AIR is the value that lets us know if the OCPD is capable of interrupting a
fault current without damage to itself or the equipment.
Fusing of PV Source Circuits
PV source circuits can use fuses and circuit breakers as com-
plementary overcurrent devices. Complementary overcurrent
devices are created to be used inside a listed device. These
overcurrent devices aid the main overcurrent device.
Standalone PV System Current-Limiting Fuses
When a battery’s overcurrent protection device has a lower
interrupt rating than the present short-circuit current, use a
current-limiting fuse. The current-limiting fuse diminishes
the battery bank’s supply of current. Current-limiting fuses have DC ratings
between 125V and 600V. They also have currents between 0.1A to 600A. They are
equipped with 20,000A DC interrupt.
Several current-limiting fuses can be used:
RK5 or RK1 (both with Class-R rejecting fuse holders or DC-rated,
fused disconnects)
DC-rated Class J or T fuses
Time-delay fuses such as the Class T fast-acting fuses
Use a current-limiting fuse and a disconnect switch in every battery bank
that has a 1,000 amp-hour capacity. It is wise to use them at lower capacities also
for additional safety.
Single-cell batteries with capac-
ities higher than 1,000 amp-hours
need unique design plans. These
batteries can create currents greater
than 20,000 AIR rating of the
current-limiting fuses.
If current-limiting fuses cannot
be used as overcurrent devices, then
substitute circuit breakers with high
interrupt ratings. These circuit
breakers cannot protect other types
of fuses or circuit breakers. Use high
interrupting circuit breakers between
the inverter and the battery bank. In
that placement, voltage drops are
minimized. The circuit breakers also
provide disconnecting means and
overcurrent protection.
TECH TIPS
Never gang fuses or circuit breakers
to enlarge current carrying capability.
According to NEC Article 240.8, this is
advisable only through manufacturer’s
assembly. It is also advisable only if
the mechanism is listed to be used in
parallel.
DC combiner with touch-safe fuse holders for safe handling of DC fuses.
Courtesy of PerfectPower, Inc.
198 ADVANCED PHOTOVOLTAIC INSTALLATIONS
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