Photovoltaic System Grounding - Solar America Board for Codes

Photovoltaic System Grounding - Solar America Board for Codes
Photovoltaic System
Grounding
Prepared by:
John C. Wiles, Jr.
Southwest Technology Development Institute
College of Engineering
New Mexico State University
October 2012
Solar America Board for Codes and Standards
www.solarabcs.org
Disclaimer
This report was prepared as an account of work sponsored by an agency of the United
States government. Neither the United States government nor any agency thereof, nor any of
their employees, makes any warranty, express or implied, or assumes any legal liability or
responsibility for the accuracy, completeness, or usefulness of any information, apparatus,
product, or process disclosed, or represents that its use would not infringe privately owned
rights. Reference herein to any specific commercial product, process, or service by trade
name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its
endorsement, recommendation, or favoring by the United States government or any agency
thereof. The views and opinions of authors expressed herein do not necessarily state or
reflect those of the United States government or any agency thereof.
Download a copy of the report:
www.solarabcs.org/systemgrounding
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Photovoltaic System Grounding
Executive Summary
Photovoltaic (PV) power systems are capable of producing hazardous voltages and
currents for decades. To ensure the safety of the public for these extended periods of
time, PV systems must be properly designed and installed using the highest standards of
workmanship.
This paper addresses the requirements for PV system grounding contained in the
U.S. National Electrical Code® (NEC®) published by the National Fire Protection Association
(NFPA). The NEC and the NEC Handbook are copyrighted by NFPA and the term NFPA-70
is a trademark owned by the NFPA (NFPA, 2011). It does not address in any detail the
various American or European standards that are used to design and produce electrical
equipment, nor does it cover the many electrical codes used in other countries.
In the United States, the NEC establishes the legal installation requirements for PV
systems, and these requirements are somewhat complex. The NEC requires that all
exposed or accessible PV equipment and circuits be properly connected to earth
(grounded) using specified methods and equipment. Source circuits in PV systems may
be grounded or ungrounded as explained in this paper.
As installed PV systems age, grounding issues emerge that impact system safety. These
issues include deteriorating electrical connections, inadequate grounding device design
and installation, and the effects of non-code compliant system installations. Many of the
required ground-fault protection devices in use today do not detect all possible ground
faults, and, in some cases, fires and equipment damage have resulted from undetected
ground faults. Both the NEC and Underwriters Laboratories (UL) hardware safety
standards for the certification/listing of equipment are being revised to address many of
these issues.
“As installed PV systems
age, grounding issues emerge
that impact system safety.
Solar America Board for Codes and Standards Report
”
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Table of Contents
DISCLAIMER.......................................................................................................................... 2
EXECUTIVE SUMMARY......................................................................................................... 3
AUTHOR BIOGRAPHY.......................................................................................................... 4
SOLAR AMERICA BOARD FOR CODES AND STANDARDS................................................. 5
ACKNOWLEDGEMENTS........................................................................................................ 5
INTRODUCTION.................................................................................................................... 6
DEFINITIONS......................................................................................................................... 7
NATIONAL ELECTRICAL CODE REQUIREMENTS....................................................... 9
SYSTEM GROUNDING........................................................................................................... 13
MICROINVERTERS AND AC PV MODULES—DIFFERENT REQUIREMENTS...................... 18
GROUND FAULTS AND GROUND-FAULT PROTECTION DEVICES...................................... 20
THE CONNECTION TO EARTH...................................................................................... 25
UTILITY VS NEC REQUIREMENTS....................................................................................... 26
ACRONYMS........................................................................................................................... 27
REFERENCES......................................................................................................................... 28
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Photovoltaic System Grounding
Author Biography
John C. Wiles, Jr.
John Wiles is a senior research engineer at the Southwest Technology Development
Institute, College of Engineering at New Mexico State University in Las Cruces, New
Mexico.
He bought his first copy of the National Electrical Code® (NEC®) in 1961 and rewired his
parent’s home to the latest NEC requirements. After graduating from West Virginia
University with a bachelor’s degree in electrical engineering, he became an officer in the
United States Air Force. He earned a master’s degree in electrical engineering and spent
24 years in the Air Force working on automatic control systems and tactical weapons
systems. He has taught college courses at West Virginia University and the U.S. Naval
Academy. He installed his first photovoltaic (PV) power system in 1984 and has been
involved in the design, installation, inspection, and testing of PV systems for 28 years.
He is a member of the Underwriters Laboratories Standards Technical Panels for PV
modules, inverters, racks, and direct current PV arc fault interrupters. He is secretary
of the PV Industry Forum, an organization that develops and submits PV proposals for
improving the NEC. He writes articles on PV and the NEC for the International Association
of Electrical Inspectors News and gives PV/NEC presentations throughout the country to PV
designers, installers, and electrical inspectors. He lives with his wife Patti, two dogs, and
a cat in an energy-efficient home with a 5-kilowatt utility-interactive PV system and full
house battery backup.
Solar America Board For Codes And Standards
The Solar America Board for Codes and Standards (Solar ABCs) is a collaborative effort
among experts to formally gather and prioritize input from the broad spectrum of
solar photovoltaic stakeholders including policy makers, manufacturers, installers, and
consumers resulting in coordinated recommendations to codes and standards making
bodies for existing and new solar technologies. The U.S. Department of Energy funds
Solar ABCs as part of its commitment to facilitate widespread adoption of safe, reliable,
and cost-effective solar technologies.
For more information, visit the Solar ABCs website:
www.SolarABCs.org
Acknowledgements
The author thanks the following individuals for their critical review and comments on
this paper: Greg Ball, BEW Engineering; Ward Bower; Bill Brooks, Brooks Engineering;
Marv Dargatz, Solar Edge; Jim Eichner, Schneider Electric; Marvin Hamon, Hamon
Engineering; Pete Jackson, City of Bakersfield; Sarah Kurtz, National Renewable Energy
Laboratory; Rhonda Parkhurst, City of Palo Alto; Dan Rice, Conergy; Michael Sheehan,
Interstate Renewable Energy Council; Don Warfield, Ameresco; and Brian Wiley.
Solar America Board for Codes and Standards Report
5
Introduction
Proper grounding of a photovoltaic (PV) power system is critical to ensuring the safety
of the public during the installation’s decades-long life. Although all components of a
PV system may not be fully functional for this period of time, the basic PV module can
produce potentially dangerous currents and voltages for the life of the system. Effective,
code-compliant, properly maintained grounding helps ensure the overall safety of the
system, even if it is no longer producing usable power.
More than a century ago, the United States and most of the Americas elected to use
grounded electrical systems, in which one of the circuit conductors is connected to
the earth. The rest of the world (ROW) for the most part chose to employ ungrounded
electrical systems, in which none of the circuit conductors are connected to earth. The
normally non-energized metal surfaces of electrical equipment are, however, required to
be connected to earth in the ROW.
This paper addresses the requirements for PV system grounding contained in the U.S.
National Electrical Code® (NEC®) published by the National Fire Protection Association
(NFPA). The NEC and the NEC Handbook are copyrighted by NFPA and the term NFPA-70
is a trademark owned by the NFPA (NFPA, 2011). It does not address in any detail the
various American or European standards that are used to design and produce electrical
equipment, nor does it cover the many electrical codes used in other countries.
The NEC requires that the authority having jurisdiction (AHJ or electrical inspector)
examine all electrical equipment for safety. Electrical equipment has generally been
standardized from an input/output interconnectivity perspective through the Nationally
Recognized Testing Laboratory (NRTL) evaluation, certification, and listing process.
The NRTL certification ensures the equipment meets all applicable safety standards.
Underwriters Laboratories (UL) coordinates the development of many of the safety
standards that apply to PV systems (e.g. UL 1703; UL 1741). The U.S. Occupational
Safety and Health Administration (OSHA) authorizes NRTLs to test and certify/list
electrical equipment to various standards. The American National Standards Institute
(ANSI) authorizes the development of standards by UL and others. Although the NEC does
not specifically require all equipment to be certified/listed, many local jurisdictions and
many AHJs establish requirements that all equipment be certified/listed because they feel
unqualified to examine uncertified/unlisted equipment for safety as the NEC requires.
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Photovoltaic System Grounding
Definitions
Before discussing the subject of grounding, the term “grounding” requires definition.
There are two types of grounding in electrical and PV systems—equipment grounding
and system grounding.
Equipment Grounding
Equipment grounding is known in the ROW as safety grounding or protective earthing.
The equipment grounding system in the United States effectively bonds (electrically
connects) all exposed non-current carrying metal parts of the electrical system together
and eventually connects these metal parts to the earth (ground).
Metal enclosures containing electrical conductors or other electrical components may
become energized as a result of insulation or mechanical failures. Energized metal
surfaces, including the metal frames of PV modules, can present electrical shock and fire
hazards.
By properly bonding exposed metal surfaces together and to the earth, the potential
difference between earth and the conductive surface during a fault condition is reduced
to near zero, reducing electric shock potential. The proper bonding to earth by the
equipment grounding system is essential, because most of the environment (including
most conductive surfaces and the earth itself) is at earth potential. The conductors used
to bond the various exposed metal surfaces together are known as equipment grounding
conductors (EGCs).
In a conventional electrical power system (utility, generator, or battery sourced), the
equipment grounding system provides a path for ground-fault currents to return to
the energy source. By allowing these currents to return to the source in an expeditious
manner, properly positioned overcurrent protective devices (OCPDs, typically fuses or
circuit breakers) are allowed to function, removing the source of the fault currents. The
National Electrical Code Requirements part of this paper describes equipment grounding
procedures used in the United States.
System Grounding
In system grounding, one of the circuit (current-carrying) conductors is bonded
(connected) to the equipment grounding system and also to earth. This is known as
functional grounding in the ROW. The circuit conductor that has been connected to the
equipment grounding system and to earth is known as the grounded conductor. The
connection between the grounded conductor and the equipment grounding system is
known as the system bonding jumper in the NEC. Only one system bonding jumper is
allowed in each separate electrical system in which the system grounded conductor is
isolated from the grounded conductors of the source or other systems. See NEC Article
100 for definitions of bonding, bonding jumpers, and system bonding jumpers. Section
250.28 expands on the proper installation of system and main bonding jumpers.
The system ground connection, made by a system bonding jumper, is the path that
allows fault currents to return to the source. If the equipment grounding system and the
system bonding jumper have sufficiently low impedance (i.e., proper conductor size and
good connections), currents that originate from an ungrounded conductor faulting to a
grounded surface or the equipment grounding system will be sufficient to trip the OCPD
supplying that circuit. PV systems, as noted below, may not perform the same under fault
conditions as other types of electrical systems.
Solar America Board for Codes and Standards Report
7
Earth Connection
The metallic device used to make contact with the earth is the grounding electrode. The
conductor that connects the central grounding point (where the equipment grounding
system is connected to the grounded circuit conductor on grounded systems) and a
grounding electrode that is in contact with the earth is known as the grounding electrode
conductor (GEC).
Solidly Grounded
The NEC, in Article 100, defines solidly grounded as being connected to ground without
inserting any resistor or impedance device in the circuit. This definition does allow for
the use of fuses, circuit breakers, and mechanical relay contacts in certain grounding
circuits, but typically precludes the use of solid-state devices such as transistors, silicon
controlled rectifiers, and bi-junction field effect transistors. This is because these devices
would likely be considered impedance devices. Although language in NEC 690.41 allows
equivalent grounding methods in listed equipment, no listing agencies certify products in
a configuration other than solidly grounded.
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Photovoltaic System Grounding
National Electrical Code Requirements
Article 250 in the NEC covers most of the grounding requirements for any electrical
system. This article is more than 30 pages long, and it is not possible to elaborate on or
restate all of these requirements here. Article 690 has other requirements for grounding
PV systems, and many parts of the NEC are revised every three years, including Articles
250 and 690. To further complicate matters, different editions of the NEC are in use in
different jurisdictions, from the most recent edition (2011) back to the 2002 NEC and
earlier versions.
For the entire text of the NEC and additional explanatory material provided by NFPA,
go to the NEC Handbook (NFPA, 2011). The Soares book on grounding published by the
International Association of Electrical Inspectors (IAEI, 2011) is also an excellent
resource on NEC requirements for grounding. For an individual installation, refer to
the NEC edition in use in that jurisdiction to establish specific project requirements.
This paper will highlight a few of the NEC requirements for both equipment and
system grounding that apply to PV systems and that are sometimes overlooked in PV
installations. Unless specifically mentioned, all references to the NEC in this paper will be
to the 2011 edition.
Good Workmanship Is an NEC Requirement
In Section 110.12, the NEC states that good workmanship is required on all electrical
installations. The phrase “in a neat and workmanlike manner” is not defined in the NEC,
but has been addressed in other material such as the National Electrical Contractors
Association Standard 1. The electrical trades teach good workmanship through on-thejob-training and in more formal courses taught in International Brotherhood of Electrical
Workers schools. The workmanship associated with the installation of PV systems is
coming under increased scrutiny by inspectors throughout the country.
Unfortunately, this scrutiny has found some less than quality PV installations. Fires have
originated because metal conduits have not been installed properly and conductors have
not been installed properly in the conduits. Conductors have had insulation damaged
during installation, and in instances when proper grounding methods or hardware was
not used, ground faults have occurred that led to fires. When the electrical system is
not installed properly and is not in compliance with NEC requirements, the safety of the
system becomes questionable, either at the time of the installation or at some future date
as the system ages and deteriorates.
Equipment Grounding
Section 690.43 of the NEC requires that PV systems have equipment grounding systems
when there are any exposed metal or conductive surfaces that may become energized.
This requirement applies to PV systems operating at any voltage, including small
standalone 12-volt PV systems and even a 6-volt, PV-powered water pump on a solar hot
water system. The exposed metal surfaces include PV module frames, metal mounting
racks, metal conduits, and enclosures for combiners, disconnects, inverters, and charge
controllers as well as other electrically conductive parts. Exposed conductors with failing
insulation as a result of a 40- or 50-year-old less-than-optimal installation could even
energize a metal roof under a PV array.
The emergence of PV modules with nonmetallic frames may simplify the grounding of PV
modules. The 690.43 requirement is a reiteration of the basic requirement for equipment
grounding found in Article 250, Part VI. For a complete assessment of the state of module
grounding according to the NEC and various UL Standards requirements, see the Solar
ABCs report Photovoltaic Module Grounding (Ball, Zgonena, & Flueckiger, 2012).
Solar America Board for Codes and Standards Report
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In many electrical power systems, the NEC allows the use of a metallic raceway such
as EMT (electrical metallic tubing) or RMC (rigid metal conduit) to be used as the EGC.
However, many PV installers and PV systems integrators choose to use a separate
EGC, which would be required if a nonmetallic raceway was used. The use of a
distinct conductor for the EGC is generally a good idea, because it does not rely on the
mechanical/electrical joints associated with metal conduit systems. Nonmetallic wiring
methods include nonmetallic rigid conduit (PVC) and nonmetallic sheathed cables (types
NM and TC).
Equipment Grounding—Finer Points
PV Direct Current Circuits. Sections 690.43, 690.45, and 690.46 cover NEC requirements
specific to the equipment grounding of PV source and PV output circuits. The basic
requirement refers to NEC section 250.110 and is outlined in the Equipment Grounding
section above. The equipment in direct current (DC) portions of the PV system may
be grounded using conductors as outlined above with appropriate connections to each
metal surface. In general, when a copper wire is connected to a metal surface to be
grounded, some sort of certified/listed grounding device must be used. This equipment
grounding connection may be a listed ground bar kit, a standard electrical lug suitable
for the application or a terminal or lead on the equipment, or a specific grounding device
certified/listed for the application. UL is developing a new PV racking standard 2703 (UL,
2010) that will establish the requirements for certifying/listing PV racking systems as
EGCs.
As in other electrical systems, the rating of the OCPD protecting a DC PV circuit is used
to determine the size of the required EGC for that circuit (NEC Table 250.122). The EGC
will typically have low enough resistance or impedance to allow timely activation of the
OCPD for that circuit when a ground-fault current flows through it.
Although this system works for AC systems supplied by stable power supplies with high
available fault currents, it may not be as effective for PV DC circuits. PV DC circuits have
limited available fault current that is a function of the incident sunlight energizing the
supply source and the configuration of the system (number of parallel circuits feeding
the faulted circuit). The available fault current may not be enough to enable the OCPD
to function under some conditions, such as during cloudy weather or in the early
morning or late afternoon. To address this issue, PV systems have ground-fault detection
requirements that account for the potentially low fault current. These requirements are
described in the Ground Faults and Ground-Fault Protection Devices part of this paper.
The current-limited nature of PV modules allows some DC circuits to be installed without
an OCPD if there isn’t sufficient fault current to damage the circuit conductors or the PV
modules themselves. When an OCPD is not required, the rated short circuit current (Isc)
for the DC PV source or output circuit is used in NEC Table 250.122 as a pseudo OCPD
and an EGC size is specified by the value of that Isc. The exception to requiring OCPD
on PV source circuits is usually applied to smaller residential PV systems with only one
PV source circuit or two strings of parallel-connected source circuits. The exception also
extends to PV output circuits, however, such as one or two parallel connected and equally
sized combiner box output circuits.
Inverters and Other Electronic Devices. UL Standard 1741 (UL, 2010) requires that
inverters have terminals, leads, or other provisions to accept EGCs for all inputs and
outputs. For a typical PV utility-interactive inverter, there should be an equipment
grounding terminal for each DC input as well as for the AC output. These terminals
may be on a single grounding busbar or may be physically separated but electrically
connected. These terminals will generally be electrically connected/bonded to the
metallic enclosure of the inverter. Charge controllers for stand-alone or multi-mode
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Photovoltaic System Grounding
systems and PV combiners will have similar provisions for EGCs. Some microinverters
and AC PV modules as well as some DC-to-DC converters may have external terminals on
the enclosure to connect the EGCs required by the NEC and UL 1741.
Conduit Bonding. When DC PV source and DC PV output circuits are installed in metal
conduits, the metal conduits must have grounding/bonding bushings if the enclosures
they connect to have concentric knockouts and if the circuits operate at more than 250
volts (NEC 250.97). Many PV systems using “string” inverters have a maximum system
voltage of more than 250 volts, and these same systems may use combiners, disconnect
switches, and inverters that have concentric knockouts. If the required grounding/bonding
bushings are not installed, electrical grounding of the conduit through the fittings may be
lost over time. The mechanical connections between the conduit and the enclosures can
loosen due to thermal and environment stresses, for example, posing a safety hazard if
ground faults occur in the conduits.
Enclosure Grounding. Commonly available disconnect switches and equipment
enclosures are listed devices, and are furnished with instructions and labels indicating
how they are to be connected to the grounding system. In section 110.3(B), the NEC
requires that these instructions (sometimes discarded by the electrician and the PV
installer) and product labels be followed. The metal of these enclosures is frequently
not sufficiently thick to hold thread cutting screws, and ordinary sheet metal screws are
no longer allowed for making grounding connections (250.8). Most of these enclosures
have a label that indicates that a specific grounding bar kit be used to make equipment
grounding connections to the enclosure. Other enclosures have a welded, threaded boss
to accept EGC connections.
In some cases, these instructions and labels are ignored and a lug is attached directly
to the enclosure surface, sometimes without removing any insulating finish (as
250.12 requires). In other cases, a sheet metal or green thread cutting screw is used
for grounding the box in PV applications. Neither method complies with the NEC
requirements as noted in the instructions and/or on the labels for these products
(110.3[B], 250.8).
Aluminum Grounding. Although steel has been used for many decades for electrical
enclosures, many new products from the PV Industry have aluminum enclosures.
Aluminum rapidly oxidizes to an insulating surface when exposed to air. If the
manufacturer has not provided a specific terminal, lead, or lug for equipment grounding,
surface preparation and oxidation protection is necessary to achieve acceptable
equipment grounding. This also applies to aluminum framed PV modules.
Not Continuous, but Green. EGCs are not required to be continuous or to be irreversibly
spliced. They may be insulated or bare depending on the installed location. When
insulated, an EGC should have insulation marked or colored green or green with yellow
stripes (200.119). For long-term PV system durability, colored insulations (red, white,
green, etc.) should be used only when absolutely necessary, because UV radiation may
result in a shortened life or fading colors. Most AHJs will allow exposed black insulated
EGC to be marked with a green marking if the installation requires an insulated EGC, as
some do over architectural metal roofs. A proposal has been submitted for the 2014 NEC
to permit this marking.
Routing for Performance and Safety. The EGCs for the DC portions of the circuit should
be routed with the circuit conductors as required by 300.3(B). Although 250.134(B) EX
2 permits EGCs on DC circuits to be run separately from the DC circuit conductors, this
may not be a good practice in PV systems.
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Overcurrent devices are tested and evaluated with a minimum of circuit inductance.
As the inductance in a circuit increases (increasing circuit time constant), however,
overcurrent devices have an increasingly difficult time clearing the fault current and
extinguishing the DC arc. The sizing requirement for overcurrent devices in PV DC
source and DC output circuits is 1.56 times the Isc. Low fault currents during periods
of low irradiance may not activate OCPD under fault conditions. The DC circuits should
be designed with every possible method applied to ensure the timely activation of
overcurrent devices under fault conditions. Keeping the fault circuit inductance low keeps
the time constant low and facilitates the operation of the overcurrent devices.
During the operation of a single-phase inverter in the stand-alone inverting mode (either
an off-grid system or a utility-interactive system with battery backup), the DC current
between the batteries and the inverter may have 120 hertz (Hz) ripple currents that are in
excess of the average DC currents. These ripple currents interact with ferrous enclosures
in the same manner as AC currents, indicating that the EGCs should be routed with the
DC circuit conductors. The allowance in the NEC (250.134[B]EX2) that the EGCs in DC
systems may be run separately from the circuit conductors should not be followed in PV
systems with battery backups.
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Photovoltaic System Grounding
SYSTEM GROUNDING
Before the 2005 edition, the NEC required that all PV systems have one of the DC circuit
conductors grounded whenever the maximum system voltage was more than 50 volts
(690.41). Section 690.35, which allows PV systems meeting certain requirements to
have ungrounded PV circuits, was added to the 2005 NEC. This change permitted the
use of utility-interactive inverters without isolation between the DC input circuits and the
AC outputs—called non-isolated inverters in the ROW—for cost savings and increased
efficiency. Most PV systems in the United States use isolated inverters with internal
transformers (isolated inverters in the ROW) as of 2012, but non-isolated inverters and
ungrounded PV systems are being certified/listed and are making market inroads.
Note that ungrounded arrays can also be used with isolated inverters. In all cases, an
ungrounded array must be provided with equivalent protection for ground faults, as
required by NEC 690.35.
Grounded Systems
A PV system is defined as a grounded system when one of the DC conductors (either
positive or negative) is connected to the grounding system, which in turn is connected to
the earth. The conductor that is grounded usually depends on the PV module technology.
Most modules can be used with a negative grounded conductor or even in an ungrounded
system, but a few PV module technologies require the positive conductor to be connected
to earth. Note that Article 690 uses the terms “system” [690.35] and “array” [690.5]
in the context of grounded or ungrounded PV DC circuits. The term “system” is more
appropriate both for consistency with AC system terminology for separately derived
systems and for clarity. There can be multiple “arrays” feeding an inverter in which the
single connection of the DC conductor to ground typically occurs.
Color Codes. The DC grounded conductor must have insulation colored white or gray
or have three white stripes if it is 6 AWG (American wire gauge) or smaller. Larger size
conductors must be marked with these colors at their termination points. Grounded PV
source and PV output conductors 6 AWG and smaller are allowed to be marked in the same
manner as larger conductors, in order to allow the use of the durable, black-insulated USE-2
and PV cable/PV wire in exposed locations within the PV array (NEC 200.6).
Grounding Electrode Conductor—Installation. On utility-interactive PV systems, the
connection between the DC grounded circuit conductor and the grounding system is usually
made through the ground-fault protection device (GFPD) internal to most non-battery based
utility-interactive inverters. UL Standard 1741 requires a provision be made for the internal
connection of a DC GEC. Microinverters that ground one of the module circuit conductors will
have an internal GFPD and will also require a GEC terminal. The connection at the inverter for
the GEC is usually a marked GEC terminal. From that terminal on the certified/listed inverter,
the PV installer must adhere to NEC requirements to make the connection to earth through a
grounding electrode. These requirements are described in Section 690.47(C), which modifies
and adds to the requirements in Article 250.
The NEC had significant changes in Section 690.47(C) of the 2005, 2008, and 2011
editions that address the DC GEC connection. The permissive requirements of this
section in the 2005 NEC or the permissive requirements in the 2008 NEC may be applied
to connect the GEC when installing a system in jurisdictions using either version.
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The 2011 NEC is easier to understand and includes all three methods for connecting the
DC GEC described in the 2005 and 2008 NEC. Section 690.47(C) from the 2011 NEC is
repeated below and may help in understanding the requirements for 690.47(C) in the
somewhat confusing 2008 NEC. Note that paragraph 690.47(C)(1) and 690.47(C)(2) in
the 2011 NEC align with 690.47(C)(1) and 690.47(C)(2) in the 2005 NEC and paragraph
690.47(C)(3) in the 2011 NEC aligns with 690.47(C) in the 2008 NEC.
690.47(C) Systems with Alternating and Direct Current Grounding Requirements.
Photovoltaic systems having dc circuits and ac circuits with no direct connection
between the dc grounded conductor and ac grounded conductor shall have a dc
grounding system. The dc grounding system shall be bonded to the ac grounding
system by one of the methods in (1), (2), or (3).
This section shall not apply to ac PV modules.
When using the methods of (C)(2) or (C) (3), the existing ac grounding electrode
system shall meet the applicable requirements of Article 250, Part III.
Informational Note No. 1: ANSI/UL 1741, Standard for Inverters, Converters and
Controllers for use in Independent Power Systems, requires that any inverter
or charge controller that has a bonding jumper between the grounded dc
conductor and the grounding system connection point have that point marked
as a grounding-electrode conductor (GEC) connection point. In PV inverters, the
terminals for the dc equipment grounding conductors and the terminals for ac
equipment grounding conductors are generally connected to, or electrically in
common with, a grounding busbar that has a marked dc GEC terminal.
Informational No.2: For utility-interactive systems, the existing premises
grounding system serves as the ac grounding system.
(1) Separate Direct-Current Grounding Electrode System Bonded to the
Alternating-Current Grounding Electrode System. A separate dc grounding
electrode or system shall be installed, and it shall be bonded directly to the ac
grounding-electrode system. The size of any bonding jumper(s) between ac
and dc systems shall be based on the larger size of the existing ac grounding
electrode conductor or the size of the dc grounding electrode conductor specified
by 250.166. The dc grounding electrode system conductor(s) or the bonding
jumpers to the ac grounding electrode system shall not be used as a substitute for
any required ac equipment grounding conductors.
(2) Common Direct-Current and Alternating-Current Grounding Electrode.
A dc grounding electrode conductor of the size specified by 250.166, shall be run
from the marked dc grounding electrode connection point to the ac grounding
electrode. Where an ac grounding electrode is not accessible, the dc grounding
electrode conductor shall be connected to the ac grounding electrode conductor
in accordance with 250.64(C)(1). This dc grounding electrode conductor shall not
be used as a substitute for any required ac equipment grounding conductors.
(3) Combined Direct-Current Grounding-Electrode Conductor and AlternatingCurrent Equipment Grounding Conductor. An unspliced, or irreversibly spliced,
combined grounding conductor shall be run from the marked dc groundingelectrode conductor connection point along with the ac circuit conductors to the
grounding busbar in the associated ac equipment. This combined grounding
conductor shall be the larger of the size specified by 250.122 or 250.166 and
shall be installed in accordance with 250.64(E).
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Photovoltaic System Grounding
Although any of the three methods of making connections to the inverter grounding
electrode terminal may be used, there are advantages and disadvantages to each.
Section 690.47(C)(1) in the 2011 NEC (similar to 690.47[C][1] in the 2005 NEC) has the
advantage of routing surges picked up by the array from nearby lightning strikes more
directly to earth than methods (C)(2) and (C)(3). Because a bonding conductor between
the new DC grounding electrode and the existing premises AC-grounding electrode is
required, the size, routing, and cost of that conductor must be considered.
Section 690.47(C)(2) in the 2011 NEC (similar to 690.47[C][2] in the 2005 NEC) uses fewer
components than the 690.47(C)(1) method and also routes surges to earth without getting
near the AC service equipment. See Figure 1 below for these two GEC routings.
Figure 1. PV Inverter Grounding Methods—2005 NEC Section 690.47(C)
Section 690.47(C)(3) in the 2011 NEC (similar to 690.47[C] in the 2008 NEC) combines the
inverter AC EGC with the DC GEC into a single conductor and thereby uses less copper.
However, this conductor is required to be bonded (GEC requirement) at the entrance and
exit of each metallic conduit and metal enclosure. For conductor sizes greater than about
6 AWG, bonding jumpers are typically used to bond conduit and enclosures, because
the conductor must remain unspliced or irreversibly spliced (GEC requirement). Also,
any lightning-induced surges picked up by the array may be routed more directly to the
service equipment and may be more likely to enter the premises wiring system than
when GECs are routed more directly to earth.
See Figure 2 for the combined EGC/GEC routing. Note that the 2008 and 2011 NEC allow
this combined conductor to be terminated at the first panel board that has a grounding
busbar with an attached GEC to a grounding electrode.
Solar America Board for Codes and Standards Report
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Figure 2. PV Inverter Grounding Methods Alternative
Note: 8 AWG is the minimum allowed, and a larger conductor may be required.
Section 690.47(D)—2008 NEC Only. Section 690.47(D) in the 2008 NEC is not entirely
clear and deals with lightning-induced surge currents in PV arrays. Because surge
reduction is not directly a safety issue, this section was removed from the 2011 NEC. The
original intent was that all ground-mounted PV arrays have a GEC routed directly from
the array frames to earth in as direct manner as possible. This would be similar to the
optional auxiliary grounding electrodes permitted by 250.54 that need not be bonded
to any other grounding electrode. Also, the intent was to have a similar array grounding
system on PV arrays mounted on buildings where the array and the inverter were on
different structures. NEC Section 250.54 is still available to those who wish to increase
system longevity through additional surge reduction techniques.
Grounding Electrode Conductor Size. GECs associated with the inverter are related to
grounding the DC part of the system and NEC 250.166 and 250.122 (for the 690.47[C]
[3] option) establishes the size. That size is primarily based on the type of grounding
electrode being used, which in turn is determined by the installation method selected
from the options in 690.47(C). Small residential and commercial PV systems in the
range of 2 to 10 kilowatts may find that only a 6 AWG GEC is needed when the installer
uses a ground rod. A 4 AWG GEC is required when the existing premises AC grounding
electrode is a concrete encased grounding electrode (UFER). Large PV systems require
careful assessments of the NEC DC requirements, the AC grounding system, the inverter
manual grounding requirements, and the utility grounding requirements when the utility
connection occurs at medium voltage. In some situations the DC GEC may have to be as
large as the largest DC conductor in the PV system.
Other sections of Article 250 such as 250.64(E) require that the GEC be continuous or
irreversibly spliced from the GEC terminal on the inverter to the grounding electrode. If
the GEC is routed through metal conduits or enclosures, the GEC must be bonded to the
conduit or the enclosure at the entrance and exit points of each conduit or enclosure to avoid
inductive (choke) effects that would prevent lightning-induced surges from reaching earth.
16
Photovoltaic System Grounding
Ungrounded Systems
When an installation meets the requirements of 690.35, the PV system can be
ungrounded. Inverters designed for use with ungrounded systems will have no
requirement for a GEC. However, the NRTLs and the manufacturers have not yet fully
understood the UL 1741 requirements, and non-isolated (transformerless) inverters are
on the market with a marked grounding electrode terminal. This creates an issue for the
PV installer because NEC 110.3(B) requires compliance with the product labels and that
would imply that a DC GEC is needed. Neither the NEC nor UL Standard 1741 specifically
require one, however. Installing the unneeded GEC will usually do no harm, other than
increase the price of the system, and may provide a lower impedance path to earth for
surge currents.
Color Codes. The NEC does not prescribe color codes for ungrounded DC conductors,
except that they may not be green, green with yellow stripes, white, gray, or have three
white stripes. By convention, red would normally be used for the positive conductor and
black for the negative conductor. However, conductors with colored insulation should be
avoided when exposed to UV radiation, as they may not be as durable as black insulated
conductors.
Ambiguity in the NEC. The current wording of 690.47(B) in the NEC relating to DC
systems leaves a hole in the NEC with respect to ungrounded PV systems. The NEC
may require a DC grounding electrode on ungrounded PV systems for the purpose of
DC equipment grounding. In actuality, with a common equipment grounding busbar
or connection between the DC equipment grounding terminals and the AC equipment
grounding terminals, the exposed metal surfaces of the DC equipment are grounded
when the AC EGC is connected to the grounding system at the AC main bonding jumper.
The following proposal for the 2014 NEC has been submitted to clarify this area:
690.47(B) (NEW)
Add a new third paragraph as follows:
Ungrounded dc PV arrays connected to utilization equipment with common ac
and dc equipment grounding terminals shall be permitted to have equipment
grounding requirements met by the ac equipment grounding system without the
requirement for a dc grounding electrode conductor or grounding system.
Substantiation:
The first paragraph of 690.47(B), as currently written, applies to stand-alone
ungrounded dc PV systems where a new grounding electrode and grounding
electrode conductor are required. There is no requirement directly addressing the
ungrounded PV array connected to a utility-interactive inverter as allowed by 690.35.
Most ungrounded PV arrays will be connected to utility-interactive inverters and those
inverters have common ac and dc equipment grounding terminals. The PV array dc
equipment grounding conductors, when connected to such inverters, have the array
dc equipment grounding conductors connected to earth through the ac equipment
grounding system and the existing ac grounding system. Additional grounding
electrodes and grounding electrode conductors are not required, but may be used.
Of course, meeting system equipment grounding requirements in this manner may not
be the best method to reduce potential damage from lightning-induced current surges and
overvoltage. (Note the use of the phrase “shall be permitted,” which indicates that this will be
an option.) The optional system grounding of 250.54 should be considered in high lightning
areas, although a fully engineered and listed lightning protection system would be a better
choice than simply adding electrodes.
Solar America Board for Codes and Standards Report
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Microinverters And Ac Pv Modules—
Different Requirements
PV modules and microinverters combined/assembled in the field or at the dealer or
distributor may not meet the intent, definition, or requirements associated with true AC
PV modules as defined in 690.2 and in 690.6. These combinations of devices also may
not meet the requirements for an AC PV module in UL Standard 1741.
As of early 2012, there is no specific size associated with either microinverters or
AC PV modules. The power outputs are increasing with nearly every new product. In
most cases, NEC requirements that apply to the larger string and central inverters will
also apply to the microinverter. Microinverters have similar AC output characteristics,
connections, and code requirements to AC PV modules.
Always follow the instructions supplied with the listed product (NEC 110.3[B]). The
material below is not a substitute for compliance with the instruction manual, the NEC,
or local codes.
Grounding. Both the AC PV module and the microinverter will require equipment
grounding connections if there is any exposed metal in these devices. A GEC connection
is required when the microinverter connects either the negative or positive inverter input
conductor (from the PV module) to ground through the inverter case. Many microinverter
manufacturers will ground the positive DC conductor to allow the inverter to be paired
with the widest range of PV modules.
Equipment/Safety Grounding. The AC output circuit cable of some microinverters does
not have an EGC. This EGC must be started (originated) in the transition box on the roof
where each set of inverters has the final factory AC output cable connected to another
wiring system. This AC EGC must be routed back to an AC grounding point as it is in
any other AC circuit. There is no requirement that it be unspliced, and the size will
typically be determined per Table 250.122. The AC EGC must also be routed to each
metal inverter enclosure either through the AC output cable or as a separate conductor
connected to a lug on that enclosure.
System Grounding. True AC PV modules, with no readily accessible DC conductors or DC
disconnects, will normally not require a GEC or GEC terminal.
Under UL Standard 1741, if the microinverter isolates the DC grounded input conductor
(assuming a grounded PV module) from the AC output (with a transformer or other
method), the microinverter must have a DC GEC running from the grounding electrode
terminal on the microinverter case to a DC grounding electrode. If the microinverter
operates the PV module as an ungrounded system (neither positive nor negative
connected to ground), then no GEC would be required.
Section 690.47(C) in the 2008 NEC and 690.47(C)(3) in the 2011 NEC permit the use of a
combined AC EGC and DC GEC from the inverter. UL 1741 requires the DC GEC terminal
on the outside of the inverter. If this combined conductor option is elected, then the most
stringent requirements that apply to either the EGC or the GEC must be followed. When
routed in conduit (for physical protection), a GEC as small as 8 AWG may be allowed
(250.166). The combined conductor may be this size assuming that the required EGC is
8 AWG or less (250.122). Section 250.64 requires that the GEC (and hence the combined
conductor) be bonded to the input and output of each metal conduit (where used) as
well as any metal enclosure that it is routed through until it gets to the main grounding
18
Photovoltaic System Grounding
bar in the AC supply equipment. The bonding requirement and 8 AWG size would rule
out the use of 10 AWG type non-metallic sheathed cable for the AC output circuit. The
bonding requirement may also be cumbersome to implement multiple times, and the
routing of this combined conductor may induce lightning surges to enter the main load
center and other branch circuits. Furthermore, if the existing AC grounding electrode for
the structure is encased in concrete, the GEC from the inverter may have to be a 4 AWG
conductor (250.166).
The alternate methods of routing the GEC allowed in 690.47(C) in the 2011 NEC might
provide better solutions. These solutions and their pros and cons were covered above in
the Systems Grounding part of this report.
AC PV Module Grounding—A Gray Area. Combinations of a microinverter and a PV
module with exposed DC connectors and DC conductors between the PV module and
the microinverter are being certified/listed as AC PV modules. Some of these products
have instruction manuals that say the microinverter may not be removed from the PV
module. Other manuals give specific instructions for removing the microinverter from
the PV module for repair. At issue is the definition of an AC PV module as a factory
assembled unit and the potential need to meet all DC NEC requirements for these
products with exposed DC connectors and DC conductors. Connectors are subject to
loosening, corrosion, or intentional opening in the field. Connectors and conductors are
also exposed to environmental degradation, ground faults, and animal damage.
Another concern is the microinverter-to-PV module frame bonding when the mechanical/
electrical connection is broken in the field. When the microinverter is replaced, the
bonding connection must be reestablished following the instructions in the product
manual, which have been evaluated during the NRTL certification/listing of the product.
In the future, these issues will be addressed in UL 1741 and possibly in the NEC.
At this time, the best strategy for installers is to follow the instruction manual and labels
found on the certified/listed product as required by NEC section 110.3(B)
Solar America Board for Codes and Standards Report
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Ground Faults And Ground-Fault
Protection Devices
Ground Faults—More Than the NEC Definition
The NEC defines a ground fault as an unintentional connection between an ungrounded
conductor and a grounded conductor or surface (Article 100 Definitions). Although
this definition is suitable and adequate for electrical systems sourced by the utility,
generator, or batteries, it does not fully address the limited fault current scenarios
found in PV systems.
A ground fault in a PV system (and other electrical systems) also occurs when any
circuit conductor (either ungrounded or grounded) comes in contact with an EGC or
grounded surface. Because there is only one system bonding jumper in each system,
when a ground fault occurs with a grounded conductor, the current that normally
flows only in the grounded circuit conductor will now also flow in the EGC and the
main bonding jumper. The current will flow in the two paths in inverse proportion
to the resistance of the parallel paths. Currents in unintended paths can start fires
if conductors or other metallic paths overheat or if the voltage at an open circuit is
sufficient to sustain an arc.
Ground-Fault Protection
Because PV modules are current-limited sources and PV current is a function of the
environmental effects of irradiance and temperature, overcurrent protection in PV circuits
is not designed and installed like it is in voltage-sourced power systems. Normally, OCPD
in PV systems is 156% of the rated Isc of the module. In many fault scenarios, there is
insufficient available Isc to activate OCPD. Although OCPDs are required to deal with
some overcurrent issues in PV systems, other methods are required to effectively deal
with ground faults in the DC circuits.
Section 690.5 of the 1987 NEC added new requirements intended to reduce fire hazards
resulting from ground faults in PV systems mounted on the roofs of dwellings. The intent
is not to provide shock protection, because the 5 milliamperes (the current level required
for AC ground-fault circuit interrupters [GFCI] anti-shock protection) level of protection
would not be possible on a PV array with distributed leakage currents. The requirement is
also not to be confused with a DC GFPD. The GFPD is intended to deal only with ground
faults (both grounded conductor and ungrounded conductor) and not line-to-line faults.
The requirements for the GFPD were modified in subsequent revisions of the NEC. The
requirements for the device in the current NEC are:
•
•
•
•
Detect a ground fault
Interrupt the fault current
Indicate that there was a ground fault
Open the ungrounded PV conductors or cause the inverter or charge controller to
stop exporting power.
In the 2008 NEC, the GFPD requirement was extended to all PV systems with a few
minor exceptions.
In a manner similar to other requirements in the NEC, there is no specific mention
of how to implement the requirement. It should also be noted that neither is there
any specification on the magnitude of the ground-fault current nor an exemption for
grounded conductor ground faults.
20
Photovoltaic System Grounding
Most currently available inverters, both stand-alone and utility-interactive, use a transformer
that isolates the DC grounded circuit conductor (usually the negative) from the AC grounded
circuit conductor (usually the neutral). With this transformer isolation, the DC side of a PV
system may be considered similar, but not identical, to a separately derived system and, as
such, must have a single DC bonding connection (DC system bonding jumper) that connects
the DC grounded circuit conductor to a common grounding point where the DC EGCs and
the DC GEC are connected. Like grounded AC systems, only a single DC system bonding
connection is allowed. If more than one bonding connection were allowed on either the AC
or DC side of the system, unwanted currents would flow in the EGCs, which violates NEC
Section 250.6.
Due to the above isolation and the fact that the DC circuits of the PV system are bonded to
ground in one place only, there will normally be no current (only leakage current) flowing to
ground. Today, a typical GFPD detects excessive current flowing to ground using an OCPD or
current sensor placed in the DC system bonding conductor or a differential current sensor
placed so that it senses the difference between the PV+ and PV- currents coming from the
PV array to the inverter.
Currently, GFPDs are available as both separate devices for adding to stand-alone PV systems
and as internal circuits integrated into most utility-interactive inverters or other PV equipment.
The GFPD that monitors current in the DC system bonding jumper often serves as that
connection and opens the connection when a ground fault is detected. In other designs, the
DC system bonding jumper is not opened to interrupt the fault currents, and the GFPD acts
by disconnecting both of the current-carrying conductors from the PV array.
In any ground-fault scenario on the DC side of a grounded PV system, ground-fault currents
from any source (PV modules or batteries in stand-alone systems) must eventually flow
through the DC system bonding jumper on their way from the energy source through the fault
and back to the energy source. This includes single ground faults involving the ungrounded
conductor or the grounded conductor faulting to ground. In grounded conductor ground faults,
the fault path creates parallel paths for the grounded conductor currents—one path through
the grounded conductor and one path through the fault path. The fault path current will flow
through the DC bonding connection. Double ground faults, either in a single source or output
circuit or in a separate source or output circuit, are beyond the ability of any equipment to
deal with at this time, and are not addressed in the NEC or in UL standards.
To meet the NEC Section 690.5 requirements, a GFPD using the DC system bonding jumper
has a 0.5- to 5-amp overcurrent device installed in that bonding connection. When the DC
ground-fault currents exceed the trip current rating of the device, it opens. By opening, the
overcurrent device interrupts the ground-fault current as required in NEC Section 690.5. If a
circuit breaker serves as the overcurrent device, the tripped position of the breaker handle
provides the indicating function. When a fuse is used, an additional electronic monitoring
circuit in the inverter provides an indication that there has been a ground fault. In some cases,
a mechanical blown fuse indicator activating an associated switch provides a blown fuse
indication. The indication function is also an NEC 690.5 requirement. Automatic resetting is
not allowed for these devices.
In some designs, in which the GFPD uses a circuit breaker as the sensing device, an additional
circuit breaker may be mechanically connected (common handle/common trip) to the sensing
circuit breaker. These types of GFPDs are commonly found in stand-alone PV systems. This
additional circuit breaker, usually rated at 100 amps and used as a switch rather than an
overcurrent device, is connected in series with the ungrounded circuit conductor from the
PV array. Multiple poles of a single circuit breaker can be used to simultaneously disconnect
different subarrays of the same system. In this manner, when a ground fault is sensed and
interrupted, the added circuit breaker(s) disconnects the PV array from the rest of the circuit,
providing an additional indication that something has happened that needs attention.
Solar America Board for Codes and Standards Report
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Even if the GFPD uses an additional circuit breaker pole in the ungrounded PV conductor,
that circuit breaker may not be used as the PV disconnect because in normal use, turning
off this breaker would unground the system and this is undesirable and could be unsafe
in non-fault situations. Furthermore, such an operation is prohibited in Section 690.13 of
the NEC. In the GFPD installed in utility-interactive inverters using a fuse as the sensing
element, the electronic controls in the inverter indicating that there has been a fault also
turn the inverter off and provide an indication that the fault has occurred.
It should be noted that the DC GFPD in many stand-alone systems detects and interrupts
ground faults that occur anywhere in the DC wiring, and the GFPD may be located
anywhere in the DC system. Because the normal location for the DC bonding connection
is at or near the DC disconnect, this bonding connection is usually made at the DC power
center where there is ready access to the DC GEC connection point. Utility-interactive
inverters or DC power centers on stand-alone systems are the most logical places for
GFPDs. There is no significant reason to install them at the PV module location. This
configuration would increase the length of the DC GEC and complicate its routing. To
achieve significant additional safety enhancements would require a GFPD at every
module. Equipment to do this does not exist and there are no requirements for such
equipment.
GFPD devices using differential current measurements or other sensing techniques could
be mounted at the inverter input or in PV DC combiners. When mounted in the DC
combiner, they could more readily isolate sections of a faulted array while leaving the
non-faulted sections in an operational state.
Figure 3 shows both positive (red) and negative (blue) ground faults and the paths that
the fault currents take in a GFPD monitoring the current in the DC system bonding
jumper. This particular system is a grounded system and the negative conductor is the
grounded circuit conductor. As noted above, all ground-fault currents must pass through
the DC system bonding connection where the GFPD sensing device is often located.
Figure 3: Ground-Fault Current Paths for Different Fault Conditions
22
Photovoltaic System Grounding
These devices are capable of interrupting ground faults occurring anywhere in the DC
system, including faults at the PV array or anywhere in the DC wiring from the PV
module to the inverter and to the battery in stand-alone systems. Systems that have
isolation components such as some charge controllers or DC-to-DC converters may
have a more limited view of fault locations. All of this can be done from many possible
locations in the DC circuit. Including these GFPDs on all PV systems reduces the potential
for fires.
Keeping the PV source and output conductors outside of a dwelling until the point of
first penetration and requiring the readily accessible DC disconnect at this point also
enhances the safety of the system. See Section 690.14 of the NEC for details. The
2005 NEC and subsequent editions of the NEC allow conductors in metallic raceways
to be routed inside the structure (690.31[E]). The use of metallic raceways for the DC
source and output circuits will facilitate ground-fault detection if the conductors in those
grounded conduits fault to the conduit.
During a ground fault, if the DC system bonding connection is opened or all conductors
are disconnected from a faulted circuit, and if the ground fault cures itself for some
reason (e.g., an arc extinguishes), the faulted circuit in the DC system remains
ungrounded until the GFPD system is reset. An ungrounded conductor-to-ground fault
may allow the grounded conductor (now ungrounded) to go to the open-circuit voltage
with respect to ground. This is addressed by the marking requirements of Section
690.5(C). A very high value (>50k) resistor is usually built into the GFPD and this resistor
bleeds off static electric charges and keeps the PV system loosely referenced to ground
(but not solidly grounded) during ground fault actions. The resistor value is selected so
that any fault currents still flowing are only a few milliamps and are too low to be a
fire hazard.
But Problems Still Exist. Although the NEC and UL standards require that ground faults
be detected, some implementations of the GFPD may not recognize ground faults in
all circuits throughout the PV array. Active analysis of these issues is ongoing and UL
Standards and/or the NEC will be modified to rectify this problem, which is addressed in
the following paragraphs.
During the daily cycle of zero current to rated current in bright sun, an ungrounded
conductor ground fault will usually source enough current to activate the ground-fault
detector. The detection current for the ground-fault system as currently implemented
must be set high enough (0.5 to 5 amps) to prevent nuisance trips due to expected and
normal leakage currents. These leakage currents increase in wet weather, with array
size, and as the array ages. However, lower currents (below the detection value) that are
associated with grounded conductor and ungrounded ground faults will go undetected.
In the case of grounded conductor ground faults, the faults may exist for significant
periods of time before being detected or found because there is typically no loss of
performance because the currents from the source still get to the inverter. A ground fault
in an ungrounded conductor that is below the detection level of the GFPD may be sensed
as a loss of power in that circuit, however, provided that the data monitoring system has
sufficient sensitivity and granularity.
If a grounded conductor ground fault exists and a second ground fault occurs in an
ungrounded conductor, the GFPD will activate, but this action may force high fault
currents through the grounded conductor ground fault. This grounded conductor fault
path may be incapable of withstanding these high currents and pose a fire hazard. See
the Solar ABCs paper on the Ground-fault Protection Blind spot (Brooks, 2012).
Second ungrounded conductor ground faults in a system with an initial ungrounded
conductor fault can be treated as an independent event and usually will not interact with
the first fault. The exception is that the two ground-fault currents may add to a sufficient
value to be sensed by the GFPD.
Solar America Board for Codes and Standards Report
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Ungrounded Systems. Section 690.35 of the NEC also requires ungrounded systems to
have a GFPD. These detection systems are mainly electronic in nature and may not
have the issues of the GFPD on grounded systems. These systems have to operate with
leakage currents and impedances and those factors will change with system age and
environmental conditions. The most common GFPD approach in ROW is electronic
impedance measurements between array conductors and ground, a strategy that has
proven effective. Monitoring and tracking the impedances over time may allow ground
faults to be more readily detected in the presence of time varying leakage currents.
With equipment modifications and redesign, and when implemented prior to exporting
current from the array, such devices could be applied to existing grounded PV systems.
24
Photovoltaic System Grounding
THE CONNECTION TO EARTH
The grounding electrode is the final connection to the earth and carries currents from
lighting-induced surges as well as from possible cross connections with other electrical
power systems to the earth. The NEC discusses a number of different metallic materials
and components that may be used as grounding electrodes as well as various restrictions
on their installation. Using more than one of these electrodes (encouraged by the NEC)
forms a grounding electrode system.
Grounding electrodes include rods, pipes, plates, rings, concrete encased electrodes,
building steel, and other devices. Rod-type electrodes must be stainless steel or copper or
zinc coated steel and may use other materials when certified/listed.
The resistance from the grounding electrode to earth must be 25 ohms or less (250.53[A]
[2]). If this criterion is not met, an additional grounding electrode must be added more
than six feet away to form a grounding electrode system. There is no requirement
to measure this resistance to ground and it is rarely measured because it requires
specialized equipment. For that reason, many local codes, particularly in the dry
Southwest, require that two grounding electrodes always be installed.
GEC Size. The apparent quality of the grounding electrode connection to earth, all other
things being equal, appears to be the rationale for sizing the GEC. The better the presumed
contact with earth, the larger the required GEC. Section 250.166 establishes the sizes for a
DC GEC and 250.66 establishes the size for an AC GEC. Both appear to require increased
sizes for the grounding electrodes that provide the better contact with earth.
No Fault Currents. It should be noted that ground-fault currents do not normally travel
through the GEC to the grounding electrode and then to the earth. These groundfault currents typically flow through the equipment grounding system and the circuit
conductors. An exception occurs when a grounding electrode system involves building
steel where multiple conductive paths in the equipment grounding system and grounding
electrode system may be involved with fault currents.
Solar America Board for Codes and Standards Report
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UTILITY VS NEC REQUIREMENTS
The NEC covers the premises wiring requirements up to the “service point,” which is an
NEC definition for the point at which utility requirements end and NEC requirements
begin. Utility electrical installation requirements are established by the National Electrical
Safety Code (NESC) (a registered trademark of the Institute of Electrical and Electronic
Engineers [IEEE]) and various other IEEE standards.
The three-phase multi-grounded system is referenced in both the NESC and the NEC. The
NESC sets the ground rules for practical safeguarding of persons during the installation,
operation, or maintenance of electric supply and communication lines and associated
equipment. The NESC contains the basic provisions that are considered necessary for the
safety of employees and the public under the specified conditions.
In the grounding arena, if the utility has grounded any of the service conductors in their
distribution system (usually at the local distribution transformer secondary), the NEC
requires that these grounded conductors be connected to earth (grounded) again at the
service disconnect. There is typically no EGC between the premises electrical system and
the utility electrical system. In some cases, like a three-wire delta service, there may not
even be a grounded circuit conductor such as the grounded neutral conductor on a fourwire wye service.
On larger systems, particularly those with an interface at medium voltage, utility
grounding requirements may be applicable to the grounding methods used at the service
entrance. IEEE 142—The Green Book (IEEE, 2007) provides some of the highly technical
explanations for grounding these types of systems and some utilities may require that
these practices be followed before allowing the PV interconnection.
26
Photovoltaic System Grounding
Acronyms
AC
alternating current
AHJ
authority having jurisdiction
ANSI American National Standards Institute
AWG American wire gauge
DC
direct current
EGC
equipment grounding conductor
GEC
grounding electrode conductor
GFCI
ground-fault circuit interrupter (AC Systems)
GFPD
ground-fault protection device
Hz
hertz
IEEE
Institute of Electrical and Electronic Engineers
Isc short circuit current
NEC
National Electrical Code
NESC
National Electrical Safety Code
NFPA
National Fire Protection Association
NRTLs Nationally Recognized Testing Laboratories
OCPDovercurrent protective device
OSHA U.S. Occupational Safety and Health Administration
ROW
rest of world
UL
Underwriters Laboratories
Solar America Board for Codes and Standards Report
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References
Ball, G., Zgonena T., & Flueckiger C. (2012). Photovoltaic module grounding: Issues and
recommendations. Solar ABCs. www.solarabcs.org/grounding
Brooks, B. (2012). The ground-fault protection blind spot: Safety concern for larger PV
systems in the U.S. Solar ABCs. www.solarabcs.org/blindspot
Institute of Electrical and Electronic Engineers (IEEE). (2007). Recommended practice for
grounding of industrial and commercial power systems (IEEE 142-The Green Book).
http://ieeexplore.ieee.org/xpl/mostRecentIssue.jsp?punumber=4396961
International Association of Electrical Inspectors. (2011). Soares grounding and bonding.
National Fire Protection Association (NFPA). (2011). National Electrical Code® (NEC®),
NFPA 70.
National Fire Protection Association (NFPA). (2011). National Electrical Code Handbook.
Underwriters Laboratories (UL). (2002). UL 1703: Standard for safety for flat-plate
photovoltaic modules and panels. (Revised dated May 8, 2012). http://ulstandardsinfonet.
ul.com/scopes/1703.html
UL. (2010). UL 1741: Standard for safety for inverters, converters, controllers and
interconnection system equipment for use with distributed energy resources.
http://ulstandardsinfonet.ul.com/scopes/scopes.asp?fn=1741.html
UL. (2010). UL 2703: Rack mounting systems and clamping devices for flat-plate photovoltaic
modules and panels. http://ulstandardsinfonet.ul.com/outscope/2703.html
28
Photovoltaic System Grounding
Solar America Board for Codes and Standards
www.solarabcs.org
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