MICROTEMP® Thermal Fuses

MICROTEMP®
Thermal Fuses
Product Information and
Application Notes
Upper Limit Temperature Protection
MICROTEMP®, the original thermal fuse from Therm-O-Disc, offers the broadest combination of globally certified
temperatures and electrical loads, as well as the broadest range of packages, mountings and design configurations
on the market today.
MICROTEMP Features:
•
Globally certified temperatures and electrical loads
•
One-shot operation cuts off electrical power
•
Current interruption capacity up to 25 amps @ 250VAC
•
Low resistance
•
Compact size
•
RoHS compliant
Operating Principle
The active trigger mechanism of the thermal fuse is an exclusively formulated, electrically nonconductive pellet. Under
normal operating temperatures, the solid pellet holds spring loaded contacts closed. When a predetermined temperature is reached, the pellet melts, allowing the compression spring to relax. The trip spring then slides the contact away
from the lead and the circuit is opened.
After the thermal fuse opens a circuit, the fuse needs to be replaced. This replacement procedure must include correction of the fault condition before the product is operated again.
G4
G5
Z6
G6
Typical
Resistive
Rating at 250VAC
10A
20A
Higher Tm Than
G6
Details found
on page
3
5
7
16A
16A
More Temperature
Ratings Than Z6
G7
G8
S9
5A
25A
15A
See pages 14-15
Application Notes - Page 16
Mountings & Configurations - Page 9
NOTE: The terms thermal fuse, thermal cutoff, thermal link, and TCO are synonymous and may be used
interchangeably throughout the catalog.
2
G4 MICROTEMP Thermal Fuses
G4 MICROTEMP - the Original Thermal Fuse
Providing reliable back-up protection for temperature controlling thermostats and other over-temperature conditions,
the G4 series MICROTEMP thermal fuse is the industry standard for over-temperature protection. The G4 is rated for
continuous operating currents up to 10 amps @ 250VAC.
Benefits
Features
Applications
• The industry standard
for over-temperature
protection
• One shot operation cuts
off electrical power
• Portable Appliance
• 10A/250VAC,
15A/120VAC,
5A/24VDC
• HVAC
• Low Resistance
• Water Heater
• Compact size
• Other
• Available in a wide range
of temperatures to offer
design flexibility in your
application
• Available in mounted and
packaged designs
• Major Appliance
• Power Supplies
3
G4 MICROTEMP Product Information
Before
Operation
After
Operation
Operating Temperature Summary
Standard Dimensions
Standard Leads
A
Overall Length ± .12” (±3.0mm)*
2.51” (63.8mm)
B
Case Lead Length ± .06” (±1.5mm)
1.38” (34.9mm)
C
Case Lead Diameter
0.040” (1.0mm)
Case Lead Material
Tin Plated Copper
D
E/F
072
57
100
073
58
100
077
62
300
084
69
220
091
76
300
093
78
300
Isolated Lead Material
Silver Plated Copper
098
83
300
Case Dimensions, Including Epoxy
.58“ L x .158” D
(14.7mm x 4.0mm)
104
89
200
110
95
240
117
102
240
121
106
300
128
113
205
134
119
205
141
126
205
144
129
300
152
137
205
158
143
240
167
152
210
172
157
310
184
169
240
190
175
350
192
177
210
205
190
310
216
200
450
229
200
450
450
E
REF.
D
B
F
C
REF.
Electrical Ratings
Agency
CCC
Tm°C
0.040” (1.0mm)
A
VDE
Th°C
Isolated Lead Diameter
* Overall length available up to 5.83” (148mm)
UL/CSA
Tf°C
Resistive
Inductive
10A/250VAC
15A/120VAC
5A/24VDC
8A/250VAC
14A/120VAC
10A/250VAC
8A/250VAC
240
200
8A/250VAC
257
220
10A/250VAC
PSE JET
10A/250VAC
S JET*
30A/16VDC**
Korea
10A/250VAC
470
Tf = Functioning open temperature +0/-5 C
Th = Maximum temperature of the thermal fuse,
measured at the case end, at which the thermal fuse can
be maintained for a period of at least168 hours without
opening
Tm = Maximum overshoot temperature. Temperature up
to which the open thermal fuse will not change state
0
*For S JET ratings for Japan replace ‘G’ with ‘S’ as first letter of nomenclature
**Load agency approved for 172C and 240C temperatures only
4
G5 MICROTEMP Thermal Fuses
G5 MICROTEMP - the Original Thermal Fuse
Designed for higher voltage and current applications, the G5 MICROTEMP Thermal Fuse is rated for operating currents up
to 20 amps @ 277 VAC. The internal construction of the G5 is designed for interrupting higher currents and withstanding
higher temperatures than other models.
Benefits
Features
Applications
• Designed for higher
voltage and current
applications
• One shot operation cuts
off electrical power
• Portable Appliance
• 20A/250VAC,
20A/277VAC,
25A/120VAC
• HVAC
• High Overshoot
Temperature Tm
• Automotive
• Compact Size
• Other
• Available in a wide range
of temperatures to offer
design flexibility in your
application
• Available in mounted and
packaged designs
• Major Appliance
• Power Supplies
• Water Heater
5
G5 MICROTEMP Product Information
Before
Operation
After
Operation
Standard Dimensions
Operating Temperature Summary
Standard Leads
Tf°C
Th°C
Tm°C
072
073
077
084
091
093
098
104
110
117
121
128
134
141
144
152
158
167
172
184
190
192
205
216
229
240
57
58
62
69
76
78
83
89
95
102
106
113
119
126
129
137
143
152
157
169
175
177
190
200
200
200
410
410
410
220
430
410
410
225
225
410
410
235
410
350
410
410
410
410
410
410
410
350
410
410
410
410
A
Overall Length ± .12” (±3.0mm)*
2.51” (63.8mm)
B
Case Lead Length ± .06” (±1.5mm)
1.38” (34.9mm)
C
D
E/F
Case Lead Diameter
0.040” (1.0mm)
Case Lead Material
Tin Plated Copper
Isolated Lead Diameter
0.040” (1.0mm)
Isolated Lead Material
Silver Plated Copper
Case Dimensions, Including Epoxy
.58“ L x .158” D
(14.7mm x 4.0mm)
* Overall length available up to 5.83” (148mm)
A
E
REF.
D
B
F
C
REF.
Electrical Ratings
Agency
Resistive
UL/CSA
20A/250VAC
25A/120AC
21A/240VAC
20A/277VAC
VDE
20A/250VAC
CCC
20A/250VAC
PSE JET
15A/250VAC
Korea
16A/250VAC
Tf = Functioning open temperature +0/-50 C
Th = Maximum temperature of the thermal fuse, measured
at the case end, at which the thermal fuse can be maintained
for a period of at least168 hours without opening
Tm = Maximum overshoot temperature. Temperature up to
which the open thermal fuse will not change state
6
Z6 MICROTEMP Thermal Fuses
Z6 MICROTEMP - the Original Thermal Fuse
MICROTEMP thermal fuses offer an accurate, reliable solution to the need for upper limit temperature protection against
overheating by interrupting an electrical circuit when operating temperatures exceed the rated temperature. Designed to
meet European voltage and current applications, the Z6 is rated for operating currents up to 16 amps @ 250 VAC.
Benefits
Features
Applications
• Same excellent quality and reliability
you have come to expect in
MICROTEMP thermal fuses
• One shot operation cuts
off electrical power
• 16A/250VAC
• Low Resistance
• Major Appliance
• Fast thermal response
• High Overshoot Temperature Tm
ratings
• Compact Size
• Portable Appliance
• HVAC
• Water Heater
• Hair Care
• Other
7
Z6 MICROTEMP Product Information
Before
Operation
After
Operation
Operating Temperature Summary
Standard Dimensions
Standard Leads
Tf °C
Th °C
Tm °C
083
280
A
Overall Length ± .12” (±3.0mm)*
2.51” (63.8mm)
098
B
Case Lead Length ± .06” (±1.5mm)
1.38” (34.9mm)
104
089
260
C
Case Lead Diameter
0.040” (1.0mm)
117
102
275
Case Lead Material
D
E/F
Tin Plated Copper
121
106
380
Isolated Lead Diameter
0.040” (1.0mm)
144
134
380
Isolated Lead Material
Silver Plated Copper
152
142
380
Case Dimensions, Including Epoxy
.58“ L x .158” D
(14.7mm x 4.0mm)
167
157
380
184
174
380
229
200
380
240
200
* Overall length available up to 5.83” (148mm)
A
E
REF.
D
380
Tf = Functioning open temperature +0/-5 C
Th = Maximum temperature of the thermal fuse, measured
at the case end, at which the thermal fuse can be maintained
for a period of at least168 hours without opening
Tm = Maximum overshoot temperature. Temperature up to
which the open thermal fuse will not change state
0
B
F
C
REF.
Electric Rating
Agency
Resistive
UL
16A/250VAC
CSA
16A/250VAC
VDE
16A/250VAC
CCC
16A/250VAC
8
MICROTEMP Mountings & Configurations
MICROTEMP thermal fuses are available in a wide range of mountings and configurations to meet a variety of application
requirements.
The G Series represents the world standard in thermal fuses. MICROTEMP TCO’s were the first chemical-pellet spring-type
TCO ever developed and continue to be the thermal cutoff of choice for many years.
9
Packaged Thermal Fuses
MICROTEMP thermal fuses also come in a variety of mounted and packaged options. Designed primarily for
HVAC heating applications, the GXAM04 and GXAM06 packages mount a standard thermal fuse on a high
temperature ceramic base.
The popular GXAP packages consist of a thermal fuse epoxy-potted into a plastic insulating mounting case.
The assembly can be supplied with various case materials, shapes and terminations. They can be easily
replaced in the field without disturbing the rest of the circuit.
Packaged TCO Material Specifications
Type
Base Material
Material Rating
Temperature °C
Maximum Tf°C
Temperature °C
GXAP02
PPS Glass Filled
220
192
GXAP04
PBT Glass Filled
120
134
GXAP05
PBT Glass Filled
120
134
GXAP10
PPS Mineral Filled
220
192
GXAP12
PBT Glass Filled
120
134
GXAM04
Ceramic DIN VDE 0335, C221
>250
257
GXAM06
Ceramic DIN VDE 0335, C221
>250
257
GXAM11
PBT
120
134
GXAM06
GXAP04
GXAP02
GXAP10
GXAP05
GXAM04
GXAM11
GXAP12
10
Lead Configurations
Thermal fuses can be furnished with virtually any lead configuration specified for an application. Lead
curls are available to match most screw sizes along with varying lead lengths and lead forms. All types of
terminations, such as quick connects, ring terminals and blade terminals are available at additional cost.
In addition, tape and reel packaging can be specified to meet high volume requirements.
Lead Cutting
Minimum Dimensions - Inches (mm)
A
B
C
0.95 (24.2)
0.22 (5.6)
0.73 (18.6)
11
Tape & Reel Packaging
Dimensions - Inches (mm)
Item
A
B
C
D
E
F
G
G4AA0901TTTC
2.52 (63.8)
1.38 (34.9)
2.062 (52.4)
1.031 (26.2)
2.859 (72.6)
0.200 (5.1)
–
G5AA0901TTTC
2.52 (63.8)
1.38 (34.9)
2.062 (52.4)
1.031 (26.2)
2.859 (72.6)
0.200 (5.1)
1.13 (28.7) Min.
G7FA0900TTTC
3.26 (82.9)
1.38 (34.9)
2.760 (70.1)
1.630 (41.4)
3.600 (91.4)
0.197 (5.0)
1.13 (28.7) Min.
Additional packaging configurations available
12
Product Nomenclature
As shown above, Therm-O-Disc MICROTEMP TCOs follow a consistent product nomenclature that identifies
the basic product type, lead wire size, special features and packaging options. For example, a standard G4
series TCO calibrated to open at 192°C would have a part number G4A00192C.
MICROTEMP TCO Product Markings
XXXXXXXX
MICROTEMP®
PZZZZZ
G Z X XX
GZXXXXRR
TF TTTC
Special customer identification (when required, up to 9 characters)
Registered trademark
Manufacturing plant (P); date code
Primary part number
Secondary part number
Maximum open temperature °C
Underwriters Labs logo
13
Temperature Ratings
MICROTEMP thermal fuses are available in a wide range of opening temperatures, providing designers a high degree
of flexibility. The proper calibration will be affected by application variables such as I2R self heating of the thermal fuse,
heat transfer through insulation and heat dissipation due to heat sinking and air flow. Thermocoupled thermal fuse
samples, that match the physical and electrical characteristics of a functional thermal fuse, are available to help evaluate application specific variables.
G4A
G5A
G6A
G7F
G8A
Z6A
Tf
ºC
Th ºC
Tm ºC
Th ºC
Tm ºC
Th ºC
Tm ºC
Th ºC
Tm ºC
Th ºC
Tm ºC
Th ºC
Tm ºC
72
57
100
57
410
47
100
-
-
47
410
-
-
73
58
100
58
410
48
100
-
-
48
410
-
-
77
62
300
62
410
62
300
62
125
62
410
-
-
84
69
220
69
220
69
220
69
125
69
220
-
-
91
76
300
76
430
76
300
-
-
-
-
-
-
93
78
300
78
410
-
-
78
140
78
410
-
-
98
83
300
83
410
83
300
83
140
83
410
83
280
104
89
200
89
225
89
200
-
-
89
225
89
260
110
95
240
95
225
-
-
95
140
95
225
-
-
117
102
240
102
410
102
240
102
150
102
410
102
275
121
106
300
106
410
106
300
106
150
106
410
106
380
128
113
205
113
235
113
205
113
150
113
235
-
-
134
119
205
119
410
-
-
119
175
-
-
-
-
141
126
205
126
350
-
-
126
175
-
-
-
-
144
129
300
129
410
129
300
129
175
119
410
134
380
152
137
205
137
410
127
205
137
175
-
-
142
380
158
143
240
143
410
-
-
143
200
-
-
-
-
167
152
210
152
410
-
-
152
200
152
410
157
380
172
157
310
157
410
-
-
157
200
-
-
-
-
184
169
240
169
410
169
210
169
200
169
410
174
380
190
175
350
175
410
-
-
175
270
-
-
-
-
192
177
210
177
350
167
210
177
210
177
350
-
-
205
190
310
190
410
-
-
-
-
-
-
-
-
216
200
450
200
410
-
-
-
-
-
-
-
-
229
200
450
200
410
200
375
-
-
200
410
200
380
240
200
450
200
410
200
450
-
-
200
410
200
380
257
220
470
-
-
-
-
-
-
-
-
-
-
Tf = Functioning open temperature +0/-50 C
Th = Maximum temperature of the thermal fuse, measured at the case end, at which the thermal fuse can be maintained for a period of at least168
hours without opening
Tm = Maximum overshoot temperature. Temperature up to which the open thermal fuse will not change state
NOTES:
• It is advised that TCOs are not exposed to continuous operating temperatures in excess of Tf -25°C
• Comparative tracking index (all primary TCOs): 250VAC
• G4, G5, G6, G7 and G8 series TCOs with Tf ≥175°C comply with UL conductive heat aging (CHAT) requirements.
14
Electrical Rating Summary
Electrical Current & Voltage Rating
Series
Agency
Resistive
G4
Inductive
G5
Resistive
G6
Resistive
Z6
Resistive
20A/250VAC
21A/240VAC5
10A/250VAC 8A/250VAC 25A/120VAC
16A/250VAC 16A/250VAC
UL/CSA
15A/120VAC 14A/120VAC 21A/240VAC
5A/24VDC
20A/277VAC
G7
Resistive
5A/250VAC
5A/24VDC
VDE
10A/250VAC 8A/250VAC 20A/250VAC 16A/250VAC 16A/250VAC 5A/250VAC
5A/24VDC
5A/24VDC
CCC
8A/250VAC 20A/250VAC 16A/250VAC 16A/250VAC
10A/250VAC
5A/250VAC
PSE JET 10A/250VAC
15A250VAC 15A/250VAC 16A/250VAC 5A/250VAC
15A/16VDC3
S JET1 30A/16VDC2
Korea
10A/250VAC
16A/250VAC
5A/250VAC
1
For S JET ratings for Japan replace “G” with “S” as first letter of nomenclature
2
Load agency approved for 172C and 240C temperatures only
3
Load agency approved for 152C temperature only
4
Except for 184C rating
5
For CSA only
Inductive
G8
Resistive
S9
Resistive
4.5A/250VAC4 20A/277VAC
4.5A/120VAC4 25A/250VAC
4.5A/250VAC
25A/250VAC
4.5A/250VAC
25A/250VAC
25A/250VAC 15A/250VAC
50A/16VDC2
Direct Current (DC) Applications
The G4,G7 and S9 series MICROTEMP thermal fuses have published electrical ratings for direct current (DC) applications.
Current interruption capacity in DC circuits is highly application sensitive.
Therm-O-Disc recommends thorough testing of DC electrical applications.
MICROTEMP TCO Standard Dimensions
Dimensions - Inches
(Millimeters)
G4, G5, G6, Z6, G8
Series
G7
Series
S9
Series
Standard
Leads
A
B
Overall Length ± .12 (±3.0)
Case Lead Length ± .06 (±1.5)
2.51 (63.8)*
1.38 (34.9)
N/A
N/A
2.51 (64.8)
1.38 (34.9)
Long
Leads
A
B
Overall Length ± .12 (±3.0)
Case Lead Length ± .06 (±1.5)
3.26 (82.9)
1.38 (34.9)
3.26 (82.9)
1.38 (34.9)
3.26 (82.9)
1.38 (34.9)
Lead Material
& Diameter
C
D
Case Lead
Tin Plated Copper
Isolated Lead Silver Plated Copper
0.040 (1.0)
0.040 (1.0)
0.023 (.57)
0.023 (.57)
0.057 (1.4)
0.057 (1.4)
Case
Dimensions
E
F
Case Length (Reference)
Case Diameter (Reference)
0.58 (14.7)
0.158 (4.0)
0.38 (9.6)
0.118 (3.0)
0.58 (14.7)
0.158 (4.0)
*Overall length available up to 5.83” (148mm)
A
E
REF.
D
B
F
REF.
15
C
Application of Thermal Fuses
MICROTEMP® thermal cutoffs, available in a variety of standard and custom configurations, provide reliable one-shot,
over-temperature protection in a wide range of applications. Performance can be affected by installation method and
location of the thermal cutoff. Both application and installation is important in the overall performance of the product,
and thorough testing is necessary for both AC and DC applications. The following guidelines will answer most questions
concerning these two subjects.
General Considerations
Location
Sufficient time and effort must be used to determine the proper and most desirable location for a thermal fuse. The
employment of infrared thermography, or a sufficient number of thermocouples to identify the highest temperature
areas in the application during normal operation and fault conditions, should be considered. The location that provides the largest differential between these two conditions is generally most desirable.
Calibration Temperature
It is necessary to select a thermal fuse rating above the maximum temperature experienced by the thermal fuse during
normal operation, including expected short-term temperature overshoots. The temperatures experienced by the
thermal fuse during normal operation will determine the life expectancy of the thermal fuse. If the thermal fuse rating
is too close to the temperature experienced during normal operation (including overshoot temperature after opening
of a thermostat, etc.), the probability of a nuisance trip increases. Nuisance trips are caused by pellet shrinkage due to
repeated operation at temperatures near but below calibration temperature, or excessive thermal gradients across the
case of the TCO and its leads (see Thermal Gradients). The design engineer must make the decision between response
and life of the TCO based on product requirements. It is important to remember that temperatures experienced in
actual application will vary from unit to unit.
Thermal Gradients
Ideal thermal fuse placement subjects the entire thermal fuse case, leads, epoxy seal and internal components to a
uniform temperature environment.
Care should be exercised in the placement of the thermal fuse to minimize thermal gradients across the thermal fuse
body. In certain applications, the thermal fuse can be mounted in a position where heat is conducted to the body of the
thermal fuse through one of the leads, resulting in thermal gradients across the thermal fuse. Over time, the thermal
fuse life can be reduced by thermal gradients if the isolated (epoxy) lead is at a consistently lower temperature than the
16
case lead. Long term testing is recommended in determining whether these conditions exist in the application.
To minimize the effects of thermal gradients and the temperature increase of the TCO body from this heat flow, attach
the isolated (epoxy) lead, rather than the case lead, to the heat source. Thermocoupled fuses can be supplied with the
thermocouples on both ends to facilitate gradient evaluations.
Temperature Limits
The temperatures experienced during normal operation, including expected temperature overshoots, will determine
the life expectancy of the thermal fuse. Nuisance trips can results if the thermal fuse rating is too close to the temperatures experienced during normal operation. Thermal fuses of any temperature rating should not be subjected to
continuous normal temperatures in excess of 200°C. Additionally, overshoot temperatures after the opening of the
thermal fuse should be minimized to avoid possible dielectric breakdown and reconduction of the thermal fuse.
Test Procedure
Application of Thermal Fuses
A thermal measurement procedure that utilizes a thermocoupled thermal fuse can assist in determining the appropriate calibration temperature and design location of MICROTEMP™ thermal fuses. The thermocoupled thermal fuse
matches the electrical characteristics of the thermal fuse but does not have thermally responsive parts. The thermocoupled thermal fuse is supplied with a thermocouple attached to the case of the thermal fuse (see below). Thermocoupled thermal fuses can be supplied with Type J, Type T, or Type K thermocouples.
Install the thermocoupled thermal fuse in the electrical circuit that is to open in the event of a fault condition. Position
it in the area that has been selected to be protected within the product based on prior determinations of the maximum
permissible temperatures to be allowed. The thermocouple thermal fuse should be installed using the same mounting and electrical connection that will be used for functional thermal fuses in production. Connect the thermocouple
leads to a digital temperature measuring device to record temperatures. The product to be protected can now be
operated, and the normal operating temperature monitored. Note that the thermocouple thermal fuse is not a functional thermal fuse and therefore will not open the circuit in the test setup.
The figure above illustrates a typical installation of a thermocouple thermal fuse. Note that the body of the thermocouple thermal fuse is at the same potential as the connecting circuit; therefore, it must be electrically isolated from
the surface against which the fuse is mounted. Also note that the thermocouple wire is at the same potential as the
connecting circuit.
17
Installation, Processing and Handling of Thermal Fuses
To avoid a false reading of the unit under test, thermocouple wires must not make contact with each other except at
the temperature sensing junction.
Ensure that the thermocouple wire insulation will provide isolation against short circuiting and shock hazards.
The terminal of the temperature measuring instrument, to which the thermocouple is attached, will be at the same
potential as the connecting circuit wire. This instrument must be electrically isolated and considerable caution must
be exercised in its use, since one of the thermocouple terminals is frequently grounded to the instrument chassis.
Before using measuring equipment powered directly from standard line voltages, check operation manuals. Be sure
line voltages impressed on the thermocouple wires by the thermocoupled thermal fuse will not cause damage to the
instrument.
Application testing is necessary to select a properly rated thermal fuse. Test factors include, but are not limited to, the
heating effect of the current through the fuse, adjoining terminals and leads, heating or cooling effect of the terminals
and external leads, rate of temperature rise, air flow, shock, vibration and other environmental and operating conditions unique to the application. The closer the actual operating and ambient conditions can be simulated during test,
the more valid the test results will be.
The product and application being tested will determine the number of cycles that must be run to determine the maximum ‘normal’ operating temperature. ‘Overshoot’ temperatures should be included in the determination of the maximum ‘normal’ operating temperature. The overshoot temperature is often considerably higher than the temperature
reached at the moment the thermal fuse opens. The conclusion of these tests will provide the maximum ‘normal’ operating temperature at the thermal fuse (at maximum anticipated voltage, ambient temperature, etc.). The overshoot
temperature seen by the thermal fuse after the thermal fuse opens in the application must also be carefully examined
and compared to overshoot temperature Tm.
Manufacturing tolerances and variations should be carefully considered, and a sufficient number of units evaluated, to
provide statistical basis on which to determine the operating overshoot temperatures.
After obtaining the above information, test the product under fault conditions and monitor to determine that desired
fault condition temperatures are not exceeded.
Where there are a variety of fault conditions, (eg, short-circuited thermostats and transform secondaries, locked motor rotors and solenoids, high ambient temperatures, restricted or blocked airflow, etc), consideration should be given
to multiple fault conditions which could occur simultaneously during the lifetime of the product, and to faults which
may cause localized overheating in areas away from the thermal fuse.
When the fault conditions have been set up, note the temperature of the thermocoupled fuses when the maximum
desired temperature limit is reached. At this point the circuit is manually interrupted. This test should be run several
times, in several different units. In some applications, it will not be possible to ‘save’ the tested item from damage, but
only prevent the product from creating an external fire or electrical hazard. Damaged products should not be retest18
ed, since the results may not be the same as with undamaged units. The thermal fuse Tf selected should be equal to or
less than the temperature recorded at the thermal fuse at the time the maximum desired temperature is reached.
Excessive overshoot temperature after the opening of the thermal fuse may cause dielectric breakdown of the thermal fuse and allow reconduction to occur. Functional thermal fuses should be tested to verify proper operation of the
thermal fuses in the application (see MICROTEMP Thermal Fuse Operating Temperature Summary and Electrical Rating
Summary on page 14).
Substitute actual thermal fuses in a sufficient number of finished products and re-run the tests to obtain statistical verification of the results. For multiple thermal fuse applications, test functional thermal fuses under fault conditions so
that the product overheats and each thermal fuse is independently called upon to interrupt the flow of current. Each
thermal fuse should open the circuit independently of any other over-temperature limit controls, with product damage
not exceeding an acceptable level. This test should be run using the maximum voltage and current the thermal fuse
will be expected to interrupt and hold open.
The performance of a MICROTEMP thermal cutoff can be affected by installation methods such as soldering, welding,
splicing, lead bending, insulation, clamping and mounting. Certain precautions should be taken during installation to
ensure that the MICROTEMP thermal cutoff is not damaged, which may cause it to not operate in its intended manner. Likewise, care should be taken during installation to ensure that the TCO in every unit experiences the expected
temperature range environment previously determined during the calibration temperature selection. The following
guidelines should be used to minimize undesirable conditions that can result from improper installation practices.
Soldering Leads
Thermal fuse leads should be heat-sinked during the soldering operation (see below). If excessive heat is conducted by
the leads into the thermal fuse, it can shorten the life of the thermal fuse. In addition, excessive lead temperatures can
damage the epoxy and possibly result in the thermal fuse failing to open. More heat-sinking is necessary for thermal
fuses with lower Tf temperatures.
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Test samples should be x-rayed before and after the soldering operation. The size of the pellet should be measured to verify that no shrinkage has occurred during the soldering operation. The epoxy seal should retain its size and shape and not
discolor. If the pellet or the epoxy have changed size as a result of the soldering operation more heat sinking is required.
Welding Leads
The thermal fuse leads may also need to be heat sinked during a welding operation. The same precautions and tests described in the soldering section should be followed for welding leads.
Heat Sink Here
Weld Points
To avoid damaging or welding internal parts, care should be taken that none of the welding current is conducted through
the thermal fuse. A welding current of hundreds of amperes can weld the internal parts together, resulting in the thermal
fuse failing to open.
Thermal fuse leads must be supported during the weld operation to prevent damaging the thermal fuse epoxy seal.
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Splices and Terminations
Insecure splices and terminations may produce high resistance junctions which can cause self-heating (I2R) as power
is dissipated across these junctions during product operation. Heat from these areas can flow down the thermal fuse
leads and increase the temperature of the thermal fuse. Nuisance openings of the thermal fuse or degradation of the
epoxy seal can occur as a result of the heat generated by high resistance junctions. The splice or termination junction
may initially measure low resistance, but can change to a much higher resistance after several temperature cycles. It is
generally better to splice thermal fuse leads to stranded lead wires rather than solid wires as the stranded wire may be
crimped tighter and maintain better electrical contact during temperature cycling.
The temperature capabilities of the splice and/or termination should be considered. For example, solder back-up
should be considered for splices of solid wires, and terminations in applications cycled at temperatures exceeding
150°C.
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Bending Leads
When configuring leads, special care must be exercised in supporting the leads at each end near the
body of the thermal fuse so that the case will not be
distorted or the epoxy will not be cracked or broken.
At least 0.125” (3mm) should be maintained between the epoxy seal and any lead bends.
The thermal fuse may fail to open the electrical circuit under certain conditions. Distortion of the case, breaking or
cracking the seal, exposing the epoxy seal to cleaning solvents, compression of the leads and current surges that
exceed the operating specifications of the thermal fuse may cause the thermal fuse not to open. In addition, pellet
shrinkage due to thermal aging under some circumstances may also result in failure to open. Finally, a very low rate
of temperature rise may produce conditions that may also result in failure to open. Care must be taken to avoid any
mishandling or misapplication of the thermal fuse.
Although thermal fuses are highly reliable devices, a thermal fuse may fail to open for one or more of the reasons set
forth above. These conditions must be taken into account by the product design engineer in determining the level of
reliability needed for the application. If failure of the thermal fuse to open could result in personal injury or property
damage, the product design engineer may want to consider using one or more redundant thermal fuses of different
ratings to achieve the desired level of reliability. A number of consumer product design engineers have incorporated
redundant thermal fuses of different ratings in their designs for this reason.
Samples and Quotations
Thermocoupled thermal fuse samples and thermal fuse samples are readily available for determining the correct
response and desired performance in an application. Contact your nearest Therm-O-Disc sales office for more information.
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Definition of Terms
Maximum Open Temperature or Rated Functioning Temperature (Tf or TF)
The maximum temperature at which the thermal fuse changes its state of conductivity to open circuit with detection
current as the only load. The rated functioning temperature is measured during a temperature rise of approximately
0.5°C per minute.
Holding Temperature (Th or TH)
The maximum temperature of the thermal fuse, measured at the case end of the thermal fuse, at which the thermal
fuse can be maintained for a period of 168 hours without opening. NOTE: It is advised that thermal fuses are not exposed to continuous operating temperatures in excess of Tf-25°C.
Maximum Overshoot Temperature or Maximum Temperature Limit (Tm or TM): The maximum temperature at
which the thermal fuse, having changed its state of conductivity, can be maintained at twice rated voltage for a specified period of time, during which its mechanical and electrical properties will not be impaired.
Rated Voltage
The maximum voltage that can be applied to the circuit in which the thermal fuse is used.
Rated Current
The maximum current that the thermal fuse is rated to interrupt at the rated voltage.
Thermal Fuse, Thermal Cutoff, Thermal Link, and TCO
All are synonymous and may be used interchangeably throughout the catalog.
Agency Recognition
MICROTEMP™ thermal fuses are recognized by the following major agencies
MICROTEMP™ thermal fuses are recognized by the major approval agencies throughout the world for AC circuit applications. These agency electrical ratings can be used as a guideline when evaluating specific thermal fuse applications.
However, the electrical and thermal conditions to which the thermal fuse may be exposed in an application may differ
significantly from agency test conditions. Accordingly, customers should not rely solely on agency ratings but rather
must perform adequate testing on the particular application to confirm that the thermal fuse selected is appropriate
for that application and will operate as intended.
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Important Notice
Users must determine the suitability of the thermal fuse for their application, including the level of reliability required,
and are solely responsible for the function of the end-use product.
These thermal fuses contain exposed electrical components and are not intended to withstand exposure to water or
other environmental contaminants which can compromise insulating components. Such exposure may result in insulation breakdown and accompanying localized electrical heating.
A thermal fuse may remain permanently closed or open as a result of exposure to excessive mechanical, electrical,
thermal or environmental conditions or at normal end-of-life. Failure to control the operation could result in personal
injury or property damage. The user should incorporate supplemental system control features to achieve the desired
level of reliability and safety. For example, backup temperature limiting devices have been incorporated in a number of
applications for this reason.
Important Notice
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The scope of the technical and application information included in this article
is necessarily limited. Operating environments and conditions can materially
affect the operating results of Therm-O-Disc™ products.
Users must determine the suitability of any Therm-O-Disc component for their
specific application, including the level of reliability required, and are solely
responsible for the function of the end-use product. It is important to review
the Application Notes which can be found at Emerson.com/thermodisc
Emerson.com
2015TOD-48 (7/17) Emerson and Microtemp are trademarks of Emerson Electric Co. or one of its affiliated companies. ©2017 Emerson Electric Co. All rights reserved.
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