String protection fuses (-SX / -SY models only)
Sizing of fuses
The correct sizing of the
10
positive side (+) and
22
negative side (-) string
fuses to be used to protect from “return currents” is very important because
it can considerably limit 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 supplied by the strings not involved in
the fault, but connected to the same input channel, to pass through the
faulty string.
In these versions of the wiring box, you must directly connect the individual
strings coming into the inverter (do not make eld switchboards for
parallel strings). This is because the 10 positive side (+) and 22 negative
side (-) string fuses, situated on each input, are not rated to take strings
in parallel (array).
This operation can cause damage to the fuse and consequently
malfunctioning of the inverter.
The sizing of the string fuses must be made taking into account the 2
following conditions:
1. The nominal current of the fuse (Irated) must not exceed the maximum
rating of the fuse to be used in series on the strings (maximum series fuse
rating), indicated in the technical data of the PV modules in compliance
with standard EC 61730-2:
Irated < Maximum series fuse rating
2. The fuse rating (Irated) must be determined based on the string current
and on the sizing guidelines provided by the manufacturer to avoid
untimely tripping. As a general guideline, based on the photovoltaic
modules’ short circuit current (Isc), it is possible to calculate the rating of
the fuse with the following formula:
Irated > (1.4 ≈ 1.5)*Isc
-SX -SY (DC)
23
22
10
19
09
15
31
14
SX and SY DC
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