Creda M350EW tripping RCD. Help confirming diagnosis.

I recently had my fuse box ( yes, fuses ) changed to a new consumer unit and now I have a problem with the ( admittedly ancient ) cooker.

The main oven and the rings are fine. The problem is only with the grill function of the top oven/grill. The top oven is good.

The symptoms are that the grill trips the RCD when it is bright red. No matter what the temperature setting, the time from when the element glows very dull red to tripping ( bright red ) is about the same ( I'm only eyeballing it so there is a margin of error but I'm reasonably confident that this only happens when the element gets to a particular temperature ).

So to my mind this shows that the grill element has a leakage to earth ( insufficient to blow a fuse but enough to trip an RCD ). Looking at other posts here, an element tripping an RCD is a common failure. And this element is very old.

But is there any way this could be the thermostat as the failure occurs when the element gets to bright red and maybe the thermostat is switching it off and that is tripping the RCD? Would the thermostat behave differently between the top oven and grill ( same control )? Does a thermostat ever fail like this?

Thanks.

I doubt it is a problem with a stat. much more likely as you suspect a faulty element or the wiring to it. Is the temp of the grill variable as if it is & it does not trip at lower temps. then it can't be the stat. that is controlling the temp. Also check there are no wires above or behind the casing that might be shorting to earth when very hot.
Are you still in contact with your electrician as he should be able to do an insulation test of the element that would definitely identify a fault in it.

Thanks.

It doesn't seem to matter what temperature is selected for the grill, the RCD trips soon after the element gets to bright red - even with the temperature set to the maximum.

I'll check for other obvious causes ( wiring etc ) and then get a pattern element which should only have to last a year or so as the plan is to replace the entire kitchen in the very near future.

Thanks again.

I would not exclude the thermostat from possible causes.

When thermostat is cold the contacts inside are closed, the moving contact is touching the fixed contact ( terminal ) that supplies power to the element .

The contacts only open when the oven has reached the required temperature.

When the contacts open the moving contact might come into contact with something that provides a path for current to Earth. This current to Earth could be enough to trip the RCD. That "something" could be grease, moisture or other debris from the cooking process that has leaked out of the cooking chamber.

The RCD can trip if either line or neutral leaks to earth, with the latter it can make one thing some high powered item is at fault when it is not, because only when a lot of power is used is the neutral and earth at different voltages.

So in real terms you can't test high powered stuff without meters.

Thanks all.

I don't have a megger so I can't test the insulation ( except with a normal multimeter ) but it all looks good. I've ordered a pattern element as it's only £10. If I subsequently have to replace the 'stat, I'm then only down a tenner this way round.

The replacement element arrived today.

I removed the old element and my confidence in the diagnosis went almost to 100%. I spotted a patch of overheating in one corner of the element.

Anyway, got the new one fitted and it certainly seems to have fixed the problem.

Utter nightmare to do though. Massive amounts of disassembly and the rear panel supports all the internal gubbins so when the screws were removed the internals dropped about 20mm.

Interestingly, I could find no difference between the original ( factory ) element and the pattern part. With pattern parts I've usually found that unimportant details differ, but not on this. And the screw holes were perfectly aligned - which is always nice. Perhaps they came from the same factory; just without a fancy branded box!