Maintenance Exchanging the Fuses
9-6 SMA America SB3000US-4000US-11-SE1607
PV String Fuse Sizing
In any electrical system, fuses are used to protect wiring and equipment from
excessive currents that can cause damage, heating or in extreme cases even fire. If
the fuse rating is too small it could open during normal operation. If the fuse rating
is too large, it cannot provide the needed protection. In PV systems, the minimum
and maximum size of the series fuse is determined by the electrical ratings of the PV
module as well as by UL and National Electrical Code (NEC) requirements. Be sure
to consult with your PV module manufacturer for appropriate PV string fuse ratings.
The minimum size of fuses and wiring are calculated using the Short Circuit Current
Rating (Isc) of the PV module. The NEC requires that all fuses and wiring be sized
for a minimum of 1.56 times the Isc of the PV module used in the system. The proper
size PV string fuse is determined by calculating 1.56 x Isc (of the PV module) and
then rounding up to the next standard fuse size.
Example: If the Isc of the PV module equals 6.9 Adc, then the PV
string fuse size is determined by 1.56 x 6.9 = 10.76. The next
standard fuse size would be a 12A, 600Vdc fuse.
CAUTION: The string fuse size must not be greater than the
maximum fuse size rating of the PV module as provided on
the PV module manufacturer data sheet. If no maximum fuse
size is indicated, please contact the PV module manufacturer.
DC Disconnect Requirements
NEC 690.15-18 allows the use of fuse holders as a suitable means of disconnecting
PV arrays for servicing. Additional DC disconnects external to the inverter may be
required by the local authority having jurisdiction.
WARNING: Never remove a fuse while it is under
load. Electrical arcing and damage to the fuse holder
will occur if a fuse removed
under load.
PV String Fuses
The SMA DC-Disconnect is shipped with 15 A, 600 Vdc fuses in the fuse holders.
Illustration
9-3 on page 9-5 shows the string fuse holders and their corresponding
terminals.
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