Unexpected EHT arcing on PSU PCB

Here's a weird one, woke up this morning and noticed the power had tripped in parts of the house. In fact, oddly enough, two MCBs had tripped. One 32A breaker which serves one ring main, and a 10A breaker which serves a spur to the dishwasher and washing machine in the kitchen. On resetting the breakers, the washing machine showed nothing except the mains neon.

The machine is a Bosch WFP-3300 (240V)

Looking at the power supply board, there were a couple of inductors burned out, bit the most striking thing is that it's clearly flashed over at several points. The one shown here on the track side of the board has arced over an 8mm gap which means it must have generated something like 8kV somehow!

I've never seen anything like this before and am a bit puzzled as to what happened there at some point in the depths of the night while the machine was sitting there on standby. Something must have failed to cause this to happen around the chopping transistor area but I'm at a loss to explain exactly what.

Incidentally, I'm not looking at attempting to repair this board. Even if replacing the inductors (tested OC) got it working, I wouldn't be happy putting it back in with what it's been through. I've bought a new board for EU40 from Germany which is very reasonable and considerably better than the EU400 I was quoted here in the UK.

 

I think 8kV is very unlikely? Is it not just heat that's damaged the board over time or possibly a little water ingress?

 

Very unlikely I know but it has all the hallmarks of an EHT discharge. There are small chunks taken out of the component pads and as you can see from the second photo, a deposit on the casing which was directly below the PCB.
As it turned out, the seller of the replacement board would not ship to England and cancelled my order. I replaced the OC inductors and the PSU now runs and generates +5V and +12V. The front panel now lights up and initial tests indicates that it is going through all the motions except that all three water admit solenoid valves open as soon as the door interlock is closed. There are controlled by a row of 12V relays.

 

That's normal for 240V. If current is interrupted, and there is sufficient heat and metal vapour, arcs upto 1cm and more can form. The thin traces, solder etc. also will vaporize, for this reason it will spread out and blackened areas will result.