Strange Cooker Issue

[morpheus83uk]

I have a strange issue with my cooker but I dont think its to do with the electrics in the house but thought I would open it up for discussion as were looking for another oven.

Whenever the oven is being used for anything normal then its fine however after prolonged use such as for a good few hours 4 -5 hours doing meat for example it seems to trip the electrics. Not just once though it does it for a while randomly. For example it has done it instantly when I have put the electrics back on and then again and again 3 or 4 times after that. It has then been OK for a while and then tripped again and this can happen for a good while until the oven has been switched off.

It doesn't trip just the MCB which is labelled cooker but the RCD trips every time so all the sockets downstairs go off each time this happens. A few people things its the cooker as it only seems to happen with the cooker on for a long time and seem to think its a manufacturing fault.

Does anyone have any ideas on this?

Thanks

James

 

[SimonH2]

Sounds like there is a small amount of leakage - such that when combined with that from the rest of the circuits it's "about" at the level needed to trip the RCD. Just a small change will change between "works" and "trips".

It's actually quite hard to get that level of leakage by accident, it requires a resistance between live parts (which can include neutral) and some earthed metalwork.

 

[morpheus83uk]

Oh right so you think its the cooker as well then which is causing the leakage? So it may well be the appliance which is causing the fault not the electrics?

 

[flyingsparks]

I have had a similar issue before with a storage heater. After about 5 hrs it would trip the RCD. The element tested at 200M ohm when cold but after all that time it dropped to 0.01M ohm and tripped the RCD. Strange as usually elements are going to increase in resistance if anything as they dry out.

So my guess is it's the oven element which needs replacing. Probably cost you about £20 for the part, and usually you can access them from inside the oven so easy to replace..

 

[Taylortwocities]

If the MCB and RCD do not trip when the cooker is not being used, then the fault would appear to be with the cooker.
However
It it also possible that there is an inherent fault in the wiring of the cooker circuit itself, so that the problem occurs when the circuit is under load (i.e. when the cooker is switched on).

Needs a bit of detective work to track this sort of fault down.

 

[SimonH2]

But if it was a fault in the cooker circuit itself (eg a N-CPC fault) then wouldn't that normally show up without the oven being on and/or immediately it's turned on ?
I'm with flyingsparks, my guess would be damp in an element. Does sound counterintuitive that the resistance would drop after it's been hot for a while, but strange things do happen.

 

[morpheus83uk]

After some testing I found that this was a loose neutral wire in the counter side switch. Once this was tightened the issue went away.

Thanks for all your help.

James

 

[SimonH2]

That's curious - since a loose neutral shouldn't cause such an issue. Unless there is a bit of N-E leakage which is only enough to trip the RCD when the N-E voltage goes up (as it would do with resistance in the neutral connection).
I'd suggest it's worth doing an IR check on the oven - particularly N-E.

Hmm, thinks a bit ...
Leakage in the element (very) close to the neutral end could possibly do it.

However, if it's been running with a loose connection in the switch, then you need to look carefully for heat damage. It doesn't need much resistance to create significant heat which can severely damage the switch and/or cable.

 

[JohnW2]

That's curious - since a loose neutral shouldn't cause such an issue.

It's very curious but, for whatever reason (much debated and theorised, so no point in repeating!), it does seem to happen - IIRC, RF even has a video of it happening!

Kind Regards, John