PV combiner box protection [RE-wrenches]

[Eric Smiley]

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Q:
> why the manufacturers do not put series current
> limiting fuses inside the J-box on the back of the PV?

A:
Access, heat in the JB and trouble shooting. Although a properly sized
fuse should not "nuisance" blow. I recently saw a PV array where a
bypass diode failed and shorted the string, thus causing the remaining
strings to backfeed the shorted string. The fuse in the combiner blew
before the breaker at the charge controller tripped thus isolating the
fault yet allowing the remainder of the array to continue operating.
Locating the damaged module in an array of flush mounted modules on a 45
degree metal roof would have been a royal pain if there were no fuses in
the series strings or fuses only in the module J-boxes, whereas the
blown fuse in the combiner pin-pointed the module easily.

Interesting aside. In Canada, overcurrent protection is not required on
each series string (PV source circuit) if the available fault current
does not exceed the ampacity of the wire. Nonetheless, most installers
do include overcurrent protection on each series string because
off-the-shelf combiners come that way and because the thought of a fault
current potentially persisting in a PV array for a long period of time
doesn't sound very safe. The module wire may be able to carry the
short-circuit current of the array, but how much current can the module
withstand before it melts, or causes a fire? I believe this rule will
probably be ammended in the next version of our code.

Eric

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