ABB TRIO-50.0-TL-OUTD Product Manual - Izing Of Fuses

- 78 -

000533AG

5 - Installation

S

tring protection fuses (-SX / -SY models only)

S

izing 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 

fire 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  field  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 (I

rated

) 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:

I

rated

2. 

The fuse rating (I

rated

) 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 (I

sc

), it is possible to calculate the rating of 

the fuse with the following formula:

I

rated

 > (1.4 ≈ 1.5)*I

sc

-SX  -SY (DC)

23

22

10

19

09

15

31

14

SX and SY DC