Blown fuse - can't understand why

[nunodonato]

Hi everyone!

Even though my system is installed and working for many months, I've only started living off-grid full-time for about 2 weeks, so now I'm making the real tests!

I have 6 panels, 2 parallel groups of 3 panels each (in series). Each panel is 320W, with a max current of around 8 amps.
I have wired each group with 4mm wire (which is more than enough for 8amps), up to the 20A fuse, which then connects to the inverter/controller with a thicker wire.

Yesterday there was a very cloudy day and the batteries took a beating, and today there was a very sunny day. So since early morning the panels were outputting their max (but still far from the rated max).
After a few hours constantly with full sun, I started to feel a burnt smell and a few minutes later the inverter beeped signaling no power coming in from the panels.

First thing I checked was the 20A fuse box. I have one 20A fuse for Negative and one 20A fuse for positive. The negative fuse was blown, but not only that, the incoming wires from the panels were very dark and the plastic of the fuse box was even melted! These seems to suggest a significant overheat... which I don't understand why it could happen since everything was sized appropriately... (or at least I hope...)

I attach a photo of the fuse box.

any ideas on what could be the problem here? I'm going to get 4 fuses instead of 2, and connect each wire to its own fuse... even though the 2 groups together should not exceed 16amps as per the specifications of the panels...

[SunEagle]

I agree the wires should have handled the 8 amps. The burnt fuse holder and wire looks like a loose connection. It is possible that the blue wires were not properly terminated which caused them to heat up.

If you have a DC clamp on amp meter you could measure the current flow on each wire to make sure they aren't seeing high amps exceeding the wire rating.

[bcroe]

I have had several such failures. None were from excessive current. The theme seemed to
be a high resistance connection (too many mili ohms) generated a lot of heat, at max current.
I believe the daily cycling of heat and cool eventually worked connections loose, the resistance
and the heating increased, and it snowballed to failure.

My suggestion, in the first months retighten all connections a few times, you may be surprised
that they are not as tight as you did them. Be very alert for any warm spots in the wiring, at the
end of a long productive period. Smells, failure is imminent. good luck, Bruce Roe

[Mike90250]

Automotive fuses are not really great for RE energy, they can't seem to handle the hours long heavy current. The MAXI fuses with their larger blades hold up better.

[azdave]

That's very clearly a failure cause by a high-resistance connection and not an over-amp situation. You don't have to be in the solar biz to have sen that before. Could have been caused by a bad component but more likely, an install error such as the connection either under or over-tightened.

I've never been a fan of having two wires terminate at a breaker or fuse holder like that. I've seen too many times where one wire gets all the compression under the terminal screw but the other wire gets pushed to the side and is not as secure.

[peakbagger]

Most but not all breakers and terminals are rated for only one wire to be connected at a time. Connecting two will usually lead to what your photo shows. Nothing wrong with terminating the two wires with an appropriate wire nut and then running one lead into the terminal strip. You can get screw type wire nuts that look a lot more permanent than the twist on type. The nut needs to be inside the enclosure.

Almost any inspector will fail a panel with two wires going to a single lug on a breaker. Note there are some breakers that reportedly are rated for two wires but I have never seen one in that form factor.

[SunEagle]

Most but not all breakers and terminals are rated for only one wire to be connected at a time. Connecting two will usually lead to what your photo shows. Nothing wrong with terminating the two wires with an appropriate wire nut and then running one lead into the terminal strip. You can get screw type wire nuts that look a lot more permanent than the twist on type. The nut needs to be inside the enclosure.

Almost any inspector will fail a panel with two wires going to a single lug on a breaker. Note there are some breakers that reportedly are rated for two wires but I have never seen one in that form factor.

Yeah. 2 wires on the same breaker terminal is called a "double tap" and is not accepted by just about any inspector.

[bcroe]

Yeah. 2 wires on the same breaker terminal is called a "double tap" and is not accepted by just about any inspector.

And now we have a demonstration of why it is not accepted. Bruce Roe

[DavidH]

That's interesting because MidNite solar has two wires connected to the main breaker on their mini-DC. The circuit diagram is on pg 4 and then top photo on pg6. Is this something then that would fail inspection? http://www.midnitesolar.com/pdfs/mndc125_250_manual.pdf

Edit: Though this is with 5/16" lugs so is that acceptable?

[inetdog]

That's interesting because MidNite solar has two wires connected to the main breaker on their mini-DC. The circuit diagram is on pg 4 and then top photo on pg6. Is this something then that would fail inspection? http://www.midnitesolar.com/pdfs/mndc125_250_manual.pdf

Edit: Though this is with 5/16" lugs so is that acceptable?

The manufacturer of the switch, fuseholder, or breaker will test and specify how many wires can be connected to each lug as part of their UL listing process. If you follow the manufacturer's recommendations you should, with very few possible exceptions, pass inspection.
One tricky point is with ground/neutral bars in an enclosure. You can, if the listing allows, put two ground (EGC) wires under one screw but you are never allowed to put two neutral wires under one screw. The idea is that in the process of working on the neutral of a circuit you have powered off you might interrupt the neutral for a circuit that is still hot. Dangerous for the electrician among other things!

[DavidH]

Ok, that's good to know. Thanks