result is this entire circuit path
then is selectively coordinated for all overcurrents up to 200,000A.
See Figure 5.
Selective Coordination
Only Faulted Circuit
Cleared
Low-Peak
KRP-C-1200SP Fuse
Low-Peak
KRP-C1200SP Fuses
Low-Peak
LPJ-400SP Fuses
e
es n
Th e
ly s Op
n
O se
Fu
Low-Peak
LPJ-400SP Fuses
Opens
Not
Affected
Low-Peak
LPJ-100SP Fuses
Any Fault Level !
Low-Peak
LPJ-100SP Fuses
Figure 5
Any Fault Level!
Figure 4
Check the LPJ-100SP fuse coordination with the LPJ-400SP fuse.
The ampacity ratio of these fuses in this circuit path is 400:100 which
equals a 4:1 ratio. Checking the Selectivity Ratio Guide, lineside LPJ (left
column) to load-side LPJ (top horizontal row), yields a ratio of 2:1. This
indicates selective coordination for these two sets of fuses for any
overcurrent condition up to 200,000A. This means for any overcurrent on the
loadside of the LPJ-100SP fuse, only the LPJ-100SP fuse opens. The
LPJ-400SP fuse remains in operation as well as the remainder of the
system.
©2008 Cooper Bussmann
5
Selective Coordination
Fusible Lighting Panels
Fusible Lighting Panels
There are multiple suppliers of fusible switchboards, power distribution
panels and motor control centers, but there are not fusible lighting panels
available from these same suppliers. Now the Cooper Bussmann®
Quik-Spec™ Coordination Panelboard provides the fusible solution for
branch panelboard applications, making it simple and cost effective to
selectively coordinate the lighting and other branch circuits with upstream
Cooper Bussmann® fuses.
This new panelboard is available in MLO (Main Lug Only), as well as fused or
non-fused main disconnect configurations with a choice of 18, 30 and 42
branch positions in NEMA 1 or 3R enclosures to easily meet the needs for
branch or service panel installations. This branch circuit panelboard uses
the Cooper Bussmann® finger-safe CUBEFuse® (1 to 60A, UL Listed,
current-limiting, time-delay, Class J performance) for the branch circuit
protective devices as an integral part of the innovative, patented Compact
Circuit Protector Base (CCPB) fusible UL 98 disconnect available in 1-, 2- and
3-pole versions. The fused main disconnect options are either 100A or
200A indicating Class J Cooper Bussmann® Low-Peak® LPJ_SPI fuses or
60A CUBEFuse. The panel is rated 600Vac and capable of providing high
Short-Circuit Current Ratings (SCCR) up to 200kA. The footprint is the same
size as traditional panelboards: 20” W x 5 3⁄4” D x 50” or 59” H (the height
depends on configuration and number of branch circuit positions). Two key
features of this new panelboard are fuse/CCPB disconnect switch interlock
which prevents removing a fuse while energized and a CUBEFuse®/ CCPB
disconnect ampacity rejection feature which coincides with standard branch
circuit amp ratings to help ensure proper fuse replacement.
The CUBEFuse® and Low-Peak® LPJ_SPI fuses are easy to selectively
coordinate with each other and other Low-Peak® fuses that are used in
upstream power distribution panelboards and switchboards. Merely
maintain at least a 2:1 fuse amp rating ratio between upstream and
downstream Low-Peak® fuses and selective coordination is ensured up
to 200kA.
Quik-Spec™ Coordination Panelboard
For further information on this panel visit
www.cooperbussmann.com/quik-spec for Data Sheet 1160, specification,
Application Notes and more.
CUBEFuse CCPB Fused
Branch Disconnect
6
©2008 Cooper Bussmann
Selective Coordination
Fuses
Another Fuse Selective Coordination Example
Figure 6 is an example where the fuses considered initially do not meet the
minimums in the Selectivity Ratio Guide. One option is to investigate other
fuse alternatives. In doing so, it is necessary to understand the various fuse
alternatives and considerations which are not covered in this section. But
this example provides the reader the concept of investigating other
alternatives. In this example, the FRS-R-200 fuses selectively coordinate
with the FRS-R-400 fuses since they have a 2:1 ratio and the Selectivity
Ratio Guide minimum is 2:1 for FRS-R to FRS-R fuses. However, the
FRS-R-400 fuse to KRP-C-800SP fuse is a 2:1 ratio and the Selectivity Ratio
Guide requires at least a 4:1 ratio. Figure 7 is a progression of analysis that
is possible to obtain selective coordination by specifying another type of
fuse. In this case, it is important to know that the FRS-R fuses and
LPS-RK_SP fuses have the same mounting dimensions (they can be installed
in the same holders and blocks) and the LPS-RK_SP fuses have the same
overload characteristics as the FRS-R fuses. This means the LPS-RK_SP
fuses should be able to be sized for the loads in the same manner as the
FRS-R fuses. The LPS-RK_SP fuses have better current-limiting
characteristics, which results in better component protection and in most
cases, better arc-flash protection. In Figure 7, Scenario A is the initial fuse
selection that does not meet the selectivity ratios. In Scenario B, the
FRS-R-400 fuses are changed to LPS-RK-400SP fuses and will selectively
coordinate with the KRP-C-800SP fuses. However, now the FRS-R-200 fuse
and LPS-RK-400SP fuse do not meet the minimum selectivity ratio, which is
8:1 for these fuses. In Scenario C, the FRS-R-200 fuses are changed to
LPS-RK-200SP fuses and these are selectively coordinated, since the
minimum selectivity ratio is 2:1.
Figure 7
Building System Recommendation
As demonstrated in the previous section, doing an analysis for selective
coordination of a fuse system is relatively simple. However, there are many
fuse types and associated ratios. For building electrical systems, the
following Low-Peak® fuses are recommended for 1⁄10 to 6000A, 600V or less
(all but the LPN-RK_SP are rated 600V or less which means they can be
used on any system up to 600V). Low-Peak fuses all have 2:1 selectivity
ratios with any other Low-Peak fuses.
Quik-Spec™ Cordination Panelboard (branch circuit panelboard)
• TCF_RN* Class J 1 to 60A
Large ampacity circuits where fuse is greater than 600A
• KRP-C_SP Class L
601 to 6000A
Main switchboards, power distribution panelboard, MCCs, etc 600A or less
• LPJ_SP Class J
1 to 600A Smaller than LPS-RK fuses
or
• LPS-RK_SP (600V) or LPN-RK_SP (250V) Class RK1 1 to 600A
Summary — Fuse Selective Coordination
Figure 6
With modern current-limiting fuses, selective coordination can be achieved
by adhering to selectivity ratios. It is neither necessary to plot the time
current curves nor to calculate the available short-circuit currents (for
systems up to 200,000A). Just maintain at least the minimum amp rating
ratios provided in the Selectivity Ratio Guide and the system will be
selectively coordinated. This simple method is easy and quick. If the
available fault current increases due to a transformer change, the selectivity
is retained. The user should keep adequate spare fuses and the electrician
should always replace opened fuses with the same type and amp rating.
The selectivity ratios are not valid with a mixture of Cooper Bussmann®
fuses and fuses of another manufacturer. If a design does not provide
selective coordination, first investigate other Cooper Bussmann fuse types
that may have different selectivity ratios. Note: if another fuse type is
investigated, the application sizing guidelines for that fuse should also be
considered. If selective coordination still cannot be achieved, then a design
change may be necessary.
*TCF_RN is non-indicating version of the CUBEFuse®. CUBEFuse is UL Listed, Class J performance
with special finger-safe IP20 construction.
©2008 Cooper Bussmann
7
Selective Coordination
Circuit Breakers
Circuit Breaker Operation Basics
Circuit Breaker Overload Operation
Circuit breakers are mechanical overcurrent protective devices. All circuit
breakers share three common operating functions:
1. Current sensing means:
A. Thermal
B. Magnetic
C. Electronic
2. Unlatching mechanism: mechanical
3. Current/voltage interruption means (both)
A. Contact parting: mechanical
B. Arc chute
Figures 9 and 10 illustrate circuit breaker operation by a thermal bimetal
element sensing a persistent overload. The bimetal element senses overload
conditions. In some circuit breakers, the overload sensing function is
performed by electronic means. In either case, the unlatching and
interruption process is the same. Figure 9 illustrates that as the overload
persists, the bimetal sensing element bends. If the overload persists for too
long, the force exerted by the bimetal sensor on the trip bar becomes
sufficient to unlatch the circuit breaker. Figure 10 shows that once a circuit
breaker is unlatched, it is on its way to opening. The spring-loaded contacts
separate and the overload is cleared. There can be some arcing as the
contacts open, but the arcing is not as prominent as when a short-circuit
current is interrupted.
The circuit breaker’s physics of operation is significantly different from that
of a fuse. First, the circuit breaker senses the overcurrent. If the overcurrent
persists for too long, the sensing means causes or signals the unlatching of
the contact mechanism. The unlatching function permits a mechanism to
start the contacts to part. As the contacts start to part, the current is
stretched through the air and arcing between the contacts commences. The
further the contacts separate the longer the arc, which aids in interrupting
the overcurrent. However, in most cases, especially for fault current, the
contacts alone are not sufficient to interrupt. The arcing is thrown to the arc
chute which aids in stretching and cooling the arc so that interruption can be
made. Figure 8 shows a simplified model with the three operating functions
shown for a thermal magnetic circuit breaker, which is the most commonly
used circuit breaker. Also, it should be noted that there are various contact
mechanism designs that can significantly affect the interruption process.
Figure 9
Figure 8
Figure 10
8
©2008 Cooper Bussmann
Selective Coordination
Circuit Breakers
Circuit Breaker Instantaneous Trip Operation
Figures 11, 12 and 13 illustrate circuit breaker instantaneous trip operation
due to a short-circuit current. The magnetic element senses higher level
overcurrent conditions. This element is often referred to as the instantaneous
trip, which means the circuit breaker is opening without intentional delay. In
some circuit breakers, the instantaneous trip sensing is performed by
electronic means. In either case, the unlatching and interruption process is
the same as illustrated in Figures 12 and 13. Figure 11 illustrates the high
rate of change of current due to a short-circuit causing the trip bar to be
pulled toward the magnetic element. If the fault current is high enough, the
strong force causes the trip bar to exert enough force to unlatch the circuit
breaker. This is a rapid event and is referred to as instantaneous trip.
Figure 12 shows that once unlatched, the contacts are permitted to start to
part. It is important to understand that once a circuit breaker is unlatched it
will open. However, the current interruption does not commence until the
contacts start to part. As the contacts start to part, the current continues to
flow through the air (arcing current) between the stationary contact and the
movable contact. At some point, the arc is thrown to the arc chute, which
stretches and cools the arc. The speed of opening the contacts depends on
the circuit breaker design. The total time of the current interruption for circuit
breaker instantaneous tripping is dependent on the specific design and
condition of the mechanisms. Smaller amp rated circuit breakers may clear
in 1⁄2 to 1 cycle or less. Larger amp rated circuit breakers may clear in a
range typically from 1 to 3 cycles, depending on the design. Circuit breakers
that are listed and marked as current-limiting can interrupt in 1⁄2 cycle or less
when the fault current is in the circuit breaker’s current-limiting range. With
the assistance of the arc chute, as well as the alternating current
running its normal course of crossing zero, and the contacts traveling a
sufficient distance, the fault current is interrupted (see Figure 13). There can
be a tremendous amount of energy released in the contact interruption path
and arc chute during the current interruption process. As a consequence,
circuit breakers are designed to have specific interrupting ratings at specific
voltage ratings. For instance, a circuit breaker may have a 14,000A IR
at 480Vac and 25,000A IR at 240Vac.
Figure 11
Figure 12
Figure 13
©2008 Cooper Bussmann
9
Selective Coordination
Circuit Breakers
1000
800
600
400 Ampere Circuit Breaker
400
300
d Re
rloa
200
Minimum
Unlatching
Time
gion
10
Typical Circuit Breaker Time-Current Characteristic Curve
Ove
Maximum
Interrrupting Time
100
80
Average Unlatching Times
Breaker Tripping Magnetically
Current in
RMS Amps
5,000
10,000
15,000
20,000
25,000
Time in
Seconds
0.0045
0.0029
0.0024
0.0020
0.0017
60
Interrupting Rating
40
RMS Sym.
240V
480V
600V
30
20
Amps
42,000
30,000
22,000
10
8
6
Adjustable Magnetic
Instantaneous Trip
Set at 10 Times
I.T. = 10X
(± 10% Band)
4
3
2
1
.8
Adjustable
Instantaneous Trip
Set at 5 Times
I.T. = 5X
(± 25% Band)
.6
.4
.3
.2
.1
.08
.06
Maximum
Interrupting
Time
.04
.03
.02
Instantanous Region
.01
.008
.006
.004
Interrupting
Rating
at 480 Volt
.003
80,000
60,000
100,000
40,000
30,000
20,000
8000
6000
10,000
4000
3000
800
1000
600
400
300
.001
2000
Average Unlatching
Times for
Instantaneous Tripping
.002
200
1. Overload Region: overloads typically can be tolerated by the circuit
components for relatively longer times than faults and therefore, the opening
times are in the range of seconds and minutes. As can be seen, the
overload region has a wide tolerance band, which means the breaker should
open within that area for a particular overload current.
2. Instantaneous Region: the circuit breaker will open as quickly as
possible. The instantaneous trip (IT) setting indicates the multiple of the full
load rating at which the circuit breaker starts to operate in its instantaneous
region. Circuit breakers with instantaneous trips either have (1) fixed
instantaneous trip settings or (2) adjustable instantaneous trip settings. The
instantaneous region is represented in Figure 14, and for this example, is
shown to be adjustable from 5x to 10x the breaker amp rating. When the
breaker senses an overcurrent in the instantaneous region, it releases the
latch which holds the contacts closed (unlatches). Unlatching permits the
contact parting process to start.
The unlatching time is represented by the curve labeled “average unlatching
times for instantaneous tripping” (this is the continuation of the
instantaneous trip curve below 0.01 second). The manufacturer of the
circuit breaker in Figure 14 also published a table of unlatching times for
various currents (upper right). Unfortunately, most circuit breaker
manufacturers no longer publish the unlatching times for their circuit
breakers. However, all circuit breakers have an unlatching characteristic, so
learning about the unlatching characteristic is fundamental in understanding
how circuit breakers perform. Unlatching frees or releases the spring loaded
contacts to start the process of parting. After unlatching, the overcurrent is
not cleared until the breaker contacts are mechanically separated and the
arc is extinguished (represented in Figure 14 as the maximum interrupting
time). Consequently, there is a wide range of time from unlatching to
interruption as is indicated by the wide band between the unlatching time
curve and the maximum interrupting time curve. This wide range of time
adversely affects the ability of circuit breakers with instantaneous trips to
selectively coordinate when the overcurrent magnitude is in the
instantaneous trip range.
Many of the lower amp rated circuit breakers (100A and 150A frame CBs)
have non-adjustable or fixed instantaneous trip settings. For larger molded
case, insulated case and power breakers the instantaneous trip setting can
usually be adjusted by an external dial. Two instantaneous trip settings for a
400A breaker are shown in Figure 14. The instantaneous trip region, drawn
with the solid line, represents an IT = 5x, or five times 400A = 2000A. At this
setting, the circuit breaker will trip instantaneously on currents of
approximately 2000A or more. The ± 25% band represents the area in which
it is uncertain whether the overload trip or the instantaneous trip will operate
to clear the overcurrent. The dashed portion represents the same 400A
breaker with an IT = 10x, or 10 times 400A = 4000A. At this setting the
overload trip will operate up to approximately 4000 amps (±10%).
Overcurrents greater than 4000A (±10%) would be sensed by the
instantaneous setting. The ± 25% and ±10% band mentioned in this
paragraph represents a tolerance. This tolerance can vary by circuit breaker
manufacturer and type.
TIME IN SECONDS
When using molded case circuit breakers of this type, there are three basic
curve considerations that must be understood (see Figure 14). These are:
1. Overload region
2. Instantaneous region with unlatching
3. Interrupting rating
The IT of a circuit breaker is typically set at its lowest setting when shipped
from the factory. Note that most published circuit breaker time-current
curves show the vertical time axis from 0.01 second up to about 100 or
1000 seconds. The published curves do not provide the instantaneous
unlatching characteristic. However, if a circuit breaker has an instantaneous
trip, it has unlatching times usually less than 0.01 second.
Some circuit breakers have short time-delay trip settings (STD). These will
be discussed later in this section. The short time-delay trip option can be
used in conjunction with (1) an instantaneous trip settings or (2) without
instantaneous trip settings. Typically, molded case circuit breakers and
insulated case circuit breakers that have short time-delay settings have an
instantaneous trip override. This means at some fault current level, the
instantaneous trip operates to protect the circuit breaker. Low voltage
power circuit breakers can be specified with a short time-delay setting
which does not inherently incorporate an instantaneous trip override.
100
Circuit Breaker Curves
CURRENT IN AMPERES
Figure 14
©2008 Cooper Bussmann
Selective Coordination
Circuit Breakers
Interrupting Rating: The interrupting rating is represented on the drawing
by a vertical line at the right end of the curve. The interrupting rating for
circuit breakers varies based on the voltage level; see the interrupting rating
table in Figure 14 which lists the interrupting ratings for this specific circuit
breaker. For coordination purposes, the vertical line is often drawn at the
fault current level in lieu of the interrupting rating (if the interrupting rating is
greater than the available short-circuit current). However, if the fault current
is above the interrupting rating, a misapplication and violation of NEC® 110.9
is evident. In Figure 14, the circuit breaker interrupting rating at 480 volts is
30,000 amps. The marked interrupting rating on a three-pole circuit breaker
is a three-pole rating and not a single-pole rating (refer to Single-Pole
Interrupting Capability section for more information).
Achieving Selective Coordination with Low Voltage
Circuit Breakers
To achieve selective coordination with low voltage circuit breakers, no
overlap of time-current curves (including the unlatching time) is permitted up
to the available short-circuit current. The ability of circuit breakers to
achieve coordination depends upon the type of circuit breakers selected;
amp ratings, settings and options of the circuit breakers, and the available
short-circuit currents. The type of circuit breaker selected could be one of
three types: circuit breakers with instantaneous trips; circuit breakers with
short time-delay but incorporating instantaneous overrides; or circuit
breakers with short time-delays (no instantaneous override). In this section,
various alternative circuit breaker schemes will be discussed in relation to
assessing for selective coordination.
Two Instantaneous Trip Circuit Breakers
Figure 15 illustrates a 90 amp circuit breaker and an upstream 400 amp
circuit breaker having an instantaneous trip setting of 5x (5 times 400A =
2000A). The minimum instantaneous trip current for the 400A circuit breaker
could be as low as 2000A times 0.75 = 1500A (± 25% band). If a fault
above 1500 amps occurs on the loadside of the 90 amp breaker, both
breakers could open. The 90 amp breaker may unlatch before the 400 amp
breaker. However, before the 90 amp breaker can part its contacts and clear
the fault current, the 400 amp breaker could have unlatched and
started the irreversible contact parting process.
Assume a 4000A short-circuit exists on the loadside of the 90A circuit
breaker. The sequence of events would be as follows:
1. The 90A breaker will unlatch (Point A) and free the breaker mechanism to start
the contact parting process.
2. The 400A breaker will unlatch (Point B) and it, too, would begin the contact
parting process. Once a breaker unlatches, it will open. At the unlatching point,
the process is irreversible. It is similar to pulling a trigger on a gun.
3. At Point C, the 90A breaker will have completely interrupted the fault current.
4. At Point D, the 400A breaker also will have opened, which unnecessarily
disrupts power to all other loads.
©2008 Cooper Bussmann
11
Selective Coordination
Circuit Breakers
The norm in the industry is to display circuit breaker curves for times from
0.01 second to about 100 or 1000 seconds. So typically the circuit breaker
curves are not shown with the unlatching curves as in Figure 15. The
following Figure 16 illustrates a 400A (IT = 7x) circuit breaker feeding a
100A circuit breaker. However, this curve, which is the industry norm, does
not show the circuit breaker characteristics below 0.01 second. For
coordination analysis, the interpretation of this curve is that these two circuit
breakers are selectively coordinated for overcurrents less than approximately
2100A (arrow on Figure 16). For overcurrents greater than 2100A, these two
circuit breakers, with these settings, would not be coordinated.
The following is an excerpt from IEEE 1015-2006 “Blue Book” Applying
Low-Voltage Circuit Breakers Used in Industrial and Commercial Power
Systems, page 145 5.5.3 Series MCCBs:
“Selective coordination is limited to currents below the instantaneous pickup
of the lineside circuit breaker. For any fault downstream of the loadside
MCCB having a current greater than the instantaneous pickup of the lineside
MCCB, both circuit breakers trip, and power is interrupted to unfaulted
circuits fed by the lineside circuit breaker.”
1000
800
600
400
400A
300
200
90A
100
80
4000A
60
40
30
20
400Amp Circuit Breaker
I.T. = 5X
90Amp
Circuit Breaker
10
8
6
4
TIME IN SECONDS
3
2
1
.8
.6
.4
.3
.2
.1
.08
.06
.04
.03
•
.02
•
D
C
.01
.008
.006
B
•
A•
.004
.003
.002
80,000
100,000
40,000
60,000
30,000
20,000
6000
4,000A
8000
10,000
1,500A
CURRENT IN AMPERES
3000
2000
800
1000
600
400
300
200
80
100
60
40
30
20
10
.001
14,000A 30,000A
I.R.
I.R.
Figure 15
These two specific circuit breakers with the settings as stated are selectively
coordinated for any overcurrent up to approximately 1500A. However, this is
a non-selective system where fault currents are above 1,500 amps,
causing a blackout to all the loads fed by the 400 amp breaker. As
mentioned previously, this is typical for molded case circuit breakers due to
the instantaneous trip and wide band of operation on medium to high fault
conditions. In addition, this can affect other larger upstream circuit breakers
depending upon the size and the instantaneous setting of the circuit
breakers upstream and the magnitude of the fault current.
As published by one circuit breaker manufacturer: “One should not overlook
the fact that when a high fault current occurs on a circuit having several
circuit breakers in series, the instantaneous trip on all breakers may operate.
Therefore, in cases where several breakers are in series, the larger upstream
breaker may start to unlatch before the smaller downstream breaker has
cleared the fault. This means that for faults in this range, a main breaker
may open when it would be desirable for only the feeder breaker to open.”
This is typically referred to in the industry as a "cascading effect."
12
Figure 16
©2008 Cooper Bussmann
Selective Coordination
Circuit Breakers
Interpreting Circuit Breaker Curves for Selective
Coordination
Figure 17 is the one-line diagram that will be used for the next couple of
examples. It has three molded case circuit breakers in series: 1200A main,
400A feeder with the 100A branch circuit. The other circuit breakers on the
one-line diagram supply other circuits and loads. The fault current path from
the power source is depicted by the red arrows/lines on the one-line
diagram. For the coordination analysis, faults on both the branch circuit and
feeder must be analyzed.
When the curves of two circuit breakers cross over in their instantaneous trip
region, then the drawing indicates that the two circuit breakers do not
coordinate for fault currents greater than this cross over point.
For instance, interpreting the curves for the 100A circuit breaker and the
400A circuit breaker. Their curves intersect in the instantaneous region
starting at approximately 3600A. The 1200A circuit breaker curve intersects
the 100A and 400A circuit breaker curves at approximately 6500A.
Analysis for branch circuit fault:
For a branch circuit fault current less than 3600A on the loadside of the
100A circuit breaker, the 400A and 1200A circuit breakers will be
selectively coordinated with the 100A circuit breaker. If the fault current is
greater than 3600A, then the 400A feeder circuit breaker unnecessarily
opens and there is a lack of coordination.
If the branch circuit fault is greater than 6500A, then the 1200A main
circuit breaker unnecessarily opens, which is a lack of coordination
between the 100A, 400A and 1200A circuit breakers. The reason is, for a
fault of greater than 6500A, all three of these circuit breakers are in their
instantaneous trip region. Both the 400A and 1200A circuit breakers can
unlatch before the 100A circuit breaker clears the fault current.
Analysis for feeder circuit fault:
For any feeder fault less than 6500 amps on the loadside of the 400A
circuit breaker, the 400A and 1200A circuit breakers will be selectively
coordinated. For feeder faults greater than 6500A, the 1200A circuit
breaker is not coordinated with the 400A feeder circuit breaker.
Conclusion for Figures 17 and 18 coordination analysis:
If the maximum available short-circuit current at the 100A branch circuit is
less than 3600A and the maximum available short-circuit current at the
400A feeder circuit is less than 6500A, then the circuit path (100A, 400A,
and 1200A) is selectively coordinated. If the maximum available short-circuit
current exceeds either of these values, the circuit path is not selectively
coordinated.
How does this affect the electrical system? Look at the one-line diagram in
Figure 19. For any fault current greater than approximately 6500A on the
loadside of the 100A circuit breaker, the 1200A and 400A circuit breakers
open as well as the 100A circuit breaker. The yellow shading indicates that
all three circuit breakers open - branch circuit, feeder and main. In addition,
all the loads fed by the other circuit breakers, denoted by the hash shading,
are blacked out unnecessarily. This is due to the lack of coordination
between the 100A, 400A and 1200A circuit breakers.
Figure 17
Figure 18
Figure 19
©2008 Cooper Bussmann
13
Selective Coordination
Circuit Breakers
Interpreting Curves with Current-Limiting Circuit
Breakers
Figure 20 is a coordination curve of a 60A current-limiting circuit breaker fed
by a 300A circuit breaker. This is a standard industry curve showing times
from 0.01 second and greater. For coordination analysis, this is
interpreted as the 300A circuit breaker coordinates with the 60A circuit
breaker for overcurrents less than 2100A (location of arrow). For
overcurrents greater than 2100A, the 300A circuit breaker is not coordinated
with the 60A circuit breaker. Figure 21, which shows times from 0.001
second and greater, illustrates the unlatching and clearing characteristics for
the 60A and 300A circuit breakers. Notice the 60A and 300A circuit
breaker curves overlap. (The unlatching characteristics for Figure 21 were
established by using past published data on a typical molded case circuit
breaker and referencing IEEE P1015 “Blue Book” for examples of unlatching
times for current-limiting circuit breakers.)
Figure 20
Figure 21
14
©2008 Cooper Bussmann
Selective Coordination
Circuit Breakers
CB Coordination:
Simplified Method Without Time-Current Curves
It is not necessary to draw the curves to assess circuit breaker coordination
when the circuit breakers are of the instantaneous trip type. There is a
simple method to determine the highest short-circuit current or short-circuit
amps (ISCA) at which circuit breakers will selectively coordinate. Simply
multiply the instantaneous trip setting by the circuit breaker amp rating. The
product of a circuit breaker’s instantaneous trip setting and its amp rating is
the approximate point at which a circuit breaker enters its instantaneous trip
region. This method is applicable to the instantaneous trip only, not the
overload region. However, in most cases, the circuit breaker overload
regions will coordinate. This simple method can be used as a first test in
assessing if a system is selectively coordinated. There may be other means
to determine higher values of ISCA where circuit breakers selectively
coordinate (such as manufacturer’s tables), but this is a practical, easy
method.
As explained previously, there is a tolerance where the instantaneous trip
initially picks up. A vertical band depicts the instantaneous trip pickup
tolerance. The following will illustrate this simple method ignoring the
tolerances. Then the simple method with the tolerances will be illustrated.
Ignoring the Tolerances
For this first example of the easy method, we will ignore the instantaneous
trip pickup tolerance band. However, the fault values where the circuit
breakers are selectively coordinated will differ from the same example when
using the curves in the previous section.
Using the simple method for the example in Figure 17, the 400A circuit
breaker has its instantaneous trip (IT) set at 10 times its amp rating (10x).
Therefore for fault currents above 10 x 400A = 4000 amps, the 400A circuit
breaker will unlatch in its instantaneous trip region, thereby opening. The
same could be determined for the 1200A circuit breaker, which has its
instantaneous trip set at 6x its amp rating. Therefore, for fault currents above
7200 amps (6 x 1200 = 7200A), the 1200A circuit breaker unlatches in its
instantaneous trip region, thereby opening.
The coordination analysis of the circuit breakers merely requires knowing
what the numbers mean.
General note: Many 100A and 150A frame circuit breakers have fixed
instantaneous trips which are not adjustable. For these circuit breakers the
fixed instantaneous trip will typically “pickup” between 800 to 1300 amps.
For adjustable circuit breakers, the instantaneous trip adjustment range can
vary depending upon frame size, manufacturer and type.
Typically adjustable settings of 4 to 10 times the amp rating are available
(check manufacturers’ data for specific circuit breakers). Circuit breakers are
generally shipped from the factory at the lowest adjustable instantaneous
trip setting. This setting should not be changed without a detailed analysis of
how it will affect the overall electrical system protection, coordination and
personnel safety.
With the Tolerances
This second example of the easy method will include the instantaneous trip
pickup tolerance band. This is a more accurate determination. The
tolerance is ±. However, for this simple method, it is only necessary to
consider the negative tolerance.
Information needed for each feeder and main circuit breaker (CB):
1. CB’s amp rating or amp setting
2. CB’s instantaneous trip setting (IT)
• Most feeder and main CBs have adjustable IT settings with varying
ranges from 3 to 12X
• Some CBs have fixed IT settings
• Some newer feeder CBs have fixed IT set at 20X
3. CB’s IT pickup percentage (%) tolerance
4. If CB IT pickup % tolerance is not known, here are some worst case*
practical rules of thumb:
• Thermal magnetic (high trip setting): ± 20%
• Thermal magnetic (low trip setting): ± 25%
• Electronic trip:
± 10%
* Based on numerous samples taken from leading CB manufacturers’ data.
Equation:
% tolerance**
ISCA Coordination < (CB amp rating x IT setting) x (1 )
100
Analysis for branch circuit faults:
In Figure 17, for a branch circuit fault less than 4000A on the loadside of
the 100A circuit breaker, the 400A and 1200A circuit breakers will be
selectively coordinated with the 100A circuit breaker. If the fault current is
greater than 4000A, then the 400A feeder circuit breaker unnecessarily
opens and there is a lack of coordination.
If the branch circuit fault is greater than 7200A, then the 1200A main
circuit breaker may unnecessarily open, which is a lack of coordination
between the 100A, 400A and 1200A circuit breakers. The reason is: for a
fault of greater than 7200A, all three of these circuit breakers are in their
instantaneous trip region. Both the 400A and 1200A circuit breakers can
unlatch before the 100A circuit breaker clears the fault current.
ISCA Coordination is the maximum short-circuit overcurrent at which the
circuit breaker will selectively coordinate with downstream circuit breakers.
** Use actual CB % tolerance, otherwise use assumed worst case % tolerances
For faults on the loadside of the 400A circuit breaker:
For any feeder fault less than 7200 amps on the loadside of the 400A
circuit breaker, the 400A and 1200A circuit breakers will be selectively
coordinated. For feeder faults greater than 7200A, the 1200A circuit
breaker is not coordinated with the 400A feeder circuit breaker.
©2008 Cooper Bussmann
15
Selective Coordination
Circuit Breakers
Example 1: See the one-line in Figure 22
Feeder: 200A Thermal magnetic CB with IT set at 10x and ± 20% IT pickup
tolerance
Main: 800A Electronic trip CB with IT set at 10X and ±10% IT pickup
tolerance
Calculations:
Feeder: 200A CB with IT set at 10x and ± 20% IT pickup tolerance
ISCA Coordination < (200 x 10) x (1 - 20% )
100
ISCA Coordination <
(2000)
x (1 - 0.20) = 2000A x 0.8
ISCA Coordination <
1600A
see Figure 22
Result: For overcurrents less than 1600A, the 200A CB will selectively
coordinate with the downstream CBs in the instantaneous region. For
overcurrents 1600A or greater, the 200A CB will not coordinate with
downstream circuit breakers.
Main: 800A CB with IT set at 10x and ± 10% IT pickup tolerance
ISCA Coordination < (800 x 10) x (1 - 10% )
100
Figure 22
ISCA Coordination <
(8000)
x (1 - 0.10) = 8000A x 0.9
ISCA Coordination <
7200A
see Figure 22
Result: For overcurrents less than 7200A, the 800A CB will selectively
coordinate with the downstream CBs in the instantaneous region. For
overcurrents 7200A or greater, the 800A CB will not coordinate with
downstream circuit breakers.
Figure 22 shows the time-current curves of this example. This example
illustrates that when assessing selective coordination for circuit breakers
with instantaneous trips, it is not necessary to plot the time-current curves.
Example 2:
The following is another example for the one-line diagram in Figure 23.
Using this simple method the values are easy to calculate and are shown in
the following table. Once you know the equation, you can do the simple
math and complete the table. It is not necessary to draw the curves,
However, the curves are shown in Figure 23.
CB Amp
Rating
16
IT
Setting
Tolerance
1000
6x
±10%
Coordinates
Up to ISCA
5,400A
400
10x
±20%
3,200A
100
–
NA
Figure 23
©2008 Cooper Bussmann
Selective Coordination
Circuit Breakers
Circuit Breaker Selective Coordination Tables
With selective coordination requirements more prevalent in the NEC®, in
recent years many circuit breaker manufacturers are publishing circuit
breaker-to-circuit breaker selective coordination tables based on testing.
These tables are for circuit breakers with instantaneous trips. The tables
typically have a format of a lineside circuit breaker feeding a loadside circuit
breaker and the values are maximum available short-circuit currents for
which the circuit breakers selectively coordinate. If these tables are used, be
sure to understand the parameters of the testing and the specifics on the
circuit breaker settings. Figure 24 shows the benefit of the table values
versus interpreting the curves for the 200A circuit breaker coordinating with
a 30A circuit breaker. Interpreting the curves shows the 200A circuit
breaker selectively coordinates with the 30A circuit breaker up to 1500A.
The selective coordination table published by the manufacturer of these
specific circuit breakers shows that they selectively coordinate up to 2700A.
Figure 25
Circuit Breakers with Short Time-Delay and
Instantaneous Override
Figure 24
Fixed High Magnetic Circuit Breakers
In recent years fixed high magnetic circuit breakers have been introduced
with the intent to provide more flexibility in achieving selective coordination.
Figure 25 illustrates a 200A fixed high magnetic trip circuit breaker. By
interpreting the curves, a normal 200A circuit breaker would selectively
coordinate with the 30 amp branch circuit breaker up to 1500A. This feeder
200A fixed high magnetic trip circuit breaker selectively coordinates with the
30A branch circuit breaker up to 3200A. This allows molded case circuit
breakers to selectively coordinate on circuits with higher available
short-circuit currents.
©2008 Cooper Bussmann
Some electronic trip molded case circuit breakers (MCCB) and most
insulated case circuit breakers (ICCB) offer short time-delay (STD) features.
This allows a circuit breaker the ability to delay tripping on fault currents for
a period of time, typically 6 to 30 cycles. However, with electronic trip
molded case circuit breakers and insulated case circuit breakers with
short time-delay setting (STD), an instantaneous trip override mechanism is
typically built in to protect the circuit breaker. This instantaneous override
function will override the STD for medium- to high-level faults. The
instantaneous override for these devices is typically 8 to 12 times the rating
of the circuit breaker and will “kick in” for faults equal to or greater than the
override setting (factory set and not adjustable). Thus, while short
time-delay in molded case and insulated case circuit breakers can improve
coordination in the low-level fault regions, it may not be able to assure
coordination for medium- and high-level fault conditions. This can be seen
in Figure 26; the 800A MCCB has a STD with an IT override (activates at 8
times for this manufacturer’s circuit breaker) and selectively coordinates with
the 100A downstream circuit breaker up to 6400A. As the overlap suggests,
for any fault condition greater than 6400A these two circuit breakers are not
coordinated: both devices may open. Because of this instantaneous
override, nonselective tripping can exist above 6400A.
17
Selective Coordination
Circuit Breakers
Figure 26
Figure 27
Low Voltage Power Circuit Breakers (LVPCB) with
Short Time-Delay
1. MCCBs and ICCBs with instantaneous trip settings
2. Circuit breakers coordinated to manufacturer’s tested coordination
tables. These tables can enable circuit breakers to coordinate for fault
currents higher than shown on the time-current curves.
3. MCCBs with fixed high magnetic trip or larger frame size may allow
higher instantaneous trip
4. CBs with short time-delay having instantaneous trip override:
• MCCBs and ICCBs with short time-delay settings have an instantaneous
trip override that opens the CB instantaneously for higher fault currents
(8x to12x amp rating)
• ICCBs may have higher instantaneous override settings than MCCBs
5. LVPCBs with short time-delay (with no instantaneous override)
Short time-delay, with settings from 6 to 30 cycles, is also available on low
voltage power circuit breakers. However, with low voltage power circuit
breakers an instantaneous override is not required. Thus, low voltage power
circuit breakers with short time-delay can “hold on” to faults for up to 30
cycles. Figure 27 illustrates a 30A molded case circuit breaker fed by a
200A LVPCB and 800A LVPCB. The 200A and 800A circuit breakers have
short time settings that provide selective coordination. The 200A circuit
breaker has a STD set at 6 cycles and the 800A circuit breaker has a STD
set at 20 cycles. The curves can be plotted to ensure the circuit breakers
do not intersect at any point. If there is intersection, investigate different
short time-delay settings. The interrupting ratings for the circuit breakers
with short time-delay may be less than the same circuit breaker with an
instantaneous trip.
Summary for Circuit Breaker Selective Coordination
It is possible to design electrical systems with circuit breakers and achieve
selective coordination. It requires analysis and proper choice of circuit
breaker types and options. In most cases it is necessary to calculate the
available short-circuit currents at the point of application of each circuit
breaker, a coordination analysis (plotting of curves) and proper interpretation
of the results for each circuit path. Following is a list that provides methods
for using circuit breakers to achieve selective coordination, with the least
expensive options appearing at the top:
18
Notes:
• The instantaneous trip of upstream circuit breakers must be greater than
the available short-circuit current for alternatives 1, 3, and 4
• Some options may require larger frame size or different type CBs
• Exercise, maintenance and testing should be performed periodically or
after fault interruption to retain proper clearing times and the coordination
scheme
In alternatives 1 through 4, if selective coordination can be achieved, it is job
or application specific; i.e., the designer must do the analysis for each
application or job. If the available short-circuit current increases due to
system changes, the selective coordination may no longer be valid. During
installation, the contractor must set the circuit breakers correctly.
©2008 Cooper Bussmann
Selective Coordination
Fuse & Circuit Breaker Mixture
System with Mixture of Fuses and Circuit Breakers
For downstream fuses and upstream circuit breakers, it is not a simple
matter to determine if a fuse and circuit breaker will be selectively
coordinated. Even if the plot of the time current curves for a downstream
fuse and an upstream circuit breaker show that the curves do not cross,
selective coordination may not be possible beyond a certain fault current.
The only sure way to determine whether these two devices will coordinate is
to test the devices together. The Cooper Bussmann Paul P. Gubany Center for
High-power Technology is available to perform this testing. Look under
Cooper Bussmann® Services at www.CooperBussmann.com.
Figure 28 shows an example: the curve is a 400A circuit breaker with a
downstream 100A fuse. Coordination is shown in the time-current curve up
to about 3000A (current axis is 10x). Coordination cannot be ensured above
this value without laboratory testing. This is because the fuse may not clear
the fault prior to unlatching of the upstream circuit breaker.
If a fuse is upstream and a circuit breaker is downstream, at some point the
fuse time-current characteristic crosses the circuit breaker time-current
characteristic. For short-circuit currents at that cross-over point and higher,
the upstream fuse is not coordinated with the down stream circuit breaker.
Figure 29 shows a 400A fuse with downstream 100A circuit breaker.
Coordination is not possible above approximately 5,000 amps as shown in
the overlap of the time-current curves (the current axis is 10x).
Figure 29
Figure 28
©2008 Cooper Bussmann
19
Selective Coordination
Mandatory Selective Coordination Requirements
Introduction
For building electrical systems, the topic of selective coordination of
over current protective devices can be segmented into two areas:
(1) where it is a desirable design consideration and
(2) where it is a mandatory NEC® requirement.
In most cases, selective coordination is a desirable design consideration and
not a NEC® requirement. However, it is in the best interest of the building
owner or tenants to have selectively coordinated overcurrent protection to
avoid unnecessary blackouts. Selective coordination should be evaluated in
the context of the reliability desired for the power system to deliver power to
the loads. In today’s modern commercial, institutional and manufacturing
building systems, what owner would not want a selectively coordinated
system?
Selective coordination is mandatory per the NEC® for a few applications. In
some building systems, there are vital loads that are important for life safety,
national security or business reasons. Continuity of power to these loads
and the reliability of the power supply to these loads is a high priority. The
sections of the NEC® defining selective coordination and those requiring the
overcurrent protection devices in the circuit paths supplying these vital loads
to be selectively coordinated are as follows:
Article 100 Definitions
Coordination (Selective).
Localization of an overcurrent condition to restrict outages to the circuit
or equipment affected, accomplished by the choice of overcurrent
protective devices and their ratings or settings.
Article 517 Healthcare Facilities
517.26 Application of Other Articles.
The essential electrical system shall meet the requirements of Article
700, except as amended by Article 517.
(Note: Article 517 has no amendment to the selective coordination
requirement, therefore selective coordination is required.)
Article 700 Emergency Systems
700.27 Coordination.
Emergency system(s) overcurrent devices shall be selectively coordinated
with all supply side overcurrent protective devices.
Exception: Selective coordination shall not be required in (1) or (2):
(1) Between transformer primary and secondary overcurrent protective
devices, where only one overcurrent protective device or set of
overcurrent protective devices exist(s) on the transformer secondary
(2) Between overcurrent protective devices of the same size (ampere
rating) in series
Article 701 Legally Required Standby Systems
701.18. Coordination.
Legally required standby system(s) overcurrent devices shall be
selectively coordinated with all supply side overcurrent protective
devices.
Exception: Selective coordination shall not be required in (1) or (2):
(1) Between transformer primary and secondary overcurrent protective
devices, where only one overcurrent protective device or set of
overcurrent protective devices exist(s) on the transformer secondary
(2) Between overcurrent protective devices of the same size (ampere
rating) in series
Article 708 Critical Operations Power Systems
708.54 Selective Coordination
Critical operations power system(s) overcurrent devices shall be
selectively coordinated with all supply side overcurrent protective
devices.
Selective coordination for elevator applications is covered in a separate
section of this publication. The following addresses the selective
coordination requirements for these other vital applications.
Article 620 Elevators
620.62 Selective Coordination
Where more than one driving machine disconnecting means is supplied
by a single feeder, the overcurrent devices in each disconnecting means
shall be selectively coordinated with any other supply side overcurrent
protective devices.
700.9(B)(5)(b), Exception.
Overcurrent protection shall be permitted at the source or for the
equipment, provided the overcurrent protection is selectively coordinated
with the down stream overcurrent protection.
20
©2008 Cooper Bussmann
Selective Coordination
Why Selective Coordination is Mandatory
Why Selective Coordination is Mandatory:
It Fills the Reliability “Hole”
The NEC® has mandatory selective coordination requirements for the
following systems:
• Emergency Systems- Article 700: 700.27
• Legally Required Standby Systems- Article 701: 701.18
• Critical Operations Power Systems- Article 708: 708.54
• Healthcare Article 517: 517.26 Required for Essential Electrical Systems
(In addition, selective coordination is required in elevator circuits (620.62), which is not discussed
in depth in this section.)
Notice these requirements are not in NEC® Chapters 1 through 4, such as
Articles 210 Branch Circuits, 215 Feeders, or 240 Overcurrent Protection.
Chapters 1 through 4 requirements pertain generally to all premise electrical
installations. Instead, these requirements are in Chapters 5 and 7 which are
under special occupancies and special conditions, respectively. Special
attention is given to these systems in the NEC® and they have some unique
requirements. Articles 700, 701, 708, and 517 are for circuits and systems
that are intended to deliver reliable power for loads that are vital to life
safety, public safety or national security. Reliability for these systems in the
above articles has to be greater than the reliability for the normal systems
covered by Chapters 1 through 4.
Articles 700, 701, 708, and 517 are unique. They have more restrictive
minimum requirements (versus the general requirements for normal
systems) in order for these systems to provide more reliable power to vital
loads. Selective coordination is one of the requirements that support higher
reliability. To make the point, here are just a few of the more restrictive
minimum requirements in Article 700:
• Periodic testing, maintenance and record retention
• Alternate power sources
• Wiring from emergency source to emergency loads shall be separate
from all other wiring
• Special fire protection for wiring
• Locating wiring to avoid outage due to physical damage during fires,
floods, vandalism, etc.
• Automatic transfer switches (ATS) with sophisticated sensors,
monitors and controls
• Separate ATSs and load segmenting (emergency, legally required
standby and optional standby) with sophisticated load shedding, if
required
Article 708 (COPS) also has a similar list of restrictive requirements with the
intent of providing a reliable power system.
Reviewing portions of the scopes of these Articles provides further insight.
Article 700: Emergency Systems
“700.1 Scope. The provisions of this article apply to the electrical safety of
the installation, operation, and maintenance…” The inclusion of operation
and maintenance indicates that reliability of these systems is very important.
For these systems, installation requirements alone are not sufficient. These
systems must operate when needed so this Article includes operational and
maintenance requirements. Why? The following statement from the scope
is clear: “Essential for safety of human life.” For instance, in times of
emergency, these loads are critical to evacuate a mass of people from
a building.
Article 708: Critical Operations Power Systems (COPS)
“708.1 Scope. FPN No. 1: Critical operations power systems are generally
installed in vital infrastructure facilities that, if destroyed or incapacitated,
would disrupt national security, the economy, public health or safety; and
where enhanced electrical infrastructure for continuity of operation has been
deemed necessary by governmental authority.” Due to recent events such as
9/11 and Hurricane Katrina, Homeland Security requested that NFPA develop
electrical requirements for systems that are vital to the public. The newly
created Article 708 (COPS) includes requirements, such as selective
coordination, that are minimum requirements for electrical systems that are
important for national security.
©2008 Cooper Bussmann
Why have these special, more restrictive requirements? The reason these
articles for special systems exist is that the electrical industry, the standard
making bodies, the technical code panel members and Homeland Security
feel special rules are needed to ensure minimum requirements for delivering
reliable power for designated vital loads. To better understand why we have
more restrictive requirements, focus on the loads that are being served by
these special systems. There are a few vital loads that pertain to life safety,
public safety and national security. For instance, 700.1 Scope FPN states
“FPN No. 3: Emergency systems are generally installed in places of assembly
where artificial illumination is required for safe exiting and for panic control
in buildings subject to occupancy by large numbers of persons, such as
hotels, theaters, sports arenas, healthcare facilities and similar institutions.
Emergency systems may also provide power for such functions as ventilation
where essential to maintain life, fire detection and alarm systems, elevators,
fire pumps, public safety communications systems, industrial processes
where current interruption would produce serious life safety or health
hazards, and similar functions.”
The requirements for these systems are intended to increase the system
reliability to deliver power and thereby increase the availability of these vital
loads during emergencies, disasters and the like.
21
Selective Coordination
Why Selective Coordination is Mandatory
Code Making Panels (CMPs) decide whether an item is a requirement or a
design consideration. Requirements are in the body of the NEC® under a
Chapter, Article and Section. A design consideration or an unenforceable
point of interest is a “Fine Print Note” (FPN). Code Making Panels make the
decision as to whether an important criterion is worthy either as an
informative FPN or as a NEC® requirement. Until 2005, selective
coordination was a FPN in Articles 700 and 701. During the 2005 NEC®
cycle, Code Making Panel 13 made the decision to convert selective
coordination from a Fine Print Note (desirable design consideration) to a
Section requirement written in mandatory performance language in order to
ensure the outcome the technical panel deemed necessary. The Code
Making Panel decided that selective coordination as a FPN was not
sufficient. Our society was changing, our culture was changing and our
building systems have evolved to a greater dependency on electricity. It was
time to make selective coordination a requirement. Their panel statement
included: “The panel agrees that selective coordination of emergency system
overcurrent devices with the supply side overcurrent devices will provide for
a more reliable emergency system.”
Let’s take a closer look at what may have prompted CMP 13 to change
selective coordination from a FPN to a requirement (700.27 and 701.18)
during the 2005 NEC® cycle and then for CMP 20 to include selective
coordination as a requirement (708.54) for Critical Operations Power
Systems in the new Article 708 for 2008 NEC®. The very first requirement
in the NEC® is a good place to start. This requirement is the root of every
requirement in the NEC®:
“90.1 Purpose. (A) Practical Safeguarding. The purpose of this Code is the
practical safeguarding of persons and property from hazards arising from the
use of electricity.”
A hazard would exist if power were not supplied to the loads that are vital to
assist a mass of people while evacuating a building in an emergency. The
NEC® has detailed requirements to address this issue. Selective
coordination is one of the requirements that ensure reliability for these
special systems. This is one of those examples where the NEC® requirement
is putting an emphasis on protecting people, similar to GFCIs.
Let’s dig a little deeper into the rationale to make selective coordination a
requirement. Until the 2005 NEC®, there was a
“hole” in the requirements of Article 700 and
701; a performance issue that reduced the
reliability of these systems was not addressed.
As already discussed, these Articles have many
special requirements that are intended to keep
the power flowing to a few vital loads. An
emergency system could have redundant power
sources, automatic transfer switches with load
shedding, location of wiring to minimize outages
22
from floods, special fire protection provisions, no ground fault protection on
the alternate source, testing, maintenance, etc. Yet the whole or part of the
system could unnecessarily be left without power because the overcurrent
protection was not selectively coordinated. These requirements for high
reliability systems had a piece that could negate the intended reliability for
these special systems. This had to be fixed. The 2005 NEC® remedied that
“hole” by inclusion of the selective coordination requirements for Articles
700 and 701 and indirectly 517 for Healthcare Essential Electric Systems.
The substantiation for the original 2005 NEC® proposal for 700.27 provides
the reasons. For better understanding, this substantiation is separated into
three segments below.
The Need is illustrated by the fact that there were already many existing
special requirements with the intent of ensuring more reliable emergency
power systems:
“This article specifically mandates that the emergency circuits be separated
from the normal circuits as shown in [Section] 700.9(B) and that the wiring
be specifically located to minimize system hazards as shown in [Section]
700.9(C), all of which reduce the probability of faults, or failures to the
system so it will be operational when called upon. With the interaction of
this Article for emergency lighting for egress, it is imperative that the lighting
system remain operational in an emergency. Failure of one component must
not result in a condition where a means of egress will be in total darkness as
shown in [Section] 700.16…”
This part of the substantiation identifies the existing “hole” that should be
rectified to ensure a more reliable system:
“Selectively coordinated overcurrent protective devices will provide a system
that will support all these requirements and principles. With properly selected
overcurrent protective devices, a fault in the emergency system will be
localized to the overcurrent protective device nearest the fault, allowing the
remainder of the system to be functional…”
This part proposes that the solution is to convert from a Fine Print Note
design consideration to a requirement:
“Due to the critical nature of the emergency system uptime, selective
coordination must be mandated for emergency systems. This can be
accomplished by both fuses and circuit breakers based on the system design
and the selection of the appropriate overcurrent protective devices.”
It was not a fuse or circuit breaker issue; since either technology can provide
selective coordination. What was needed was the mandate to design the
electrical distribution system so that the fuses and circuit breakers would
provide selective coordination. Without this as a requirement, electrical
distribution systems are designed and installed without regard to how the
overcurrent protective devices interact and this can negatively impact the
system reliability for delivering power to these vital loads.
The Code Making Panel action was to accept this proposal in principle and in
part. The panel deleted the Fine Print Note and rewrote and accepted the
following requirement text with a vote of 13 to 1.
700.27 Coordination. “Emergency system(s) overcurrent devices shall be
selectively coordinated with all supply side overcurrent protective devices."
It is important to note the panel expressly used the word “all.”
The Code Making Panel 13 statement provides the panel’s reasoning: “The
panel agrees that selective coordination of emergency system
overcurrent devices with the supply side overcurrent devices will
provide for a more reliable emergency system…” The take away from
the panel’s action is that selective coordination equals reliability. Acceptance
of this requirement plugged the “hole” that had previously existed.
©2008 Cooper Bussmann
Selective Coordination
Why Selective Coordination is Mandatory
In the comment stage, this new requirement was challenged but was not
overturned. Some people incorrectly characterized this as a circuit breaker
versus fuse issue. At the NFPA Annual Meeting, a motion was brought forth
to delete this requirement for the 2005 NEC®. The same comments, both
pro and con, that were brought up in the proposal and comment stages were
discussed. After the discussion, the motion to delete this new requirement
failed. So in the 2005 NEC®, selective coordination was required in
emergency and legally required standby systems. In addition, since selective
coordination was required in 700.27, it was required in healthcare essential
electrical systems.
The selective coordination requirements
expanded in the 2008 NEC®. A new Article 708
Critical Operations Power Systems (COPS) was
developed by the newly created Code Making Panel
20 and the message carried through. The COPS
scope encompasses electrical systems designated
for national security and public safety. Is there a
need for these systems to deliver
reliable power? Absolutely, there is a need. If there is a need for reliable
power, then there is a need for selective coordination. CMP 20 included a
requirement for selective coordination in Article 708:
708.54 Selective Coordination “Critical operations power system(s)
overcurrent devices shall be selectively coordinated with all supply side
overcurrent protective devices.”
Also, in the 2008 NEC® cycle, the selective coordination requirements in
700.27 (emergency systems), 701.18 (legally required standby systems), and
620.62 (elevator circuits) were challenged. In the proposal and comment
stages, there were plenty of pro and con submittals. All rationale was
presented, debated and discussed in this Code cycle. All selective
coordination requirements were retained with 700.27 and 701.18 adding two
clarifying exceptions. Neither exception reduced life safety because no
additional parts of the electrical system would be shut down unnecessarily.
To understand the support for these requirements by the
national industry experts on the technical committee, the following is
official voting from the 2008 NEC® comment stage:
• Code Making Panel 12 voted unanimously (11–0) to retain the
requirement for selective coordination in elevator circuits (620.62)
• Code Making Panel 13 voted 11–2 to add exceptions to 700.27 and
701.18 for two devices of the same amp rating in series, and single
devices on the primary and secondary of a transformer
• Code Making Panel 20 voted 16–0 (three times) and 15–1 (one time)
to reject all attempts to reduce or eliminate this key life safety
requirement (708.54)
During the 2008 NEC® proposal stage, CMP 13 reaffirmed the selective
coordination and communicated several key positions in their statement. In
this case, the panel statement clearly communicates the panel action and
position. Proposal 13-135 proposed the elimination of the selective
coordination requirement for 700.27 and moving the language back to a Fine
Print Note. This proposal was rejected 9 to 4.
Panel Statement: “This proposal removes the selective coordination
requirement from the mandatory text and places it in a non-mandatory FPN.
The requirement for selective coordination for emergency system overcurrent
devices should remain in the mandatory text. Selective coordination
increases the reliability of the emergency system. The current wording of
the NEC® is adequate. The instantaneous portion of the time-current
curve is no less important than the long time portion. Selective
coordination is achievable with the equipment available now.”
©2008 Cooper Bussmann
Special note: some people are still advocating lessening or diluting the
requirement to wording similar to “for times greater than 0.1 second”. This
would only provide selective coordination for overloads, would not cover
most ground faults or arcing faults, and would definitely not cover high level
short-circuit currents. It certainly would reduce the reliability of these power
systems. CMP 13 considered all these type proposals and by their above
statement, clearly stated that the selective coordination requirement is for all
levels of overcurrent, irrespective of the operating time of an overcurrent
device.
During the 2008 NEC® comment stage, Code Making Panel 20 reaffirmed
the selective coordination requirement based on system reliability. Comment
20-13 proposed the deletion of the 708.54 selective coordination
requirement. This comment was rejected 16 to 0.
Panel Statement: “The overriding theme of Article 585 (renumbered to
708) is to keep the power on for vital loads. Selective coordination is
obviously essential for the continuity of service required in critical
operations power systems. Selective coordination increases the
reliability of the COPS system.”
Inevitably, costs are discussed even though the first requirement in the
NEC®, 90.1, tells us the NEC® is concerned about safety, even if not efficient
or convenient. For designing and installing selectively coordinated
overcurrent protective devices, the cost may not necessarily be greater. That
depends on the design. It is important to keep in mind that the requirements
in the whole of Articles 700, 701, 517, and 708 result in extra work and cost.
An alternate power source with additional electrical distribution gear,
automatic transfer switches, sophisticated sensors, monitoring, control and
other provisions costs more and takes additional engineering effort. These
systems also require extra time and money to test, maintain, and retain
records. The extra cost is expected in order to provide more reliability for
these special systems compared to normal systems. For mission critical
business operations, such as data servers, financial applications and
communication industry centers, electrical distribution system design and
equipment selection for selective coordination is the norm. No less should
be expected for the few important loads that are critical for life safety. If we
do it to protect our vital business assets, why can’t we do it to protect our
people?
Summarizing
Selective coordination for elevator circuits has been a requirement since the
1993 NEC® and the industry has adjusted to compliance. For two NEC®
cycles, opposition to the 700.27 and 701.18 requirements has vigorously
worked on removing or diluting these selective coordination requirements.
However, during this time, the requirements have been reaffirmed and
expanded with Article 708 (COPS) in the 2008 NEC®. Now three Code
Making Panels have inserted selective coordination requirements in four
Articles of the NEC®.
23
Selective Coordination
Selective Coordination System Considerations
These Articles provide the minimum requirements for these special systems
essential for life safety, public safety and national security. We obtain insight
as to why selective coordination is a requirement by studying the panel
statements. The panel’s statements make clear these are special systems
where reliability is of utmost importance and selective coordination increases
the system reliability to deliver power to these few vital loads.
In our modern buildings, there is a greater dependence on electricity and the
NEC® requirements must adjust to this greater dependency and complexity.
This is evidenced by Homeland Security approaching NFPA and requesting
the NEC® include requirements for Critical Operations Power Systems. The
reliability of electrical systems supplying vital loads must be greater than
that of the systems supplying power to normal loads. Hence, the
reason for having Articles 700, 701, 708, and 517. People’s health and
safety as well as possibly national security and public safety rely on the
power to these vital loads, even under adverse conditions such as fires,
earthquakes, hurricanes and man-made catastrophes. Selective
coordination of all the overcurrent protective devices for the circuits
supplying these loads adds another assurance of reliability: it fills the “hole”.
Last, a quote from an October 2007 Electrical Construction & Maintenance
magazine article sums it up well. James S. Nasby is engineering director for
Master Control Systems, Inc. and was the NEMA representative on Code
Panel 13 for the 2005 and 2008 NEC® cycles. “In response, Nasby asks
detractors (of selective coordination requirements) to list the essential
emergency systems they’d want to risk going offline. He says it’s difficult to
calculate risk when it’s your family on the top floor of a high-rise hotel.
‘Typically, no building owners will install anymore emergency services than
are required, and what is required for that building is important’ Nasby says.
‘You don’t want to lose lights in the stairwell or the emergency elevators, and
you don’t want a fault on one of these services to take out anything else...
The premise of distribution systems is that a fault on one circuit doesn’t
propagate upstream – and that’s what this is asking for.’”
Selective Coordination System
Considerations
Classifications, Codes, Standards, and the AHJ
There are various Codes and standards that are applicable for one or more of
the various types of systems. Most notable is the National Electrical Code
(NEC®). The applicable NEC® Articles are 700 Emergency Systems, 701
Legally Required Standby Systems and 708 Critical Operations Power
Systems. The NEC® does not designate which vital loads have to be served
by these systems. Typically NFPA 101 (Life Safety Code) provides guidance
on the vital loads to be classified as served by emergency and legally
required standby systems. Vital loads served by COPS systems are
designated by a government authority or an owner may choose to comply.
NEC® Article 702 covers Optional Standby Systems and NEC® Article 517
covers Healthcare Facilities.
24
Vital Load Classifications
Emergency systems are considered in places of assembly where artificial
illumination is required, for areas where panic control is needed (such as
hotels, theaters, sports arenas, healthcare facilities) and similar institutions,
and where interruption of power to a vital load could cause human injury or
death. Emergency loads may include emergency and egress lighting,
ventilation and pressurization systems, fire detection and alarm systems,
elevators, fire pumps, public safety communications, or industrial process
loads where interruption could cause severe safety hazards. For instance,
emergency lighting is essential to prevent injury or loss of life during
evacuation situations where the normal lighting is lost. NEC® Article 700
provides the electrical systems requirements. 700.27 contains the
requirement for selective coordination.
Legally required standby systems are intended to supply power to
selected loads in the event of failure of the normal source. Legally required
standby systems typically serve loads in heating and refrigeration,
communication systems, ventilation and smoke removal systems, sewage
disposal, lighting systems and industrial processes where interruption could
cause safety hazards. NEC® Article 701 provides the electrical system
requirements. 701.18 contains the requirement for selective coordination.
Where hazardous materials are manufactured, processed, dispensed or
stored, the loads that may be classified to be supplied by emergency or
legally required standby systems include ventilation, treatment systems,
temperature control, alarm, detection or other electrically operated systems.
The essential electrical systems of healthcare facilities include the loads
on the critical branch, life safety branch and equipment branch. The
emergency system of a hospital is made up of two branches of the electrical
system that are essential for life safety and for the health and welfare of
patients receiving critical care and life support. These two branches are the
life safety branch and the critical system branch. NEC® Article 517 provides
the electrical system requirements and 517.26 refers to Article 700
requirements. Selective coordination is a requirement for essential electrical
systems based on 517.26 since there is no amendment to the selective
coordination requirement in Article 517.
Critical Operations Power Systems (COPS) are systems intended to provide
continuity of power to vital operations loads. COPS are intended to be
installed in facilities where continuity of operations is important for national
security, the economy or public safety. These systems will be classified COPS
by government jurisdiction or facility management. The type of loads may be
any and all types considered vital to a facility or organization, including data
centers and communications centers. New NEC® Article 708 provides the
requirements. 708.54 contains the requirement for selective coordination.
©2008 Cooper Bussmann
Selective Coordination
System Considerations
Optional Power Systems are for supplying loads with backup power, but the
loads are not classified as required to be supplied by emergency systems,
legally required standby systems or COPS systems. These can supply loads
that are not critical for life safety. These may be data center loads,
computer facility loads, critical manufacturing process loads or other loads
where the building occupant wants backup power. NEC® Article 702
provides the requirements and selective coordination is not mandatory for
these circuits. However, many businesses place their mission critical loads
on these systems and it is best practice to provide selectively coordinated
overcurrent protection for these circuit paths.
Figure 30
Alternate Power Systems
Since availability of power for these loads is so important, these loads are
supplied by a normal electrical power source and an alternate electrical
power source. These systems typically have transfer switches for the
purpose of transferring the source of power feeding the loads from the
normal source to the alternate source or vice versa. For the emergency
system, legally required standby system and critical operation power system
loads, the transfer switch is required to be automatic. For optional power
system loads, the transfer switch is permitted to be manually operated. The
transfer switches are typically configured so that one or more transfer
switches supply only emergency loads and another one or more transfer
switches supply only legally required standby loads, and one or more
transfer switches supply the optional loads (see Figure 30). The systems are
automated such that if normal power on the lineside terminals of a transfer
switch is lost for any reason, the alternate source is called into action and a
transfer is made to the alternate source supply. If for some reason the
alternate power source supply can not meet the connected load demand, the
loads are shed in reverse order of their priorities. First the optional standby
loads are shed, and then if more shedding is necessary, the legally required
standby loads are shed. For instance, in Figure 30, suppose the generator
had sufficient capacity to meet the entire load demand of the three load
classifications, but when called into action the generator malfunctioned and
could only supply a fraction of its rating. If the normal power was lost and
the generator output was limited, the system would shed the optional
standby loads and if necessary, the legally required standby loads.
©2008 Cooper Bussmann
There are numerous types of electrical power sources that can be utilized as
the alternative source, such as generators (many fuel types available) and
stored energy battery systems. Uninterruptible Power Systems (UPS) are
also often used. The selection of the alternate power source type(s) and
possibly stored energy/conversion equipment, such as UPS systems, are
based on many factors. Two of the most important criteria are:
1. After the normal power is lost, the time required for the alternative power
system to commence delivering power to the vital loads.
2. The time duration that the alternative system must continue to deliver power to
the vital loads.
In some systems, multiple types of alternative power source equipment are
utilized: one type to quickly pick up the load and another type that takes
longer to start but can supply electrical power for long time periods. For
instance, a natural gas generator may be used in combination with a UPS
system (with batteries). If the normal power is lost, a UPS can deliver power
very rapidly for a quick transition. A generator takes longer to come on line
and is capable of delivering power, (depending on the fuel capacity) for long
time periods.
The following table provides the NEC® requirements on the maximum time
the systems are permitted to initiate delivering current to the loads. Other
Codes and standards may have requirements, also.
System
Classification
Emergency
Legally Required
Standby
Maximum Time to Initiate
Delivering Current to Loads
Within time required for application,
but not to exceed 10 seconds
Within time required for application
but not to exceed 60 seconds
NEC®
Section
700.12
701.11
25
Selective Coordination
System Considerations
Normal Path and Alternate Path
Which OCPDs Have to Be Selectively Coordinated
Since availability of power for these vital loads is so important, these loads
are supplied by a normal electrical power source and an alternate electrical
power source. Selective coordination is about the continuance of power to
vital loads. These vital loads (supplied by the emergency systems, legally
required standby systems, critical operations power systems, and healthcare
facilities essential electrical systems) can be powered through the normal
source or through the alternate source. Selective coordination is required for
both the alternate power circuit path (Figure 31) and normal power circuit
path (Figure 32). The requirements state selective coordination is required,
“with all supply side overcurrent protective devices.”
The Code text for the selective coordination requirements in 700.27 is
carefully worded, stating that all emergency OCPDs shall selectively
coordinate with all supply side overcurrent devices. This helps ensure that
these vital loads are not disrupted, whether fed from the normal source or
the alternate source. Wording for 701.18 legally required standby systems
and 708.54 critical operations power systems is similar except for the
system type nomenclature.
Figure 33 illustrates that all emergency overcurrent protective devices must
be selectively coordinated through to the alternate power source. In
addition, the emergency overcurrent protective devices on the loadside of the
transfer switch must selectively coordinate with the overcurrent protective
devices in the normal circuit path. However, based on the requirement
wording, there is a difference on the minimum requirement for the
overcurrent protective devices in the normal source path that are on the
lineside of the transfer switch. This same requirement is in 701.18 for
Legally Required Standby Systems and 708.54 for Critical Operations Power
Systems. Read the following 700.27 requirement and the practical
application of the requirement example. Best engineering practice would be
to have them be selectively coordinated.
Selective Coordination Includes the
Entire Circuit Path, Through Both Sources
Normal
Source
From a vital load to the
alternate source, the OCPDs
shall be selectively
coordinated
Alternate
Source
N
E
ATS
Panel
Figure 31
For a vital load to the normal
source main, the OCPDs shall
be selectively coordinated
“Emergency system(s)
overcurrent devices shall be
selectively coordinated with
all supply side overcurrent
protective devices”
This wording is inclusive of
the normal source path
OCPDs
Normal
Source
Alternate
Source
N
E
ATS
Panel
Figure 32
26
©2008 Cooper Bussmann
Selective Coordination
System Considerations
NEC® 700.27 “Emergency system(s) overcurrent devices shall be selectively
coordinated with all supply side overcurrent protective devices.”
This wording is inclusive of the alternate path and normal source path
overcurrent devices for each emergency load.
Practical Application of Requirement Example:
• OCPD 1 Must selectively coordinate with OCPD’s 2, 3, 4, 5, 6
• OCPD 2 Must selectively coordinate with OCPD’s 3, 4, 5, 6
• OCPD 3 Must selectively coordinate with OCPD 4
• OCPD 5 Does not have to selectively coordinate with OCPD 6
With this specific wording, the analysis effort evaluating the normal source
OCPDs can be much easier. Although it is permitted to have OCPD 5 not
selectively coordinate with OCPD 6, the best engineering practice would be
to have them be selectively coordinated.
Figure 34
Lack of Selective Coordination
Example 1:
Figure 35 illustrates Example 1 where the power is from the normal source
(ATS is switched to normal source). In this example, a fault opens the feeder
overcurrent protective device (OCPD) as well as the branch circuit OCPD.
The cause is the branch circuit OCPD is not selectively coordinated with the
feeder OCPD for the full range of overcurrents at the point of application of
the branch circuit OCPD. Because voltage is still present at the normal
connection of the ATS, the generator will not automatically start and the ATS
will not automatically transfer. The load on the faulted branch circuit is
rightfully de-energized. However, the other emergency loads supplied by this
feeder will incur an unnecessary loss of power. This would not comply with
700.27, 701.18, or 708.54 if this were an emergency system, legally
required standby system or critical operations power system.
Example 1 Non-Coordinated
Figure 33
•
This Practical Application of Requirement Example and figure are reprinted with permission from
necdigest® article Keep The Power On For Vital Loads December 2007 Copyright© 2007, National
Fire Protection Association, Quincy, MA. This material is not the official position of the NFPA on the
referenced subject, which is represented only by the standard in its entirety.
•
•
Exceptions
700.27 and 708.18 have two exceptions for selective coordination that are
shown in Figure 34. Neither exception reduces life safety because no
additional parts of the electrical system would be shut down unnecessarily.
The hashed OCPDs in both circuits shown in Figure 34 do not have to be
selectively coordinated with each other. 708.54 does not have these two
exceptions. However, in application of 708.54, it is essentially the same.
Code Panel 20 (which is responsible for 708.54) that considered the circuit
circumstances for the hashed OCPDs in Figure 34 to comply with the
selective coordination requirement (considering the requirement in context
with the definition).
©2008 Cooper Bussmann
Figure 35
27
Selective Coordination
System Considerations
Example 2:
If the emergency overcurrent protective devices are not selectively
coordinated with the normal path overcurrent protective devices, a fault in
the emergency system can cause the OCPDs to cascade open thereby
unnecessarily opening the normal path feeder OCPD or possibly main OCPD.
Figure 36 illustrates this scenario. If this occurs, all the vital loads are
unnecessarily without power at least temporarily. Since the power is lost to
the ATS normal lineside termination, the generator is signaled to start. When
the generator starts and the loads transfer to the alternate source, some vital
loads will continue to be unnecessarily blacked out due to the emergency
feeder OCPD’s lack of selective coordination (it is still open). In addition, this
action reduces the reliability of the system since there is some probability
that the generator may not start or the transfer switch may not transfer.
Alternate
Source
Normal
Source
N
E
ATS
Panel
Example 2 Non-Coordinated System
Consequences
• Non-coordinated OCPDs
• Blackout all emergency loads
temporarily (shaded)
Figure 37
• Transfer activated
• Unnecessary blackout
persists (hashed)
• Reliability concerns whether
generator or transfer equipment
operate properly– why
increase possibility of
unwanted outcome?
OCPD Opens
Fault
Alternate
Source
Normal
Source
Full Range of Overcurrents
N
E
ATS
Panel
Fault
Figure 36
Evaluate for the Worst Case Fault Current
In assessing whether the overcurrent protective devices are selectively
coordinated in the circuit path for these vital loads, it is important that the
available short-circuit current from the normal source be considered (see
Figure 37). Generally, the normal source can deliver much more short-circuit
current than the emergency generators. This is required per 700.5(A)
Capacity and Rating. …”The emergency system equipment shall be suitable
for the maximum available fault current at its terminals.”
To comply, the overcurrent protective devices must selectively coordinate for
the full range of overcurrents possible for the application: overloads and
short-circuits which include ground faults, arcing faults and bolted faults. It is
not selective coordination if the fuses or circuit breakers are coordinated
only for overloads and low level fault currents. The fuses or circuit breakers
must also be selectively coordinated for the maximum short-circuit current
available at each point of application. “The instantaneous portion of the
time-current curve is no less important than the long time portion” is
extracted from a Code Making Panel 13 statement where the panel rejected
a comment to eliminate the selective coordination requirement. High- and
medium-level faults may not occur as frequently as overloads and very lowlevel faults, but they can and do occur. High- and medium-level faults will be
more likely during fires, attacks on buildings, building failures or as systems
age, or if proper maintenance is not regularly performed. Selective
coordination has a very clear and unambiguous definition. Either overcurrent
protective devices in a circuit path are selectively coordinated for the full
range of overcurrents for the application or they are not. The words
“optimized selective coordination,” “selectively coordinated for times greater
than 0.1 second,” or other similar wording are merely attempts to not meet
the selective coordination requirements. And terms like “selective
coordination where practicable” are unenforceable. For more information on
this, see this publication’s section on Selective Coordination Objections and
Misunderstandings.
Ground Fault Protection Relays
If a circuit path includes a Ground Fault Protection Relay (GFPR), then the
selective coordination analysis should include the GFPRs. One approach is to
first do the fuse or circuit breaker selective coordination analysis as
described in the previous sections. (This includes all type of overcurrents).
Then do a separate analysis for how the fuses or circuit breakers and GFPRs
coordinate for ground faults. For more information see the section on Ground
Fault Protection: Coordination Considerations.
28
©2008 Cooper Bussmann
Selective Coordination
System Considerations
Faster Restoration & Increased Safety
Beside minimizing an outage to only the part of the circuit path that needs to
be removed due to an overcurrent condition, selective coordination also
ensures faster restoration of power when only the closest upstream
overcurrent protective device opens on an overcurrent. When the electrician
arrives to investigate the cause, correct the problem and restore power, the
electrician does not have to spend time locating upstream overcurrent
protective devices that unnecessarily opened. This also increases safety by
avoiding reclosing or replacing upstream OCPDs that have unnecessarily
cascaded open; electrical equipment closer to the source typically has higher
arc-flash hazard risk categories.
Ensuring Compliance
Achieving overcurrent protective device selective coordination requires
proper engineering, specification and installation of the required overcurrent
protective devices. It is possible for both fusible systems and circuit breaker
systems to be selectively coordinated with proper analysis and selection.
Selective coordination is best resolved in the design phase. Depending on
the load needs and types of overcurrent protective devices, there is flexibility
in the design phase to investigate various alternatives. After equipment is
installed it can be costly to “fix” a system that lacks selective coordination.
It is the professional engineer’s fiduciary responsibility to selectively
coordinate the emergency, legally required standby and critical operations
power systems. Once the distribution system is designed, without thought
given to selective coordination, it is often too late to delegate the
responsibility to the electrical contractor or equipment supplier. It is most
efficient therefore, if the system is designed with selective coordination in
mind, and not delegated to the electrical contractor, nor to the equipment
supplier.
The contractor must install the proper overcurrent protective devices per the
engineer’s specifications and approved submittals. If the system uses circuit
breakers, the installer needs to ensure the circuit breaker settings (long
time-delay, short time-delay and instantaneous trip) are set per the
engineer’s coordination analysis. Circuit breakers are typically shipped from
the manufacturer with the short time-delay and instantaneous trip settings
on low or the minimum; these settings usually require adjustment to the
engineer’s selective coordination analysis.
The following is a Selective Coordination Check List that may be useful.
Cooper Bussmann grants permission to copy and use this check list.
©2008 Cooper Bussmann
29
Check List
SELECTIVE COORDINATION REQUIREMENTS INSPECTION CHECK LIST
ISSUED BY:
This form provides documentation to ensure compliance with the following NFPA 70, National Electrical Code®
requirements for selective coordination found directly in articles 620, 700, 701 & 708, and indirectly in Article 517.
JOB #:
NAME:
LOCATION:
FIRM:
COMPLIANCE CHECKLIST
Several sections in the Code require all supply side overcurrent protective devices to be selectively coordinated in the
circuits supplying life-safety-related loads. These loads are those supplied by elevator circuits (620.62), emergency
systems (700.9(B)(5)(b) Exception & 700.27), legally required standby systems (701.18), and critical operations power
systems (708.54). These requirements have been taken into account and the installation has been designed to meet the
following sections for the normal and alternate circuit paths to the applicable loads. This analysis included the full range
of overcurrents possible, taking into account the worst case available short-circuit current from the normal source or
alternate source (whichever is greater). (Check all that apply below).
1. Verify Selective Coordination for the System Type ARTICLE 620 – ELEVATORS, DUMBWAITERS, ESCALATORS, MOVING WALKWAYS,
WHEELCHAIR LIFTS AND STAIRWAY CHAIR LIFTS
620.62 Selective Coordination. Where more than one driving machine disconnecting
means is supplied by a single feeder, the overcurrent devices in each disconnecting
means shall be selectively coordinated with any other supply side overcurrent
protective devices.
ARTICLE 700 – EMERGENCY SYSTEMS
700.27 Coordination. Emergency system overcurrent protective devices shall be
selectively coordinated with all supply side overcurrent protective devices.
(exception for single devices on the primary and secondary of a transformer and 2
devices of the same amp rating in series)
ARTICLE 701 – LEGALLY REQUIRED STANDBY SYSTEMS
701.18 Coordination. Legally required standby system overcurrent protective devices
shall be selectively coordinated with all supply side overcurrent protective devices.
(exception for single devices on the primary and secondary of a transformer and 2
devices of the sameamp rating in series)
ARTICLE 708 – CRITICAL OPERATIONS POWER SYSTEMS (COPS)
708.54 Coordination. Critical operations power system overcurrent protective devices
shall be selectively coordinated with all supply side overcurrent protective devices.
ARTICLE 517 – HEALTHCARE FACILITIES
517.26 Application of Other Articles. The essential electrical system shall meet
the requirements of Article 700, except as amended by Article 517.
(Article 517 does notamend the selective coordination requirements of Article 700)
YES
YES
YES
YES
YES
NO
NO
NO
NO
NO
N/A
N/A
N/A
N/A
N/A
2. Verify Selective Coordination for the Overcurrent Protective Device Type An analysis shall include the available short-circuit currents and interpretation of the overcurrent
protective device characteristics utilizing industry practices. Fuse selective coordination can be
demonstrated by the fuse manufacturer’s selectivity ratio guide, and a complete short-circuit
current study is not required if the available short-circuit current is shown to be less than or
equal to 200kA or the fuse interrupting ratings, whichever is lower. Circuit breaker selective
coordination can be demonstrated by the circuit breaker manufacturer’s selective coordination
tables in conjunction with the available short-circuit currents applicable. The analysis shall be
retained and submitted upon request.
Signature
30
Date
YES
NO
N/A
P.E. Seal
Selective Coordination
Fuse and Circuit Breaker Choices for Selective Coordination
MCCBs/ICCBs
Fuses
LVPCBs
Instantaneous
Trip
Fix High
Magnetic
Instantaneous
Trip
Short Time-Delay
With
Instantaneous
Override
Yes
Yes
Yes
Short Time-Delay
Settings
(STD)
Short-Circuit Current
(ISCA) Calculations
Needed
No
Selectivity Ratios
Applicable to 200kA*
Ease of Coordination
Analysis
Simplest:
Use Fuse Selectivity
Ratios
Job Specific:
Limited to ISCA
Calculated for
Specific Job
Not Limited
All Systems
(Up to 200,000A*)
Limited
Lower ISCA Systems
(Larger Frame CBs
May Help)
Limited
Expands Range of
ISCA Systems
Limited
Lower ISCA Systems
(Larger Frame CBs
May Help)
Only Limited to Systems
Where ISCA Exceeds CB
Interrupting Ratings
Cost
Low to Medium
Low to Medium
Low to Medium
Medium
High
Applicable Even if
Transformer
Changes
(ISCA Increases)
Yes
(Up to 200,000A*)
No
(Must Reverify)
No
(Must Reverify)
No
(Must Reverify)
Yes
(Verify ISCA Within CB
Interrupting Rating and
Short Time Rating)
Takes More Work (Use One of Below):
• CB Manufacturers’ Coordination Tables
• Simple Analysis Rules
• Curves (Commercial Software Packages): Interpret Properly
No
(ISCA Needed for CB
Interrupting Ratings)
Simple:
Set Short Time-Delay
Bands Properly
*Or fuse interrupting rating, whichever is lower.
This simple table for Fuse and Circuit Breaker: Choices for Selective
Coordination provides a summary of what has been covered in this section
on selective coordination and includes practical considerations in the design
effort and identifies limitations.
Overcurrent Protective Device Choices are across the chart’s top row and
include:
1. Fuses: modern current-limiting fuses
2. MCCBs/ICCBs: molded case circuit breakers or insulated case circuit
breakers:
a. With instantaneous trips
b. With fixed high magnetic instantaneous trips
c. With short time-delay (STD) and instantaneous override
3. LVPCB: low voltage power circuit breakers with short time-delay (no
instantaneous trip)
©2008 Cooper Bussmann
The left column has five considerations for selective coordination.
Short-Circuit Current (ISCA) Calculations Needed:
• With fuses, there is no need to calculate the short-circuit current in
most cases. As long as the main transformer secondary can not
deliver an available short-circuit current more than 200,000A*, just
use the selectivity ratios. This saves a great deal of time and lowers
engineering cost.
• With LVPCBs utilizing STDs and no instantaneous trip, it is not
necessary to calculate the short-circuit current in many cases. It is
necessary if the short-circuit current exceeds the interrupting rating or
short-time rating for any circuit breaker. A quick check of the
available short-circuit current at the main transformer secondary will
determine if a detailed short-circuit current study is required.
• With MCCBs and ICCBs it is necessary to calculate the available shortcircuit currents at each point a circuit breaker is applied.
31
Selective Coordination
Ease of Coordination Analysis:
• With fuses, just use the selectivity ratio guide which is applicable for
the full range of overcurrents up to the fuses’ interrupting ratings or
200,000A, which is lower. This saves a great deal of time and lowers
the engineering cost.
• With LVPCBs, utilizing STDs and no instantaneous trip, it is a matter of
selecting short time-delay bands that do not intersect. However, it is
easy to achieve selective coordination.
• With MCCBs and ICCBs it is necessary to do a detailed analysis. This
is because the available short-circuit currents may trip the upstream
circuit breakers. The method entails knowing the available
short-circuit current at each CB point of application and determining if
the circuit breakers are selectively coordinated or not. Three
methods are:
1. Circuit breaker coordination tables (published by each CB
manufacturer).
2. Analysis method (without plotting curves) presented in a previous
section.
3. Using a commercial software package that plots the curves
(necessary to interpret the curves properly).
Applicable Even if Transformer Changes
(ISCA increases):
• With fuses, even if there is a system change that increases the
short-circuit current, such as when the main transformer gets
changed, selective coordination is retained (up to 200,000A*).
• With LVPCBs, utilizing STDs and no instantaneous trip, the selective
coordination is also retained. In this case, it is necessary to verify the
higher short-circuit current does not now exceed the interrupting or
short time-delay rating for any circuit breaker.
• With MCCBs and ICCBs selective coordination may be negated if the
short-circuit current increases due to a system change. It is
necessary to perform a new short-circuit current study and revisit the
selective coordination analysis to verify if selective coordination is
still valid.
Note: If the system includes ground fault protection relays, selective
coordination must be analyzed with these protective devices, also. See the
section on Selective Coordination: Ground Fault Protection Relays.
Job Specific: Limited to ISCA Calculated for
Specific Job
• With fuses, the selective coordination scheme determined is not
limited just to that specific job since it is a matter of utilizing the
selectivity ratios. The same specification of fuse types and sizes
could be utilized for another project as long as the short-circuit
current is not greater than 200,000A*.
• With LVPCBs, utilizing STDs and no instantaneous trip, the selective
coordination scheme determined is not limited just to that specific job
since it is a matter of specifying STD bands that do not intersect.
Once determined, the same specification of circuit breaker types and
settings could be used on another project, as long as the short-circuit
current does not exceed any circuit breaker interrupting or short
time-delay rating.
• With MCCBs and ICCBs the selective coordination scheme that is
selectively coordinated for one project is not necessarily transferable
to another project. The reason is that even if the same circuit
breakers are used, each project will have its own specific available
short-circuit currents. Therefore, using these type circuit breakers
requires each project to have a short-circuit current and coordination
analysis.
Cost:
• This row is a rough estimate of the cost range of the electrical
equipment.
*Or fuse interrupting rating, whichever is lower.
32
©2008 Cooper Bussmann
Selective Coordination
Selective Coordination Objections & Misunderstandings
Selective Coordination Objections and
Misunderstandings
Mandatory selective coordination required in the NEC® for the circuit paths
of some vital loads requires some changes in the industry. As with any
change, there are those who are quick adopters and they have moved on,
ensuring their design, installations and inspections comply. Others have
been more reluctant to change. Although selective coordination is an easy
concept to understand, the devil can be in the details. This section presents
the most common objections voiced in opposition to the selective
coordination requirements with accompanying clarifying facts. As with any
complex subject, it is easy to provide general statements that support or
oppose a position. As one digs deeper into the objections, the reality
becomes:
1. For many of the objections, there are remedies or technologies that are
suitable solutions
2. Some of the objections are not accurate
3. For other objections, since selective coordination is now mandatory,
selective coordination is a higher priority
All these arguments as to why mandatory selective coordination
requirements should be deleted or diluted have been thoroughly presented,
discussed and debated in the technical Code panels as well as in other
industry forums for more than two Code cycles. For elevator circuits,
selective coordination has been a mandatory requirement since the 1993
NEC®. Three Code panels have made selective coordination a mandatory
requirement because it increases the system reliability for powering vital life
safety loads and it is achievable with existing technology. In addition, as is
typical with significant industry changes, manufacturers are responding with
new products that make it easier and less costly to comply.
To answer the broad question why selective coordination is needed as a
NEC® requirement, see the section on: Why Selective Coordination is
Mandatory: It fills the reliability “Hole.”
©2008 Cooper Bussmann
Objection 1
Changing the requirement for selective coordination to times of 0.1 second
and greater is a better method.
Clarifying Facts to Objection 1
A. The Code Making Panels have already considered this option and
rejected it. The real question that has already been answered by the
industry experts on three National Electrical Code panels is what level
of coordination is required to provide system reliability to supply power
to vital loads. Their answer is selective coordination, for the full range
of overcurrents. Selective coordination can not be specified by time
parameters as some are promoting. Selective coordination is a matter
of the available fault current and how characteristics of the various
overcurrent protective devices in series in the circuit path perform
relative to one another. Selective coordination is for the full range of
overcurrents that the specific system is capable of delivering. In reality
it comes down to this:
• Fuses: if the fuses comply with the fuse manufacturer’s selectivity
ratios, the fuses selectively coordinate for fault currents up to
200,000A or the fuses interrupting rating, whichever is lower. There is
no need to limit reliability to times of only 0.1 second and longer.
• Circuit breakers: the fault current level in the specific system/location
determines the type of circuit breakers that would be the most cost
effective and still selectively coordinate. If there are low available fault
currents, then molded case circuit breakers may comply. If the fault
current is in a higher range, then molded case circuit breakers with
fixed high magnetic instantaneous trips may comply. If not, then
short time-delay circuit breakers may be necessary. See the section
on Achieving Selective Coordination with Low Voltage Circuit Breakers
for more details on the various options for different levels of fault
current. As with fusible systems, circuit breaker solutions are available
to provide selective coordination for all available fault currents.
B. The argument to consider coordination for times only greater than 0.1
second is merely a tactic to circumvent the detailed engineering
required to ensure a more reliable system for life safety. It is only half
of the story. It clearly is intended to ignore circuit breaker
instantaneous trip settings when analyzing selective coordination. This
will provide coordination for primarily only overloads and it will not
even ensure selective coordination for low level arcing fault currents
on many systems. This is purely a ploy by some individuals who do
not want to alter their typical “cookie cutter” designs to meet the new
higher reliability requirements. If all levels of short-circuit currents are
not an important criteria, why be concerned with complying with the
interrupting rating requirements of NEC® 110.9 or short-circuit current
rating requirements such as 110.10? Code panel 13 recognized that
selective coordination has to be for the full range of overcurrents. In a
panel statement rejecting a proposal to modify the selective
coordination requirement, included “The instantaneous portion of the
time-current curve is no less important than the long time portion.”
(The instantaneous portion covers times below 0.1 second) (Panel
Statement to Comment 13-135 during the 2005 NEC® cycle.)
C. Overcurrents in branch circuits can be either overloads or faults.
However, overcurrents in feeder circuits (distribution panels and
switchboards) tend to be faults and not overloads. As a consequence,
without selective coordination for the full range of overcurrents, feeder
faults will have a greater probability to unnecessarily blackout vital life
safety loads due to cascading overcurrent protective devices.
33
Selective Coordination
Selective Coordination Objections & Misunderstandings
D. Let’s examine this ill-advised suggestion to have selective coordination
be for only times greater than 0.1 second. Figure 38 includes a
one-line diagram and time-current curves showing only times greater
than 0.1 second. If considering only times greater than 0.1 second,
this system would be “acceptable” for any available short-circuit
current up to the interrupting ratings of the circuit breakers. Figure 39
illustrates why this is ill-advised. It shows enough of the time-current
curves where the true reliability concerns and consequences are
shown. In reality, this system is only selectively coordinated for
overcurrents on the branch circuits up to 750A and for overcurrents on
the feeder up to 2400A. Why can this be? Circuit breakers are
typically shipped from the factory with the instantaneous trip set at the
lowest setting. These 200A and 800A circuit breakers are set at the
low IT. Without some engineering effort to select appropriate
overcurrent protective device types, and their amp ratings and settings,
this system could unnecessarily blackout vital loads in a critical
situation. The proper selection of devices depends on the fault current
level and type of device. Thus, if selective coordination is considered
to be only analyzed for greater than 0.1 second, inappropriate devices
can be selected that adversely affect the capability of the system to be
selectively coordinated, reducing system reliability, for low- mediumand high-level faults. While this explanation shows the difficulties
encountered with these standard molded case thermal-magnetic circuit
breakers, there are solutions for the full range of overcurrents of a
specific system. It may be as simple as doing a selective coordination
study and adjusting the circuit breakers to higher instantaneous trip
settings. Other, more sophisticated circuit breakers are available that
selectively coordinate below 0.1 second (for the full range of
overcurrents). See the section Achieving Selective Coordination with
Low Voltage Circuit Breakers to assist in selecting the least costly
circuit breaker alternatives for the system available fault currents.
Figure 38
Figure 39
This figure shows the real limitations for this system to deliver reliable power for faults
greater than:
• 750A, the 30A CB is not coordinated with the 200A CB.
• 2400A, the 30A CB is not coordinated with the 800A CB.
• 2400A, the 200A CB is not coordinated with the 800A CB.
Objection 2
Selective coordination results in reduced electrical safety with an increased
arc-flash hazard.
Clarifying Facts to Objection 2
A. In fact, the opposite is true from a system standpoint; selective
coordination improves electrical safety for the worker. Selective
coordination isolates overcurrents to the lowest level possible, resulting
in fewer exposures to hazards for electricians. Also, since the worker
does not unnecessarily have to interface with upstream equipment
closer to the source, the arc-flash levels are often lower. The lack of
selective coordination can actually increase the arc-flash hazard for
workers because the worker will have to interface with larger amp rated
overcurrent protective devices upstream. The electrical equipment,
closer to the source, is generally protected by larger amp rated
overcurrent protective devices and has higher available short-circuit
currents, which typically results in higher arc-flash hazards. See Figures
40 and 41.
In Figure 42, assume a fault in the branch circuit opens the branch
circuit OCPD, plus it, unnecessarily opens the feeder OCPD in the
distribution panel, and the feeder OCPD in the service panel due to a
lack of selective coordination. The electrician starts trouble shooting at
the highest level in the system that is without power. At this point, the
electrician does not know that a lack of selective coordination
unnecessarily opened the feeder OCPDs in the distribution panel and
service panel. The electrician does not even know
This system would comply if the selective coordination requirement was only for
OCPDs operating characteristics of 0.1 second and greater. See Figure 39 for the real
consequence to system reliability.
34
©2008 Cooper Bussmann
Selective Coordination
Selective Coordination Objections & Misunderstandings
which overcurrent protective devices opened, where the fault occurred
and what damage may have occurred on the circuit paths. It is Federal
law that a circuit breaker shall not be reset or fuses replaced [OSHA
1910.334(b)(2)] “until it has been determined that the equipment and
circuit can be safely energized.” Even though the fault may have
occurred on the branch circuit, the fault current may have damaged the
circuit components on the feeders. Therefore, the proper electrically
safe work practices for the electrician are as follows (equipment must
be in an electrically safe work condition for this work).
Let’s assume it is a circuit breaker system. At each location in the
electrical system that he works, he must place the equipment in an
electrically safe work condition. This requires a shock hazard analysis
and flash hazard analysis for each location. In addition, at each location
the electrician must wear the proper PPE (Personal Protective
Equipment) until he has verified the equipment to be worked on is in
an electrically safe work condition. From the top, the electrician must
work through the system:
Only OCPD
in this
panel opens
Figure 40
Service Panel - Check the condition of each conductor on the feeder
circuit from the service panel to the distribution panel by individually
testing each conductor. Check the condition of the circuit breaker in the
feeder circuit of the service panel. This requires visual inspection and
testing. Since this CB opened due to a lack of selective coordination,
let’s assume these feeder conductors are in good condition and no
damage was sustained due the fault current. The electrician still does
not know the cause of the opening of the service panel feeder CB, but
he knows this circuit is safe to energize. So he moves his attention to
the distribution panel.
Selective coordination isolates overcurrents to the lowest level possible, resulting in fewer
exposures to arc-flash hazards and typically at lower energy levels for electricians. In this case, the
electrician may not have to interface with OCPDs in upstream panels.
* Illustrative example of how arc-flash hazard levels can increase for larger equipment that is
closer to the source. Actual values can vary.
Distribution Panel - He finds the sub-feeder circuit breaker that
opened. He must follow the same procedures: test the condition of
each conductor on the feeder circuit from the distribution panel to the
branch panel and check the condition of the circuit breaker in the
feeder circuit of the distribution panel. This requires visual inspection
and testing. Since this CB opened due to lack of selective coordination,
let’s assume these sub-feeder conductors are in good condition and no
damage to the circuit or circuit breaker was sustained due to the fault
current. The electrician still does not know the cause of the opening of
the distribution panel feeder OCPD, but he knows this circuit is safe to
energize. So he moves his attention to the branch panel.
Branch Panel - He finds the branch circuit breaker that opened. He
must follow the same procedure: check the condition of each
conductor on the branch circuit from the branch panel to the load and
check the condition of the circuit breaker in the branch circuit of the
branch panel. This requires visual inspection and testing. Now he finds
the root cause being a fault on this circuit. He then must repair the
circuit, test it thoroughly to ensure it is safe prior to re-energizing.
It is evident that selectively coordinated overcurrent protective devices
can not only save restoration time, it also, can reduce the arc-flash
hazards for electricians. Even if the electrican was informed of the
location of the fault when he started his troubleshooting of the circuits
in Figure 41, the conductors and circuit breakers on the feeder and
sub-feeder circuits must be verified by testing as to their suitability to
be put back into service after incurring a fault.
©2008 Cooper Bussmann
Figure 41
Lack of selective coordination can increase the arc-flash hazard. When overcurrent protective
devices cascade open, the electric worker must unnecessarily work at higher levels in the system,
where arc-flash hazards are typically higher. This also increases the trouble-shooting (power
restoration) time.
* Illustrative example of how arc-flash hazard levels can increase for larger equipment that is
closer to the source. Actual values can vary.
35
Selective Coordination
Selective Coordination Objections & Misunderstandings
B. Fuses inherently are easy to selectively coordinate and there is not a
trade-off between providing selective coordination and arc-flash hazard
reduction. With current-limiting fuses, intentional short time-delay is
not required for selective coordination. Therefore, arcing faults are
taken off-line as quickly as possible, which does not result in increased
arc-flash hazards when designing for selective coordination. Some fuse
types provide lower arc-flash hazard levels than others. For building
distribution systems, as a general rule, Low-Peak® fuses are
recommended because their selectivity ratios are 2:1 and their built-in
current limitation helps limit arc- flash hazard levels.
C. Equipment can utilize arc-flash options which deploy optic sensors that
detect arc faults and react by shunt tripping a circuit breaker or switch
which can result in lowering high arc-flash hazards.
D. To achieve selective coordination using circuit breakers, in some cases,
upstream circuit breakers have to be intentionally delayed such as
using a short time-delay. However, for this objection, it is important to
separate the electrical system normal operation from tasks such as
performing maintenance or troubling shooting. Arc-flash
considerations are not an issue during normal operation; arc-flash is a
consideration when tasks such as performing maintenance or
troubleshooting are needed. When an electrician has to perform
maintenance or troubleshooting, there are practices and circuit breaker
options that can mitigate higher arc-flash hazard levels.
1. With CBs, a control switch option referred to as an arc-flash
reducing maintenance switch, is available that by-passes the
short time-delay (imposes instantaneous trip) and which can set the
instantaneous to a low setting while work is actually being performed
on or near energized equipment. This allows the circuit breaker to
normally have a short time-delay for coordination purposes during
normal operation, but when a worker is working on energized
equipment, the circuit breaker is switched to instantaneous trip. With
the switch enabled to instantaneous trip, the arc-flash hazard is
lower than would occur with a short time-delay setting.
2. Work practices may be an option. Prior to working on the
equipment, the electrician may temporarily adjust the setting to
lower levels for a circuit breaker supplying the equipment to be
worked on. The circuit breaker setting adjustments are typically
accessible without opening the enclosure. In so doing, the arc-flash
hazard level is reduced for the time period necessary for
maintenance.
3. There are other practices and equipment to mitigate higher level
arc-flash hazards, such as remote racking, extended length racking
tools, motorized switching options, etc.
4. With CBs, zone selective interlocking is a system option that reduces
the arc-flash hazard associated with using short time-delay. This
technology makes it simple to selectively coordinate circuit breakers
and still provide lower arc-flash levels and better equipment
protection whether during normal operation or performing
maintenance on energized equipment. See Figures 42, 43, and 44.
36
Figure 42
When there is a fault on the loadside of CB3, CB3 opens instantaneously and sends a signal to CB2
and CB1 to hold off (short time-delay).
Figure 43
When there is a fault on the loadside of CB2, but on the lineside of CB3; CB2 opens instantaneously
since there is no signal from CB3 to hold off. CB2 sends a signal to CB1 to hold off
(short time-delay).
©2008 Cooper Bussmann
Selective Coordination
Selective Coordination Objections & Misunderstandings
Figure 44
When there is a fault on the loadside of CB1, but on the lineside of CB2, CB1 opens instantaneously
since there is no signal from CB3 or CB2 to hold off.
Objection 3
Bolted short-circuits or high level fault currents don’t occur very frequently,
so selective coordination should only be required for overload conditions.
Clarifying Facts to Objection 3
A. Bolted faults are not the only condition where higher fault currents can
result. Low impedance arcing faults (results in high fault current) can
and do occur. Higher-level faults are more likely in fires, natural
catastrophes, human caused catastrophes and other emergency
situations.
B. Line-to-ground arcing faults in enclosures tend to quickly escalate to
three-phase arcing faults of significant levels. Arcing faults range from
70% to 43% of the bolted ISCA available in testing performed per IEEE
Paper PCIC-99-36. The lower the bolted ISCA, the higher the arcing
fault current as a % of the bolted fault current.
C. Even low-level faults can unnecessarily open multiple levels of
overcurrent protective devices if these devices are chosen without
regard to the available fault current. Low-level fault currents can still
result in a lack of coordination between the branch and feeder devices
or feeder and main devices if proper OCPD selection and selective
coordination analysis is not done.
©2008 Cooper Bussmann
Objection 4
Selective coordination results in greater equipment short-circuit damage
when short time-delay is used.
Clarifying Facts to Objection 4
A. With current-limiting fuses, intentional short time-delay is not required
for selective coordination. Therefore, short-circuits are taken off-line
as quickly as possible; equipment damage is not increased.
B. Equipment, such as transfer switches and busways, is now available
with longer short-time withstand ratings (short-circuit current rating).
C. With CBs, zone selective interlocking allows the upstream CB to open
as quickly as possible, bypassing the short time-delay for all faults
between the two CBs, thus improving equipment protection.
Objection 5
There are no documented incidents where a lack of coordination caused a
problem.
Clarifying Facts to Objection 5
A. Incidents are suppressed (sealed) due to litigation or fears of negative
publicity.
B. Eaton/Cutler-Hammer discusses details of a serious incident in a
healthcare facility in their service newsletter Power Systems Outage in
Critical Care Publication SA.81A.01.S.E, April 1999. Key points:
• Fault on a fan (branch circuit) causes loss of power to entire
emergency system in healthcare facility.
• Switched to emergency – fault still present, tripped emergency
generator device.
• All power to critical care loads including life support and ventilation
systems lost – patients required immediate medical attention.
• Lack of coordination and maintenance was determined as cause of
loss of power.
C. Findings by informal polling: a large percentage of electricians have
experienced occurrences where a lack of OCPD selective coordination
unnecessarily blacked out portions of a system.
D. Lack of coordination is accepted by experienced electricians as
something that normally happens. Once a system is installed with
overcurrent protective devices that are not selectively coordinated, the
situation typically can only be corrected by changing out the electrical
gear: so people live with it.
E. Code Making Panel (CMP) 13 (Articles 700 and 701) panel statement
included: “The panel agrees that selective coordination of emergency
system overcurrent devices with the supply side overcurrent devices
will provide for a more reliable emergency system.” (Panel Statement
to Proposal 13-135 during the 2005 NEC® cycle.)
F. CMP 20 panel statement in 2008 NEC® cycle: “The overriding theme of
Articles 585 (renumbered to 708) is to keep the power on for vital
loads. Selective coordination is obviously essential for the continuity of
service required in critical operations power systems. Selective
coordination increases the reliability of the COPS system.” (Panel
Statement to Comment 20-13 during the 2008 NEC® cycle.)
37
Selective Coordination
Selective Coordination Objections & Misunderstandings
Objection 6
NEC® 700.27 selective coordination requirement conflicts with NFPA 110
Standard for Emergency and Standby Power Systems.
Clarifying Facts to Objection 6
A. There is no conflict. NFPA 70 encompasses the entire electrical system
and NFPA 110 has a limited scope, not even the entire emergency
system. The scope of NFPA 110 only covers the electrical system from
the generator to the load terminals of the transfer switch and includes
optional standby alternate power systems where selective coordination
is not required. The NEC® (NFPA 70) includes Article 700 the entire
emergency system, Article 701 the entire legally required standby
system, Article 702 the entire optional standby systems and Article 708
the entire critical operations power systems. See Figure 45.
B. NFPA 110 calls for optimized selective coordination. Total selective
coordination is the very best “optimization” possible.
Parallel Generators Solution:
Bus differential relaying
provides short-circuit
protection for bus &
Emergency
generators for bus fault
Source
(between CTs)
G G
Overload protection only.
Coordinates with
overload
characteristics
Normal
of downstream
OCPDs
Source
N
87B
E
N
E
N
E
Bus Differential
Relay
Fuses or CBs
selectively
coordinated with
downstream
OCPDs for all
overcurrents
Figure 46
NFPA 70 (NEC)
Normal
Source
NFPA 110
Alternate
Source
N
E
N
E
N
E
Figure 45
Objection 7
Selective coordination is not possible with multiple emergency generators in
parallel (to increase reliability).
Clarifying Fact to Objection 7
For these more complex configurations, relays and transfer switch schemes
can be utilized to achieve selective coordination. See Figure 46.
38
Objection 8
The NEC® is not a performance or a design standard, so requirements for
selective coordination have no business in the NEC®.
Clarifying Facts of Objection 8
A. NEC® provides the very minimum requirements, the starting point, or
basis for all electrical designs. NEC® doesn’t tell the engineer how to
selectively coordinate the system. The requirement is performance
based and not prescriptive.
B. The stated purpose of the NEC® is the practical safeguarding of
persons and property from hazards arising from the use of electricity.
Three Code Making Panels (12, 13, and 20) of the NEC® have
confirmed or reconfirmed their desire for selective coordination
requirements in four articles. These requirements are for a few
important loads where system reliability is deemed very critical for life
safety and national security. See the section Why Selective
Coordination is Mandatory: It fills the Reliability “Hole.”
Objection 9
Compliance with selective coordination costs more, so it has no business in
the NEC®.
Clarifying Facts to Objection 9
A. This depends on design and system requirements. Costs are not
necessarily higher.
B. There is a cost associated with continuity of service for emergency and
critical operations power systems. There can be a greater cost (lives
lost) where continuity of service is not provided.
C. If this is true, there is no need for any of Articles 700, 701, 517, and
708 because there are additional costs with the requirements in all
these Articles. The whole of these Articles increases the costs. The
costs of an alternate power source, separate wiring, automatic transfer
switches, sophisticated sensors and control schemes, periodic testing,
and other items add cost to provide a reliable system that ensures high
availability of power to these vital loads. Selective coordination is
another requirement that increases the reliability of the system to
deliver power during critical times/emergencies.
D. See the section Why Selective Coordination is Mandatory: It fills the
Reliability “Hole.”
©2008 Cooper Bussmann
Selective Coordination
Elevator Circuit
Elevator Circuits and Required
Shunt Trip Disconnect — A Simple Solution.
When sprinklers are installed in elevator hoistways, machine rooms, or
machinery spaces, ANSI/ASME A17.1 requires that the power be removed to
the affected elevator upon or prior to the activation of these sprinklers. This
is an elevator code requirement that affects the electrical installation. The
electrical installation allows this requirement to be implemented at the
disconnecting means for the elevator in NEC® 620.51(B). This requirement is
most commonly accomplished through the use of a shunt trip disconnect
and its own control power. To make this situation even more complicated,
interface with the fire alarm system along with the monitoring of
components required by NFPA 72 must be accomplished in order to activate
the shunt trip action when appropriate and as well as making sure that the
system is functional during normal operation. This requires the use of
interposing relays that must be supplied in an additional enclosure. Other
requirements that have to be met include selective coordination for multiple
elevators (620.62) and hydraulic elevators with battery lowering [620.91(C)].
There is a simple solution available for engineering consultants, contractors,
and inspectors to help comply with all of these requirements in one enclosure called the Cooper Bussmann® Power Module™.
Elevator Selective Coordination Requirement
In the 2005 NEC®, 620.62 states:
Where more than one driving machine disconnecting means is supplied
by a single feeder, the overcurrent protective devices in each
disconnecting means shall be selectively coordinated with any other
supply side overcurrent protective devices.
A design engineer must specify and the contractor must install main, feeder,
sub-feeder and branch circuit protective devices that are selectively
coordinated for all values of overloads and short-circuits.
To better understand how to assess if the overcurrent protective devices in
an electrical system are selectively coordinated refer to the Selective
Coordination Section of this publication. Below is a brief coordination
assessment of an elevator system in a circuit breaker system (Example 1)
and in a fuse system (Example 2).
Power Module™ Elevator Disconnect
All-in-One Solution for Three Disciplines
NEC®
• Selective Coordination
• Hydraulic Elevators
• Traction Elevators
NFPA 72
• Fire Safety Interface
• Component Monitoring
ANSI/ASME A17.1
• Shunt Trip
Requirement
The Power Module contains a shunt trip fusible switch together with the
components necessary to comply with the fire alarm system requirements
and shunt trip control power all in one package. For engineering consultants
this means a simplified specification. For contractors this means a simplified
installation because all that has to be done is connecting the appropriate
wires. For inspectors this becomes simplified because everything is in one
place with the same wiring every time. The fusible portion of the switch
utilizes Low-Peak® LPJ-(amp)SP fuses that protect the elevator branch
circuit from the damaging effects of short-circuit currents as well as helping
to provide an easy method of selective coordination when supplied with an
upstream Low-Peak fuse with at least a 2:1 amp rating ratio. More
information about the Cooper Bussmann Power Module can be found at
www.cooperbussmann.com.
The Quik-Spec Power Module Switch (PS) for single elevator applications
©2008 Cooper Bussmann
Using the one-line diagram above, a coordination study must be done to see
that the system complies with the 620.62 selective coordination requirement
if EL-1, EL-2, and EL-3 are elevator motors.
Go to the Selective Coordination section for a more indepth discussion on
how to analyze systems to determine if selective coordination can be
achieved.
Quik-Spec Power Module Panel (PMP) for multiple elevator applications
39
Selective Coordination
Elevator Circuit
Example 1 Circuit Breaker System
Example 2 Fusible System
In this example, molded case circuit breakers (MCCB) will be used for the
branch and feeder protective devices and an insulated case circuit breaker
(ICCB) will be used for the main protective device.
In our second example, LPJ-(amp)SP fuses will be used for the branch
protection, LPS-RK-(amp)SP fuses will be used for the feeder protection, and
KRP-C-(amp)SP fuses will be used for the main protection.
1,000
800
1,000
800
600
600
400
300
400
300
200
200
100
80
60
100
80
60
KRP-C-1600SP
LPS-RK-400SP
1600A ICCB
40
200A MCCB
100A MCCB
10
8
6
4
3
TIME IN SECONDS
30
20
TIME IN SECONDS
40
400A MCCB
30
LPS-RK-200SP
20
LPJ-100SP
10
8
6
4
3
2
2
1
.8
.6
1
.8
.6
.4
.3
.4
.3
.2
.2
.1
.08
.06
.1
.08
.06
.04
.03
.04
.03
.02
.02
Looking at the time current curves for the circuit breaker in the figure above,
where any two circuit breaker curves overlap is a lack of selective
coordination. The overlap indicates both devices open. If any fault current
greater than 750A and less than 3100A occurs at EL-1, EL-2 or EL-3, the
200A circuit breaker will open as well as the 100A branch circuit breaker this is not a selectively coordinated system and does not meet the
requirements of 620.62. This lack of selective coordination could result in
stranding passengers in elevators or not having elevators available for fire
fighters. Fault currents above 3100A will open the 400A circuit breaker as
well and faults above approximately 16,000A will open the 1600A circuit
breaker - which further illustrates the lack of coordination. For a better
understanding of how to assess circuit breaker coordination, see the section
on Circuit Breaker Coordination in this publication. A system that is not in
compliance may result in needlessly stranding passengers and creating a
serious safety hazard.
40
60,000
80,000
100,000
40,000
30,000
20,000
6,000
8,000
10,000
4,000
3,000
2,000
800
1,000
600
400
300
.01
200
BLACKOUT
(TOTAL)
100
40,000
30,000
60,000
80,000
100,000
BLACKOUT
(PARTIAL)
CURRENT IN AMPERES
20,000
8,000
10,000
6,000
4,000
3,000
2,000
800
1,000
600
300
400
200
100
.01
CURRENT IN AMPERES
To verify selective coordination, go no further than the Fuse Selectivity Ratio
Guide in the Fuse Selective Coordination section in this publication. The
Low-Peak® fuses just require a 2:1 amp rating ratio to assure selective
coordination. In this example, there is a 4:1 ratio between the main fuse
(1600A) and the first level feeder fuse (400A) and a 2:1 ratio between the
first level feeder fuse and the second level feeder fuse (200A). As well, there
is a 2:1 ratio between the second level feeder fuse and the branch circuit
fuse (100A). Since a minimum of a 2:1 ratio is satisfied at all levels for this
system, selective coordination is achieved and 620.62 is met.
As just demonstrated in the prior paragraph, the fuse time-current curves do
not have to be drawn to assess selective coordination. For illustrative
purposes, the time-current curves for this example are shown above.
©2008 Cooper Bussmann
Selective
Coordination
Ground Fault
Protection
Introduction to Ground Fault Protection
Introduction
This section covers equipment protection from ground faults using ground
fault protection relays per the NEC®, options to design systems without
ground fault relays per the NEC® and selective coordination considerations
for circuits with ground fault protection relays.
Requirements
The pertinent NEC® requirements for Ground Fault Protection Relays (GFPRs)
are located in 230.95, 215.10, 240.13, 517.17, 695.6(H), 700.26, 701.17,
and 708.52. These sections provide requirements where GFPRs must be
used as well as requirements either not allowing GFPRs to be used or the
option to not use GFPRs (where GFPRs otherwise would be required). For
instance:
• GFPRs are required on 1000A or greater service disconnects for
480/277V, solidly grounded wye systems
• If a GFPRs is on the service of a healthcare facility, then GFPRs must
be on the next level of feeders.
• GFPRs are not required for the alternate source of emergency systems
(700.26) and legally required standby systems per 701.17.
• GFPRs can not be on the circuit paths for fire pumps per 695.6(H)
• For healthcare essential electrical systems, GFPRs can not be on the
loadside of transfer switches or between the alternate source and the
transfer switch.
GFPRs are only required in a few applications. If the use of GFPRs is not
desired, in some cases, there maybe design options in which GFPRs are not
required.
GFPR
Ground fault protection relays (or sensors) are used to sense ground faults.
When the ground fault current magnitude and time reach the GFPR’s pick-up
setting, the control scheme signals the circuit disconnect to open. GFPRs
only monitor and respond to ground fault currents.
Fuses and circuit breakers respond to any type overcurrent condition:
overloads and short-circuit currents, including ground faults. Per the NEC®,
for most premise circuits, the branch circuit overcurrent protection (fuses or
circuit breakers) are permitted to provide protection for all types of
overcurrent conditions, including ground faults. However, for some very
large ampacity circuits, the NEC® requires GFPRs, which are intended to
provide equipment protection from lower magnitude ground fault currents.
Ground fault relays typically only provide equipment protection from the
effects of low magnitude ground faults. GPFRs and disconnecting means
typically are too slow for higher magnitude ground faults. Equipment
protection against the effects of higher magnitude ground faults is
dependent on the speed of response of the conventional overcurrent
protective devices (fuses or circuit breakers).
GFPRs Do Not Provide:
• People protection: GFPRs do not prevent shock. Ground fault circuit
interrupters (GFCIs) are required for certain 15 and 20A, 120V branch
circuits, and are intended to protect people.
• Ground fault prevention
• Protection against 3-phase, phase-phase, or phase-neutral faults
• Adequate protection from high level faults
©2008 Cooper Bussmann
Providing ground fault protection with a GFPR requires a sensor, monitor,
shunt trip and circuit disconnecting means. A fusible switch with shunt trip
capability can be equipped with GFPR. Figure 1 shows a bolted pressure
switch equipped with GFPR. Circuit breakers with shunt trip capability also
can be equipped in a similar manner. Some electronic trip circuit breakers
have GFPR options where the GFPR components are internal to the circuit
breaker.
Figure 1
Fusible bolted pressure switch equipped with ground fault protection relay (Courtesy
of Boltswitch, Inc.)
GFPR Characteristics and Settings
GFPRs typically have adjustable trip settings and various shaped
time-current curves. The trip setting generally consists of selecting an amp
set point from a range and selecting a time set point from a range.
Understanding a GFPR’s characteristics is important in assessing the level of
protection of the equipment and in coordination. Too often a GFPR on a
service is adjusted to a low amp and instantaneous trip setting. With this
setting, a ground fault on a 20A branch circuit may unnecessarily cause a
GFPR to open the service disconnect. If the GFPR is set properly, a fault on a
20A branch circuit would be interrupted by the 20A fuse or circuit breaker.
NEC® section 230.95 has a maximum limit for service GFPR characteristics
of 1200A setting and an operational limit of 1 second at 3000A. GFPRs are
available with various time-current shaped characteristics; some with a step
function such as two examples shown in Figure 3 and some with an inverse
time function such as shown in Figure 5. A GFPR’s time-current
characteristic curve shape, various amp set points, and various time-delay
set points permit selecting time-current characteristics to provide the level of
equipment protection needed and provide the level of coordination desired.
Selective Coordination
GFPRs should be included in a selective coordination analysis. This is
covered later in GFPR Selective Coordination Considerations. If the use of a
particular GFPR causes a lack of selective coordination, there may be other
GFPR options available or there may be alternate design options.
The following pages on ground fault protection provide more information on
the requirements and considerations for application of GFPRs.
41
Ground Fault Protection
Requirements
Where GFPRs are NOT Required
Section 230.95
Ground Fault Protection of Equipment
This Section means that 480Y/277V, solidly grounded “wye” only connected
service disconnects, 1000A and larger, must have ground fault protection in
addition to conventional overcurrent protection. A ground fault protection
relay, however, is not required on a service disconnect for a continuous
process where its opening will increase hazards (240.13). All delta connected
or high resistance grounded services are not required to have GFPR. The
maximum setting for the ground fault protection relay (or sensor) can be set
to pick up ground faults at a maximum of 1200A and actuate the main
switch or circuit breaker to disconnect all phase conductors. A ground fault
relay with a deliberate time-delay characteristic of up to 1 second, may be
specified for currents greater than or equal to 3000A. (The use of such a
relay greatly enhances system coordination and minimizes power
outages - see Figure 5).
A ground fault protection relay in itself will not limit the line-to-ground or
phase-to-phase short-circuit current. When non current limiting mechanical
protective devices such as conventional circuit breakers are used with GFPR,
all of the available short-circuit current may flow to the point of fault, limited
only by circuit impedance. Therefore, it is recommended that current-limiting
overcurrent protective devices be used in conjunction with GFPR.
This system offers:
1. Some degree of arcing and low
magnitude ground fault protection by
the GFPR operating the switch.
2. Current limitation for high magnitude
ground faults and short-circuits by
current-limiting fuses, which provides
component protection for the
switchgear.
480Y/277V.
3Ø/4W
1000 Amp or
Larger Switch
Note: This system DOES NOT provide
current limitation for high magnitude
ground faults and short-circuits.
42
1. Continuous industrial process where a non-orderly shut down would increase
hazards (section 230.95 exception and 240.13).
• Alternate source of emergency systems (700.26) and legally required standby
systems (701.17).
• For healthcare essential electrical systems, GFPRs are not permitted on the
loadside of transfer switches or between the alternate source and the
transfer switch [(517.17(B))].
2. All services or feeders where the disconnect is less than 1000 amps.
3. All 208Y/120 Volt, 3ø, 4W (wye) services or feeders.
4. All single-phase services or feeders including 240/120 Volt.
5. Resistance or impedance grounded systems, such as 480V, high resistance
grounded wye systems.
6. High or medium voltage services or feeders. (See NEC® section 240.13 and
215.10 for feeder requirements.)
7. All services or feeders on delta systems (grounded or ungrounded) such as 480
Volt, 3ø, 3W delta, or 240 Volt, 3ø, 4W delta with midpoint tap.
8. Service with six disconnects or less (section 230.71) where each disconnect is
less than 1000 amps. A 4000A service could be split into 5 - 800A switches.
9. Fire Pumps [(695.6(H))].
10. For feeders where ground fault protection is provided on the service (except for
Healthcare Facilities and COPS. See section 517.17 and 708.52.)
CurrentLimiting Fuses
GFPR
Relay
For instance, ground fault relays are not required on these systems.
480Y/277V
SWBD
Service
Disconnect
less than
1000 Amps
This system offers:
1. Some degree of arcing and low
magnitude ground fault protection by
the GFPR operating the
circuit breaker.
There are many services and feeders where 230.95, 215.10, and others
do not require or permit ground fault protection including:
480Y/277V.
3Ø/4W
1000 Amp
Circuit Breaker
or Larger
208Y/120V
Any Size
Service
Disconnect
480V
3Ø 3W
Delta
Any Size
Service
Disconnect
480V/ 277V
Six Service Disconnects
800 Amps or Less
GFPR
Relay
SWBD
©2008 Cooper Bussmann
©2008
Ground Fault Protection
Requirements
215.10. – Ground Fault Protection of Equipment
Equipment classified as a feeder disconnect must have ground fault
protection as specified in 230.95.
Service
Med. Voltage
4160V
Healthcare Facility and Critical Operations Power
Systems
1. When a ground fault protection relay is placed on the service or feeder then,
2. Ground fault protection relays must also be placed on the next level
downstream, and the upstream ground fault protection relay time band must
have a 6 cycle separation from the main ground fault relay.
Feeder w/o
GFPR
VIOLATION
480Y/277V
1000A
or Greater
1,000
800
600
Feeder
Provided
w/GFPR
Service
Med. Voltage
4160V
200
COMPLIANCE
480Y/277V
1000A
or Greater
100
80
60
A ground fault protection relay will not be required on feeder equipment
when it is provided on the supply side of the feeder (except for certain
healthcare facilities requirements, Article 517 and 708).
1000A
or
Greater
Feeder of any rating
no GFPR Required
(Except Per Article 517 & 708)
MAIN GFPR
GFPR
FDR GFPR
SWBD
40
30
TIME IN SECONDS
GFPR
COMPLIANCE
480Y/277V
480Y/277V.
3Ø/4W
400
300
20
GFPR
10
8
6
4
3
240.13. – Ground Fault Protection of Equipment
2
Equipment ground fault protection of the type required in section 230.95 is
required for each disconnect rated 1000A or more on 480Y/277V solidly
grounded wye systems, that will serve as a main disconnect for a separate
building or structure. Refer to sections 215.10 and 230.95.
1
.8
.6
MAIN GFPR
1200 Amp
12 Cycles
.4
.3
.2
©2008 Cooper
©2008
CooperBussmann
Bussmann
.02
60,000
80,000
100,000
30,000
40,000
20,000
6,000
8,000
10,000
4,000
3,000
.01
2,000
If ground fault protection is placed on the main service of a healthcare
facility (517.17) or critical operations power system (708.52), ground fault
protection relays must also be placed on the next level of feeders. The
separation between ground fault relay time bands for any feeder and main
ground fault protection relay must be at least six cycles in order to achieve
coordination between these two ground fault protection relays. Where no
ground fault protection relay is placed on the main or feeders, no ground
fault protection relays are required on the feeders or subfeeders. Therefore, if
the requirements of 230.95, 240.13, or 215.10 do not require a ground fault
protection relay and no ground fault protection relay is utilized on the main
service disconnect or feeder disconnect, then no ground fault protection
relays are required on the next level downstream. See Figure 2.
FEEDER GFPR
800 Amp
2 Cycles
.03
800
1,000
Two Levels of Ground Fault Protection
.04
600
GF PR Not
Required
400
Building B Service
1000A or Greater
480Y/277V
Minimum
6 Cycle Separation
.1
.08
.06
300
800A
480Y/277V
GF PR Not
Required
200
GF PR Not
Required
Building A Service
100
High Voltage
Service
CURRENT IN AMPS
Figure 2
Note: Merely providing coordinated ground fault protection relays does not
prevent a main service blackout caused by feeder ground faults. The
overcurrent protective devices must also be selectively coordinated. The
intent of 517.17 and 708.52 is to achieve “100 percent selectivity” for all
magnitudes of ground fault current and overcurrents. 100% selectivity
requires that the overcurrent protective devices also be selectively
coordinated for medium and high magnitude ground fault currents because
the conventional overcurrent devices may operate at these levels.
43
Ground Fault Protection
Overcurrent Protective Devices
Analysis of Ground Fault Relay
Curves and Overcurrent Device Curves
To a fuse or circuit breaker, ground fault current is sensed just as any other
current. If the ground fault current is high enough, the fuse or circuit breaker
responds before the ground fault protection relay (this depends on the GFPR
setting, overcurrent device characteristics, speed of response of the
overcurrent device and ground fault current magnitude). Therefore, when
analyzing ground fault protection, it is necessary to study the characteristics
of the GFPR and overcurrent protective device as a combination.
The combination of the GFPR and overcurrent device have a ground fault
“effective curve.” This is a composite of the ground fault relay and
overcurrent protective device curves. When analyzing line-to-ground faults,
the “effective” curve of the ground fault protection relay and conventional
overcurrent protective device must be examined.
Figure 4 below is the “effective” ground fault curve for a 1600A circuit
breaker in combination with a ground fault protection relay scheme set at
1200A and 12 cycle delay.
When comparing Figures 3 and 4 notice that for ground faults above
approximately 14,000A the fused bolted pressure switch combination has
the advantage of faster response and above 22,000A the fused switch has
the advantage of current-limitation.
1,000
800
600
MAIN GFPR
400
300
Main
G.F.R.
200
KRP-C1600SP
100
80
60
40
30
TIME IN SECONDS
KRP-C 1600SP
20
10
8
6
4
3
2
1
.8
.6
.4
.3
.2
Figure 4
.1
.08
.06
“Effective” time-current curve for line-to-ground fault with 1600A circuit breaker
and ground fault sensor setting at 1200A.
.04
.03
.02
Figure 3
60,000
80,000
100,000
30,000
40,000
20,000
6,000
8,000
10,000
4,000
3,000
2,000
800
1,000
600
400
300
200
100
.01
CURRENT IN AMPS
“Effective” time-current curve for line to ground fault with 1600A fuse and ground
fault protection relay set at 1200A.
Figure 3 above is the “effective” ground fault curve for a 1600A fuse in
combination with a ground fault relay scheme set at 1200A pickup and 12
cycle delay.
44
©2008 Cooper Bussmann
©2008
Ground Fault Protection
GFPR Selective Coordination Considerations
GFPR Selective Coordination Considerations
When ground fault protection relays are used in a system, selective
coordination should include an analysis of the circuit paths for ground faults.
As previously mentioned, GFPRs only monitor and respond to ground fault
currents. Branch circuit fuses and circuit breakers sense and respond to all
types of overcurrents. Therefore, when analyzing a circuit path for selective
coordination, GFPRs should be included. For circuit paths with GFPRs, there
are two phases in a coordination analysis:
1. Analyze the circuit paths only considering the fuses or circuit
breakers for all types of overcurrents. Previous sections in this
publication cover this in depth.
2. Analyze the circuit paths for just ground faults. In this case, the
GFPR characteristics and the fuse or circuit breaker characteristics
must be considered together. Remember, fuses and circuit breakers
monitor and respond to any type overcurrent, so they should be
factored in also. The following pages have some important
considerations for this analysis.
A. One step ground fault relaying (starts on this page)
B. Two step ground fault relaying (starting on a later page)
A. One Step Ground Fault Relaying
When a ground fault occurs on a feeder or branch circuit it is highly
desirable for the feeder or branch circuit overcurrent device to clear that
fault before the main device opens, thus preventing an unnecessary system
blackout. However, this is not always the case when a ground fault relay is
located on the main or when the overcurrent protective devices are not
selectively coordinated.
To avoid unnecessary service disruptions (or BLACKOUTS):
GFPR
GFPR
Feeder
Feeder
Feeder
Ground
Fault
Branch
Circuit
OR
Branch
Circuit
Branch Circuit
Ground Fault
Selective coordination should be investigated for low and high magnitude
ground faults. Generally on low magnitude ground faults the feeder
overcurrent device must be selective with the main ground fault relay. For
high magnitude ground faults it is necessary also to consider selective
coordination between the main overcurrent device and feeder overcurrent
device.
1. The characteristics of the main overcurrent device must be analyzed with
relation to the feeder and branch circuit overcurrent protective devices.
2. The characteristics of the feeder and/or branch circuit overcurrent devices must
be analyzed with relation to the main ground fault protection relay
characteristics.
©2008 Cooper
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CooperBussmann
Bussmann
45
Ground Fault Protection
GFPR Selective Coordination Considerations
Low Magnitude Ground Faults on Feeders —
One Step Ground Fault Relaying.
For low magnitude feeder ground faults, the feeder overcurrent protective
device can clear the circuit without disrupting the main service if the feeder
overcurrent device lies to the left of the ground fault protection relay and
does not cross at any point.
In Figures 5 and 6, the ground fault protection relay located on the main has
an operating time-delay of 18 cycles and 1200A pickup. Its inverse-time
characteristic with the maximum 1 second opening time at 3000A improves
selective coordination with downstream devices.
MAIN GFPR
Fuse System
MAIN GRPR
Main
GFPR
Figure 6
Coordination considerations for low magnitude feeder ground faults. Consider main
ground fault relay and feeder overcurrent device. A lack of coordination exists for
ground faults between 1200A and 1800A.
Circuit Breaker System
Figure 6 illustrates that for some low magnitude ground faults this 200A
circuit breaker will not coordinate with the ground fault relay. If this circuit
breaker has an adjustable instaneous trip, it may be possible to lower the
setting and achieve coordination with the GFPR.
Figure 5
Selective coordination considerations for low magnitude feeder ground faults.
Longer GFPR relay delay permits larger feeder fuse to coordinate with main relay.
Figure 5 illustrates that an inverse-time main ground fault relay may permit a
larger size feeder fuse to selectively coordinate with the ground fault relay. In
this case, the inverse time ground fault relay is set at 1200A and 18 cycle
delay. A LPS-RK-200SP amp feeder fuse coordinates with this main ground
fault relay. A JKS-400A feeder fuse, which is a non time-delay fuse,
coordinates with this same main GFPR (figure not included).
46
©2008 Cooper Bussmann
©2008
Ground Fault Protection
GFPR Selective Coordination Considerations
High Magnitude Ground Faults on Feeders —
One Step Ground Fault Relaying
For higher magnitude ground faults, it is generally necessary to consider the
characteristics of the main overcurrent protective device as well as the
ground fault relay. Conventional overcurrent protective devices, fuses or
circuit breakers, cannot differentiate between a high magnitude ground fault
or a high magnitude phase-to-phase short-circuit. Therefore, when a high
magnitude feeder ground fault occurs, the main overcurrent device must be
considered in relation to the feeder overcurrent device. To achieve selective
coordination and prevent a blackout for high magnitude ground faults, the
feeder overcurrent device must be selective with the main overcurrent
device.
approximately 17,000A). For currents greater than 17,000A, using the
Selectivity Ratio Guide presented in the Fuse Selective Coordination
Section shows that the LPS-RK-200A fuses selectively coordinate with the
KRP-C-1200SP fuses up to 200,000A for any type overcurrent including
ground fault currents.
MAIN GFPR
1,000
800
600
400
300
MAIN MAIN
GFPR
200
KRP-C1200SP
100
80
60
40
TIME IN SECONDS
30
20
LPS-RK200SP
10
8
6
4
Main
GFPR
3
2
KRP-C1200SP
1
.8
.6
.4
.3
LPS-RK200SP
.2
Figure 8
.1
.08
.06
Selective coordination considerations for high magnitude feeder ground faults
requires analysis of main and feeder overcurrent devices. In this case feeder
ground faults greater than 11,000A will cause the main circuit breaker to open
unnecessarily creating a BLACKOUT! Thus the entire service is blacked-out because
of a lack of coordination. The ground fault relay is not of concern because it has an
18 cycle delay.
.04
.03
.02
60,000
80,000
100,000
30,000
40,000
20,000
4,000
3,000
2,000
800
1,000
600
6,000
8,000
10,000
Figure 7
400
300
200
100
.01
Selective coordination considerations for high magnitude feeder ground faults
requires analysis of main and feeder overcurrent devices. In this case the fuses are
selectively coordinated so that an unnecessary blackout does not occur.
Fuse System
Figure 7 illustrates that the feeder LPS-RK-200SP 200 amp fuse
selectively coordinates with the inverse-time main GFPR for all levels of
ground faults. Also, for any type overcurrent including low level and high
level ground faults the LPS-RK-200SP fuse selectively coordinates with
the main KRP-C-1200SP 1200 amp fuses. Figure 7 fuse time-current
curves show coordination for the portion of the curves shown (up to
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CooperBussmann
Bussmann
Circuit Breaker System
CURRENT IN AMPS
Figure 8 illustrates that for feeder ground faults above 11,000A the main
service 1200A circuit breaker as well as the 200A circuit breaker will
open. This is because an 11,000A or greater fault current unlatches both
the 200A and 1200A circuit breakers. This condition will create a service
blackout when a feeder ground fault occurs.
In addition, ground faults between approximately 1200A and 1800A on the
loadside of the 200A circuit breaker will cause the GFPR to open the main
circuit breaker, thereby blacking out the entire service.
47
Ground Fault Protection
GFPR Selective Coordination Considerations
This fact is commonly overlooked when applying ground fault relays.
Generally, the short time-delay on the ground fault relay is thought to provide
coordination for higher magnitude feeder ground faults. However, as shown
by this example, the main circuit breaker operates to cause an unnecessary
blackout.
1,000
800
600
400
300
200
Note: There are several alternatives for achieving selective coordination with
circuit breakers discussed in the Circuit Breaker Selective Coordination
Section of this publication. Circuit breakers with short time-delay trip
settings were not considered in this section on GFPR selective coordination.
100
80
60
RESULT: BLACKOUT
30
TIME IN SECONDS
10
8
6
4
3
2
1
.8
.6
.4
.3
Ground Fault
11,000A
or Greater
The system in Figure 9 illustrates the typical problem concerning this point.
The main ground fault relay is set at 1200A, 18-cycle delay and the feeder
ground fault relay is set at 100A, 6-cycle delay. These ground fault relay
settings could mistakenly be interpreted to mean that feeder ground faults
would be cleared by only the feeder ground fault relay opening the feeder
disconnect. But the analysis must also include the phase overcurrent device
characteristics since these devices also respond to current.
.04
.03
.02
Figure 10
60,000
80,000
100,000
30,000
40,000
20,000
6,000
8,000
10,000
4,000
3,000
2,000
800
1,000
600
.01
400
Feeder
GFPR Relay
100A
6 Cycle Delay
BLACKOUT
AREA
.1
.08
.06
300
Main
GFPR Relay
1200A
18 Cycle Delay
200A CB
48
200A CB
.2
Circuit
Breaker Opens
Figure 9
20
200
Circuit
Breaker Opens
1200A CB
800
100
1200A CB
FDR GFPR
40
B. Two Step Ground Fault Relaying
Two step ground fault relaying includes ground fault relays on the main
service and feeders.
In many instances, this procedure can provide a higher degree of ground
fault coordination to prevent unnecessary service blackouts. Yet it is
mistakenly believed by many that two step ground fault relays assure total
ground fault coordination. For complete selective coordination of all ground
faults, the conventional overcurrent protective devices must be selectively
coordinated as well as the ground fault relays. The fact is that even with this
two step relay provision, ground fault coordination is not assured on many
systems designed with circuit breakers which incorporate instantaneous
unlatching mechanisms.
MAIN GFPR
CURRENT IN AMPS
The two step ground fault protection relays give a false sense of security.
Figure 10 above illustrates that the ground fault relays are coordinated, but
overcurrent devices are not coordinated for feeder or branch circuit ground
faults above 11,000 amps. This is indicated as the BLACKOUT AREA on the
curve. In this case, the main overcurrent device and the feeder overcurrent
device both open on a feeder circuit fault. Thus the entire system is blacked
out; even though two step ground fault relays are provided.
For healthcare facilities (517.17) and Critical Operations Power Systems
(708.52), the main and feeders are required to be 100% selectively
coordinated for all magnitudes of ground fault current - including low,
medium and high ground fault currents.
©2008 Cooper Bussmann
©2008
Ground Fault Protection
GFPR Selective Coordination Considerations
In many cases two step relays do provide a higher degree of ground fault
coordination. When properly selected, the main fuse can be selectively
coordinated with the feeder fuses. Thus on all feeder ground faults or short
circuits the feeder fuse will always open before the main fuse. When
selectively coordinated main and feeder fuses are combined with selectively
coordinated main and feeder ground fault protection relays, ground fault
coordination between the main and feeder is predictable.
1,000
800
600
400
300
200
LPS-RK 200SP
100
80
60
MAIN GFPR
FDR GFPR
40
KRP-C1200SP
KRP-C1200SP
Does Not
Open
TIME IN SECONDS
30
Main
GFPR
1200A
18 Cycle Delay
20
10
8
6
4
3
Only Feeder
Disrupted
1
.8
.6
.4
If downstream circuits must be selectively coordinated with the feeder GFPR
and overcurrent protective devices, the analysis needs to include the
downstream overcurrent protective devices.
.03
.02
Figure 12
60,000
80,000
100,000
30,000
40,000
20,000
6,000
8,000
10,000
4,000
3,000
2,000
.01
800
1,000
2. The feeder fuses are selectively coordinated with the main fuses for all ground
faults, short-circuits or overloads on the loadside of the feeder. The feeder fuses
would clear the fault before the main fuses open.
.04
600
1. The feeder ground fault relay is set at a lower time band than the main ground
fault relay, therefore the relays are coordinated.
.1
.08
.06
400
Figures 11 and 12 illustrate a selectively coordinated main and feeder for all
levels of ground faults, overloads and short-circuits. Any fault on the feeder
will not disrupt the main service.
This system offers full selective coordination for all levels of ground faults or
short-circuits.
.2
300
Figure 11
.3
200
Any Level Ground
Fault Current
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©2008
CooperBussmann
Bussmann
2
80
100
LPS-RK200SP
Feeder
GFPR
100A
6 Cycle Delay
CURRENT IN AMPS
Design Options
GFPRs are only required in a few applications. If the use of GFPRs cause
selective coordination issues, or is not desired, there are design options to
resolve the issues:
• Use inverse-time ground fault relays and set the amp set point and
time delay set point as high as practical
• Utilize a 480V high resistance grounded wye system. This type of
system does not require GFPRs. These systems also reduce the
probability of a hazardous arcing-fault starting from line-to-ground
faults; this benefits worker safety. Loads requiring neutrals must be
fed from downstream transformers, which can be 208/120V solidly
grounded wye systems or 480/277V solidly grounded wye systems
with feeder disconnects of 800A or less.
• Design 480/277V solidly grounded wye services using up to six 800A
or less disconnects (230.71).
• For circuits supplying loads where there are alternate sources, place
the automatic transfer switches close to the loads. Use smaller
transfer switches placed closer to the final panelboard or large branch
circuit loads. This option requires more transfer switches and longer
cable runs. However, it enhances the reliability of supplying power to
vital loads.
49
Ground Fault Protection
Current Limitation
The Need for Current Limitation
If ground fault protection is required, then the best protection is a switch
equipped with a ground fault protection relay scheme, a shunt trip
mechanism and current-limiting fuses. The reason is that this system will
offer protection for high magnitude ground faults as well as low magnitude
ground faults. Ground fault protection relay schemes and shunt trip
mechanisms on switches or circuit breakers can protect equipment against
extensive damage from low magnitude ground faults - this is their intended
purpose.
The National Electrical Code® requires ground fault protection for
intermediate and high ground faults as well as low grade ground faults. For
high magnitude ground faults, ground fault relay schemes operate too slowly
to prevent extensive equipment damage. The main or feeder overcurrent
devices, such as fuses or circuit breakers must clear the circuit.
Current-limiting fuses substantially limit the energy let-through for higher
magnitude ground faults and thereby offer a higher degree of protection.
Conventional circuit breakers are not current-limiting protective devices and
during higher magnitude ground faults can let through large amounts of
damaging energy.
1,000
800
600
400
300
200
100
80
60
1600A CB
40
TIME IN SECONDS
TIME IN SECONDS
30
20
10
8
6
4
3
2
1
.8
.6
.4
.3
.2
.1
.08
.06
.04
.03
Clearing characteristic for a 1600A fuse. A 20,000 amp fault is cleared by the
KRP-C-1600SP fuse in 0.019 to 0.039 second (between one and two cycles).
For currents greater than 25,000A, the fuse enters its current-limiting range.
Then the clearing time is less than one half cycle (less than 0.008 second).
50
Figure 14
60,000
80,000
100,000
40,000
30,000
20,000
6,000
8,000
10,000
4,000
600
400
300
3,000
CURRENT IN AMPS
2,000
Figure 13
200
100
.01
800
1,000
.02
CURRENT IN AMPS
Clearing characteristic for 1600A circuit breaker. A 20,000A fault is cleared by the
1600A circuit breaker in 0.05 second. The circuit breaker has a fixed operating time
for high values of current. This time is approximately 0.05 second (three cycles).
Therefore, high magnitude ground faults and short-circuits are permitted to flow for
at least three cycles.
©2008 Cooper Bussmann
©2008
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