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000438AG
5 - Installation
String protection fuses (-FS model only)
Sizing of fuses
Correctly sizing the string fuses
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to be used for protection against re-
turn currents is very important since it can signicantly reduce the risk of
re and damage to the PV generator. A “return current” can be generated
in the event of a fault and relevant short-circuit at the ends of one or more
PV modules of the system; this condition can cause all the current sup-
plied by the strings not involved in the fault, but connected to the same
input channel, to pass through the faulty string.
The following 2 conditions must be taken into account when sizing the
string fuses
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:
1. The current rating of the fuse (Irated) may not exceed the maximum
rating of the fuse to be used in series with the string (maximum series
fuse rating), specied in the technical data of the PV modules, in agree-
ment with standard IEC 61730-2
Irated < Maximum series fuse rating
2. The rating of the fuse (Irated) must be determined according to the
string current and the manufacturer’s sizing guidelines in order to avoid
unforeseen failures. As a general guide, based on the short-circuit cur-
rent (Isc) of the PV modules, the fuse rating may be calculated from the
following formula:
Irated > (1.4 ≈ 1,5)*Isc
The fuse selected must be the standard commercial size closest to the
result obtained.
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