PHOTOVOLTAIC SYSTEM GROUND-FAULT PROTECTION
When a photovoltaic system is mounted on the roof of a residential dwelling, NEC
requirements dictate the installation of ground-fault protection (detection and interrupting) devices (GFPD). However, ground-mounted systems are not required to have the
same protection since most grid-connected system inverters incorporate the required
GFPDs.
Ground-fault detection and interruption circuitry perform ground-fault current
detection, fault current isolation, and solar power load isolation by shutting down the
inverter. This technology is currently going through a developmental process, and it is
expected to become a mandatory requirement in future installations.
PV SYSTEM GROUNDING
Photovoltaic power systems that have an output of 50 V dc under open-circuit conditions are required to have one of the current-carrying conductors grounded. In electrical engineering, the terminologies used for grounding are somewhat convoluted and
confusing. In order to differentiate various grounding appellations it would be helpful
to review the following terminologies as defined in NEC Articles 100 and 250.
Grounded.Means that a conductor connects to the metallic enclosure of an electrical device housing that serves as earth.
Grounded conductor. A conductor that is intentionally grounded. In PV systems it
is usually the negative of the dc output for a two-wire system or the center-tapped
conductor of an earlier bipolar solar power array technology.
Equipment grounding conductor. A conductor that normally does not carry current
and is generally a bare copper wire that may also have a green insulator cover. The
conductor is usually connected to an equipment chassis or a metallic enclosure that
provides a dc conduction path to a ground electrode when metal parts are accidentally energized.
Grounding electrode conductor. A conductor that connects the grounded conductors
to a system grounding electrode, which is usually located only in a single location
within the project site, and does not carry current. In the event of the accidental
shorting of equipment the current is directed to the ground, which facilitates actuation of ground-fault devices.
Grounding electrode.A grounding rod, a concrete-encased ultrafiltration rate (UFR)
conductor, a grounding plate, or simply a structural steel member to which a grounding
ENTRANCE SERVICE CONSIDERATIONS FOR GRID-CONNECTED SOLAR POWER SYSTEMS 71
electrode conductor is connected. As per the NEC all PV systems—whether gridconnected or stand-alone, in order to reduce the effects of lightning and provide a
measure of personnel safety—are required to be equipped with an adequate grounding system. Incidentally, grounding of PV systems substantially reduces radiofrequency noise generated by inverter equipment.
In general, grounding conductors that connect the PV module and enclosure frames
to the ground electrode are required to carry full short-circuited current to the ground;
as such, they should be sized adequately for this purpose. As a rule, grounding conductors larger than AWG #4 are permitted to be installed or attached without special
protection measures against physical damage. However, smaller conductors are
required to be installed within a protective conduit or raceway. As mentioned earlier,
all ground electrode conductors are required to be connected to a single grounding
electrode or a grounding bus.
EQUIPMENT GROUNDING
Metallic enclosures, junction boxes, disconnect switches, and equipment used in the
entire solar power system, which could be accidentally energized are required to be
grounded. NEC Articles 690, 250, and 720 describe specific grounding requirements.
NEC Table 25.11 provides equipment grounding conductor sizes. Equipment grounding conductors similar to regular wires are required to provide 25 percent extra ground
current-carrying capacity and are sized by multiplying the calculated ground current
value by 125 percent. The conductors must also be oversized for voltage drops as
defined in NEC Article 250.122(B).
In some installations bare copper grounding conductors are attached along the railings that support the PV modules. In installations where PV current-carrying conductors are routed through metallic conduits, separate grounding conductors could be
eliminated since the metallic conduits are considered to provide proper grounding when
adequately coupled. It is, however, important to test conduit conductivity to ensure that
there are no conduction path abnormalities or unacceptable resistance values.
When a photovoltaic system is mounted on the roof of a residential dwelling, NEC
requirements dictate the installation of ground-fault protection (detection and interrupting) devices (GFPD). However, ground-mounted systems are not required to have the
same protection since most grid-connected system inverters incorporate the required
GFPDs.
Ground-fault detection and interruption circuitry perform ground-fault current
detection, fault current isolation, and solar power load isolation by shutting down the
inverter. This technology is currently going through a developmental process, and it is
expected to become a mandatory requirement in future installations.
PV SYSTEM GROUNDING
Photovoltaic power systems that have an output of 50 V dc under open-circuit conditions are required to have one of the current-carrying conductors grounded. In electrical engineering, the terminologies used for grounding are somewhat convoluted and
confusing. In order to differentiate various grounding appellations it would be helpful
to review the following terminologies as defined in NEC Articles 100 and 250.
Grounded.Means that a conductor connects to the metallic enclosure of an electrical device housing that serves as earth.
Grounded conductor. A conductor that is intentionally grounded. In PV systems it
is usually the negative of the dc output for a two-wire system or the center-tapped
conductor of an earlier bipolar solar power array technology.
Equipment grounding conductor. A conductor that normally does not carry current
and is generally a bare copper wire that may also have a green insulator cover. The
conductor is usually connected to an equipment chassis or a metallic enclosure that
provides a dc conduction path to a ground electrode when metal parts are accidentally energized.
Grounding electrode conductor. A conductor that connects the grounded conductors
to a system grounding electrode, which is usually located only in a single location
within the project site, and does not carry current. In the event of the accidental
shorting of equipment the current is directed to the ground, which facilitates actuation of ground-fault devices.
Grounding electrode.A grounding rod, a concrete-encased ultrafiltration rate (UFR)
conductor, a grounding plate, or simply a structural steel member to which a grounding
ENTRANCE SERVICE CONSIDERATIONS FOR GRID-CONNECTED SOLAR POWER SYSTEMS 71
electrode conductor is connected. As per the NEC all PV systems—whether gridconnected or stand-alone, in order to reduce the effects of lightning and provide a
measure of personnel safety—are required to be equipped with an adequate grounding system. Incidentally, grounding of PV systems substantially reduces radiofrequency noise generated by inverter equipment.
In general, grounding conductors that connect the PV module and enclosure frames
to the ground electrode are required to carry full short-circuited current to the ground;
as such, they should be sized adequately for this purpose. As a rule, grounding conductors larger than AWG #4 are permitted to be installed or attached without special
protection measures against physical damage. However, smaller conductors are
required to be installed within a protective conduit or raceway. As mentioned earlier,
all ground electrode conductors are required to be connected to a single grounding
electrode or a grounding bus.
EQUIPMENT GROUNDING
Metallic enclosures, junction boxes, disconnect switches, and equipment used in the
entire solar power system, which could be accidentally energized are required to be
grounded. NEC Articles 690, 250, and 720 describe specific grounding requirements.
NEC Table 25.11 provides equipment grounding conductor sizes. Equipment grounding conductors similar to regular wires are required to provide 25 percent extra ground
current-carrying capacity and are sized by multiplying the calculated ground current
value by 125 percent. The conductors must also be oversized for voltage drops as
defined in NEC Article 250.122(B).
In some installations bare copper grounding conductors are attached along the railings that support the PV modules. In installations where PV current-carrying conductors are routed through metallic conduits, separate grounding conductors could be
eliminated since the metallic conduits are considered to provide proper grounding when
adequately coupled. It is, however, important to test conduit conductivity to ensure that
there are no conduction path abnormalities or unacceptable resistance values.
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