Melting wires, Why?

[chilidog]

Hello,
I am away from my home at the moment so I dont have access to my inverter or panels to get all the info. I am going off what I remember when I set the
system up. I set up a 5K grid tie in system and put up the right amount of panels(watts Volts, Amps,ect), followed all the manufacture steps. I have had the system running for 6 months with
not problems and everything working fine. In Thailand we are in the hot season were temperatures in my area can be 100-106f, 35-42c. I have 2 strings in parallel with 6mm wire going about 10-13 meters. I have a breaker box for each string and 3 different breakers in each box, I cant remember what they are but one is 16DC and 2 others. When the temperatures came up
I noticed that one of my negative wires running to my breaker box from one string had gotten hot and melted the MC4 going into the breaker box and no breaker tripped. I have check the wires coming from the panels and all looks good. What would cause the negative wire to become hot enough to melt the MC4 connector? At midday I touch the other wires and they all feel normal except the negative. At morning or evening when its generating less watts the wire is normal temp.

Thanks,

[inetdog]

One thing that causes wires to overheat locally and even melt insulation is a bad (high resistance) termination. It can be a screw connection, wire nut, spring pressure, or crimp, but if for any reason it has a high resistance it can overheat the connection itself and wire running several inches from the connection. The problem will be worse if the wire is already carrying close to its rated amperage.

[jflorey2]

What would cause the negative wire to become hot enough to melt the MC4 connector?

Bad termination. Bad crimp, or wire not inserted into screw terminal, or incorrect lug, or aluminum wire used in Cu rated terminal, something like that.

[bcroe]

Perhaps the MC4 is the cause. These are not very robust, and the contacts easily oxidize
if exposed to the elements. Then you have increased contact resistance, which causes heat,
which increases resistance more, till failure. I managed to heat an MC4 to burn out that way.
Bruce Roe

[solarix]

Not all MC4 connectors are quite compatible. There was an article in SolarPro a few years ago where they found at least a dozen different brands and there were some combinations which did not mate very well. Even at best with these connectors there is only maybe a 1/4in of engagement of the contacts despite the contact pins being about 1/2in long.
Plus if a connector is unplugged under load (often times there will be labels on the panel wiring warning not to do this), an arc will result in a burned spot on the pin which you can't see and which will lead to poor conduction and overheating. I've seen more than a few melted MC4 connectrors.

[chilidog]

The MC4 connectors that came with the inverter (Zever) were pretty good quality. I ordered other MC4 connectors from online that were not as
good of quality, so that might be the issue. Could be the Chinese made connectors and explains why only one connection melted. Its have be a bit hard to source out better quality ones here.
I never unplugged any of them under a load.

[bcroe]

MC4 connectors
I never unplugged any of them under a load.

Good, because that is about the quickest way to ruin them. Bruce Roe

[chilidog]

Well I am away at work I have another 12 more days before before I get home. I talked with my wife and she had unplugged the connectors at the solar panels and I am
pretty sure it would have in under a load. I will just have to see what happened when I get home.

[chilidog]

Hello,
So I have an update and more questions. I arrived home yesterday and got a look at the problem. I activated one of the strings of panels that was off and was looking at working on the string with the melted wires. The string with the problem is not plugged in and I was testing it to make sure there was no power before working on it, but there was still 240volts from the inverter to the breaker box. I only have basic knowledge of electricity and I am wondering why there would be power coming from the inverter the breaker that is shut off and the string is off. Im wandering why nothing tripped when There was a problem and why I am still getting power on either side of the breaker with the power off on the string.

[Mike90250]

There is the possibility the breaker is bad, or that another AC wire is paralleled in bypassing that breaker. Without AC, the solar inverter should shut off.

Did you install this or an electrician ?

[inetdog]

As Mike said, one possibility is that there is another breaker (or wire) in parallel with the breaker you are looking at. Confirm that there are no batteries in this system? What is the brand and model of the inverter?
With no power going into the inverter there can be no power out. And with the breaker truly open the only explanation for the power on the load side is a parallel connection somewhere. Is is possible that the inverter output was wired to two breakers in parallel instead of getting a single breaker of the right size?
Can you open the main breaker of the panel to test?

[michaelpapa7]

Perhaps the MC4 is the cause. These are not very robust, and the contacts easily oxidize
if exposed to the elements. Then you have increased contact resistance, which causes heat,
which increases resistance more, till failure. I managed to heat an MC4 to burn out that way.
Bruce Roe

I just replaced 140 connectors, 20 of them were welded together from heat. One connector was Mc4 that was on the positive side of the solar edge optimizers the mating connector from the Canadian panel did not have a name on it and was rubber coated. All melted wires were on the positive side of the optimizers. My system as and still is showing code 25 faults on both strings. I’m about to junk the whole system that is only six years old. My inverter is also solar edge.

[bcroe]

I just replaced 140 connectors, 20 of them were welded together from heat. One connector was Mc4 that was on the positive side of the solar edge optimizers the mating connector from the Canadian panel did not have a name on it and was rubber coated. All melted wires were on the positive side of the optimizers. My system as and still is showing code 25 faults on both strings. I’m about to junk the whole system that is only six years old. My inverter is also solar edge.

With about 300 mated MC4 pairs, I have managed to burn out around 3 in
8 years. Not counting a 6 gauge wire at 21A and an AC breaker. My conclusions

Its good to tighten (in the dark of the night) all screw down terminals every half
year for a couple years, then they seem to stabilize. and

MC4s have a better chance if some NoOx is used on the contacts. And some
of that blue automotive form-a-gasket around the insulation clamp area may help
assure the seal really is water tight. NEVER leave an MC4 exposed to the
elements. I keep extras in a sealed jar.

Bruce Roe

[michaelpapa7]

Thank you for your feedback, I like the no nox idea, I did apply a small amount to the seal and nut. Without lube the nut would strip before completely tightened, after applying a small amount I was able to tighten the nut until it bottomed out. I did not put it on the threads only at sealing area. All Mc4 connectors.