Pv fuses blowing constantly

Have a new installation where the Isc of the panel is 12.97 amps, have 16 amp fuses, they don’t blow right away, but can be 12 to 24 hours, never had this problem before, even on my installation, same set up, any ideas

It could be that you see power spikes on those panels during the cold days. The specs for the PV panels are created on factory conditions, but in the wild that could vary with the temp and sun.

I know a lot of fuses do blow if you run them close to their rated current for an extended period. For PV strings the fuse should probably be a bit less than double the Isc, so try 20A fuses.

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While still calculating that the fuse will still blow before the cable melts :stuck_out_tongue:


It is 6 mm cable, though, as mentioned this is the first installation I am seeing this issue

The PV fuses are there to protect against faults. You can’t get more than Isc from a PV string though, so a fuse doesn’t do much if you have only one string (unless you have a non-isolated buck converter for your MPPT, in which case a short at the panels could short the battery through the reverse diode in the buck switch and so the fuse is very useful). When you have more than 2 strings in parallel, a fault on one string can have the Isc of all the other strings flowing back into the fault, and then the fuse should blow. So as long as it blows when you have 2 x Isc or more then you are ok.

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Apologies, it is a Sunsynk with two mppt’s, so each string is 6 panels in series

Fuses are to protect the wiring so what size is your PV cable?

I experienced the same. Isc of the PV modules is 9A, but it blows 12A fuses every few months. I changed to 16A fuses and the problem went away.

6 mm2 and around 10 meter runs

Going up to a 20A fuse should be fine.

Yep, that is what I plan to do, only my ocd mind wants to know, why only this installation??
If a 20 amp fuse fixes the problem, guess I should let sleeping dogs lie !!!

If you read my install thread, had a similar problem. What actually happened was a fuse being run at rated, or very close to capacity, heating up quite a lot and then blowing every couple of days. Go to next Fuse value. The heat will kill em. Remember them fuses should run cold, not hot.


If this install is a exact copy and paste of others where it doesn’t happen, what are the chances that you might be sitting with a bad quality batch of fuses?
Slim chance, but you never know these days with all the chip and other material shortages, shipping delays etc.

With that being said, I’ll also opt for the next bigger fuse size, 20A, 25A, remember fuses slowly heat up and then eventually go, also where fast and slow blow comes in. How long it will handle a certain current and temperature before giving up on life.

Indeed! Fuses aren’t an exact science. They take time to blow. The higher the overload current the quicker it will blow.
Check out this article: https://electrical-engineering-portal.com/fuse-characteristics-protection-circuits

Please ask the owner what voltage the Sunsynk reports for both the MPPT’s.

IEC standards on how to rate fuses…

You should be using as a minimum 1.56xIsc on the Balance of System (including fuses). Note this does not apply to Circuit breakers as this would have a slightly different calculation.

You state Isc = 12.97 then you would need to do 20.2Amps and NOT 6amps, to be honest, 16Amps is too low for even 1.25x Isc which is the bare bare minimum. I am even not going to go into the entire derating requirements discussion. (Note you can get away with 1.25x if you do not have a parallel string configuration).

You might want to rethink the sizing completely (and get the SANS/IEC standard that I have referred to in the past (IEC60364-7-712)

Mersen has a document on the web on how to do fuse sizing correctly.

Also, check the datasheet on the modules, while they mention “max” fuse rating, this is the one to usually go for straight out (unless you undersize the cables!)

So you don’t have a combiner box??

Thanks Rautenk, installed 20 amp fuses, now to wait and see.
Hi Richard, i do have surge protectors and fuses

I’m not convinced that you need fuses especially on a series system. If you look a the honorable Mr Prowse’s video (from 2min) he alludes to this: What is a PV Combiner Box? - YouTube