Page 100 - Solar Power in Building Design The Engineer's Complete Design Resource

70 SOLAR POWER SYSTEM DESIGN CONSIDERATIONS

such as USE-2 and UF-2, that are sized #6 or above to be identified with a white tape
or marker.
As mentioned earlier, all PV array frames, collector panels, disconnect switches,
inverters, and metallic enclosures should be connected together and grounded at a sin-
gle service grounding point.

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 interrupt-
ing) 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 condi-
tions are required to have one of the current-carrying conductors grounded. In electri-
cal 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 electri-
cal 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 acciden-
tally 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 actua-
tion 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