DIY Offgrid Project – MC4 Fuses

Solar PV panels are wired in series to get to the desired voltage and then these “strings” are connected in parallel to deliver the required current. Each string is supposed to have a fuse in series to protect against excessive current flowing through the string. Solar panels have several characteristics including an open circuit voltage (Voc) and a short circuit current (Isc) Isc which is typically the maximum current the panel and hence string can produce. Deployment rules require a protective fuse in series. The panels I’m using say the fuse should be no more than 15 amps. My panels can’t produce more than 10 amps so the output of the panel isn’t going to blow the fuse.

What is the fuse protecting? It’s protecting the panels from being damaged by being back-fed from several other strings and also from damage your inverter fails and drives a high current back through the panel. It is also protecting the cabling from excessive current although I’m using 4mm² cable which is rated to carry 25 amps. You have to try hard to imagine the failure scenarios on a low voltage system like mine that would cause the fuse to blow. Never the less, I have one of these fuses in series with each string of three panels.

Having wired three strings of three panels, with fuses in-line, I short-circuited them to test performance.

Nearly 20 amps Isc – short circuit current. Not bad for late in the day!

And a word of warning. These were advertised as “fuse diodes” but there is nothing “diode” about them so don’t pay a premium for that.

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