Bit of a problem in a heat wave!

So, what causes the demise of a wiring accessory that is being used at 30 or 40 degrees C? What actually fails inside?

Z.

Don't think that 30 or 40 degrees is the issue.

Best guess is cheap 20A DP switch (what make?)

Otherwise poor screw terminal tightening.

The airing clothing and bedding will protect the wall mounted switch from the heat of the tank and pipes. It will act like a thermal insulator.

Z.

The airing clothing and bedding will protect the wall mounted switch from the heat of the tank and pipes. It will act like a thermal insulator.

But also restricting the heat that can be lost from the switch - like a cable in a thermally insulated wall.

Also thermal insulation isn't quite like electrical insulation in that with electrical insulation we tend to think of it as a practically perfect insulation - once you've got a thin layer of electrical insulation the amount of 'leakage' is negligible so we can ignore it - but thermal insulation is much more of a 'semiconductor' - it may half or quarter (or whatever) the amount of heat leakage, but never entirely eliminates it. The net upshot of which is that you get a temperature gradient across thermal insulation - not just deliberate insulation but all materials - so the switch will have an ambient temperate somewhere between that of the tank and that of the outside, depending on the ratio of thermal insulation on each side of the 'sandwich' it finds itself in.

   - Andy.

The problem of running warm, 30 degrees outside and 30-40 degrees hotter inside at the terminals is dependant on materials - if switches were still made of ceramic and contacts of brass, then not a lot. Maybe the brass tarnishes, but until it gets so hot that contacts lose their 'spring' it is not an issue.

But some plastics soften when warmer than 100 or so,  and slowly distort reducing the pressure on the mating contacts, maybe then raising the resistance and the self heating. With extreme over heat, some materials may char and become sooty, and conducting,

The current rating of a switch is decided n by looking at the the end of life (so worn) contact resistance, and seeing if that leads to unacceptable heating. If it doesn't then the value engineering team  say the rating can be increased, or the contacts made smaller...

Mike.