I have two arrays for my AC500, and for the second time one of the arrays fried the MC4 connector, leaving burnt black plastic and charred wiring.
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I can only guess that there’s too much electrical resistance at the crimp point inside the MC4, causing the fire. I have the correct crimp tool which gives professional factory looking results. The only thing I can see to fix it is to start soldering all the connections.
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I doubt it’s the weather as we get so little rain that it’s headline news when it happens. We get so few thunderstorms that the Ring Community forum lit up with reports of flashing lights in the sky. “What the hell is that?” We’re just south of Area 51 so I ran outside with my camera, looked around, went back inside and reported, “Uh guys, we call that “lightning” where I’m from.” The thunderstorm was too far away to hear the thunder. 
Two observations -
- It would help to answer your Q if you noted the Voltage and Wattage you are using through your connectors.
- Not all MC4 connectors are “good” quality. I don’t know what you are using in the U.S. but some cheaper manufactured ones are known for this issue.
It might also help if you noted cable gauge, run length, panel array size and parallel/series connection etc.
Edit, Soldering can increase resistance, MC4 connectors are intended to be crimped, not soldered. Soldering can compromise water resistance and create a weak point. (mechanical stress)
Well, the AC500 input limit per array is 150V @10 amps, (1500 Watt limit). I can’t exceed that without getting a warning from the AC500.
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Voc is 123.75V and Isc is 13.89A for three Bi-facial panels in series. Maybe they are more sensitive to the “cloud lensing” effect.
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Thing is, this array is always a couple hundred watts less than my other array using 18-100 watt panels, and it’s been trouble free.
Hi St8kout AC500 is 15A Max per input not 10. So you could be getting nearly 15A if your Vmp is around 100 Volts before you hit the 1500 watts limit of the AC500. Bi-facial panels increase the current above the lmp as the rear panels add to the current. My Bi-Facial panels are listed at lmp 13A but always used to hit the 15A max of my MPPT, and I think would go a bit higher as well.
Just something to keep in mind if you are using lower gauge wire. It’s probably what Mandp said about low quality connectors though or them getting too hot.
That post was a brain fart. I looked up the wrong panels.
What I have are the 550W bifacials. 49.95Voc and 14.05 Isc. I had to buy 4 to use 3 in series to match to the AC500.
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For the money, they were not worth it. The 18-100 watt panels for my other array were cheap at the time, something like $75/pair. They consistently produce about 1495W every sunny day. I’m lucky if the three Bi-facials get to 1350W, and this is the array that fried the 2 MC-4s.
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I use nothing but 10 gauge wire. It was hard to find on sale.