Page 114 - Alternative Energy Systems in Building Design

90 SOLAR POWER SYSTEM PHYSICS AND TECHNOLOGIES

As mentioned earlier, all PV array frames, collector panels, disconnect switches,
inverters, and metallic enclosures should be connected together and grounded at a single
service grounding point.

PV system ground-fault protection When a PV system is mounted on the roof
of a residential dwelling, NEC requirements dictate the installation of ground-fault
detection and interrupting devices (GFPD). However, ground-mounted systems are
not required to have the same protection because most grid-connected system inverters
incorporate the required GFPD devices.
Ground-fault detection and interruption circuitry performs ground-fault current
detection, fault current isolation, and solar power load isolation by shutting down the
inverter. Ground-fault isolation technology is currently going through a developmental
process, and it is expected to become a mandatory requirement in future installations.

PV system grounding PV 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.
In the preceding paragraph, grounded means that a conductor connects to the metallic
enclosure of an electrical device housing that serves as earth. A grounded conductor
refers to a conductor that is intentionally grounded. In PV systems, it is usually the nega-
tive 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 is a conductor that normally does not carry current
and generally is a bare copper wires that also may have a green insulator cover. The con-
ductor usually is 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 connects the grounded conductors to a system
grounding electrode, which usually is located at a single site within the project site and
does not carry current. In the event of accidental shorting of equipment, the current is
directed to the ground, which facilitates actuation of ground-fault devices. The
grounding electrode is a grounding rod or a concrete-encased rebar (UFR) conductor,
a grounding plate, or simply a structural steel member to which a grounding electrode
conductor is connected. Based on the NEC, all PV systems, whether grid-connected
or stand-alone, are required to be equipped with an adequate grounding system to
reduce the effects of lightning and to provide a measure of personnel safety.
Incidentally, grounding of PV systems substantially reduces radiofrequency noise
generated by the inverter equipment.
In general, grounding conductors that connect 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