Since 1993 I spend all my holidays on a small farm in upper Austria. 230/400V 3-ph service, local ground rod, neutral and ground seperate. Panel about 20 years old, completely makeshift. Contains a 100mA 4 pole main GFI.Several single and 3 pole breakers. (btw I don't believe the single poles were allowed back then, but I guess double poles fusing the neutral wouldn't have fitted the already very cramped panel) One of the 3 poles feeding A small gas station, the 2nd feeding the electric range, the 3rd feeding a detached garden house several hundred feet away, fed via UF. Then some 13A breakers feeding lights and receptacles. No neutral busbar, some neutrals put directly under the load terminals of the GFI, most of them together in 2 giant block connectors. Three hot busbars (combs) feeding the various breakers. A complete mess of wires. Blue wires, grey wires, brown wires, black wires, violet wires,... all connected to the breakers. The entire panel has a wooden frame and a grounded metal cover which is only about 1 cm (3/8") away from the hot busbar.

Last year we had some heavy rain and around 11.15 pm the GFI tripped. First guess was lightning, which happens frequently out there. When we tried to reset the GFI it tripped again. So we pulled every plug and tried to reset the GFI. It tripped again. I got furious and threw every breaker. Now the GFI would reset. Then I reset the breakers one by one. When I came to the one for the garden house the GFI tripped. I left it off, reset the breaker and started to put back the plugs. When we hooked up the freezer the GFI tripped again. We didn't want to have the contents of the freezer to decay, so we put some rubber mats from a car under the feet of the freezer to avoid any ground path. Then we told everyone not to touch the freezer. The next day it was sunny and warm. When the electrician came and meggered everything he found... nothing. We pulled the mats out and everything worked. This summer we came back and were told that over the year nothing had happened. I was already a little sceptic, because the kitchen had been remodeled in the meantime. And one day I woke up because one of my brothers and the farmer rushed into my room pulling all plugs. Not hard to guess what had happened. Now there appeared to be two faulty circuits. The one for the garden house and the 1ph circuit feeding lights and receptacles in the kitchen and two bedrooms.
After some minutes the Gfi could be reset but would trip again without any apparent reason. Additionally each time we plugged in the freezer it would trip again. When my mom switched on the electric range (2 weeks old) it would also trip the GFI. Hmmmmm. I took off the panel cover and looked around. Sure a mess. Lotsa wires that were stripped too long, two or three wires under one breaker terminal, let alone the ground busbar. But still, no visible reason for the GFI tripping.
Now have a nice time.