Oven tripping RCD

[ARW]

Actually operating the oven without RCD protection would actually show if it's a LE or NE fault/

[Andy Burns]

Dragged it out today, disconnected all elements, it would run cold without tripping, high resistance from elements to earth while still cold, reconnected elements and after warming up on fan setting for a couple of minutes it tripped, disconnected elements one by one and found the bottom element is the culprit (the most expensive and for me the least used).

While warm, the earth resistance from the dodgy element was 60k, after leaving running for 20 minutes to get decently hot, it got up to about

200k, for now its wires are taped-up and left disconnected.

[Andy Burns]

Flipped the oven over and got the element out, it looks as clean as a whistle ... there seem to be only three in stock in the UK (several of the espares/4neff/partmaster websites are the same thing in disguise) all want about £50 for it.

There's one available from Holland for £13, but as mine is stamped 240V, I guess that one might actually be for 220V, so much for harmonised 230V?

[Fredxx]

Given the small 20% possible difference in power, I'd still go for it.

[Andy Burns]

Seems the Dutch website is scraping items from German eBay, and it's secondhand, so I'll pass ...

[Roger Hayter]

Any resistance you can measure with a multimeter is definitely defective. I would expect it to be gigaohms even at 500V.

[Andy Burns]

Not sure about gigaohms? I've got no way to distinguish between

100-200M and infinity.

when cold and at 9V (or whatever voltage my multimeter uses) the most I've seen is 11M, but shortly after a trip the multimeter has variously read between 60k and 250k, I'd expect it to have to get down to 7k or so to trip the RCD.

At 500V my insulation tester shows under 100k (impossible to tell how far under as the next minor graduation on the scale is zero and it wobbles around in use)

Anyway, with the variations in insulation between cold/warm/hot I'm convinced it is buggered, even though it looks ok.

[Roger Hayter]

That's what I was getting at. If the insulation tester shows anything below maximum resistance with one of those mineral insulated elements then the element is defective, and will have increasing leakage with both time and temperature. If one is lucky enough to find this out before it fails completely one should immediately change it.

[ARW]

Of course it had to be the most expensive one:-(

It probably has moisture in it.

As Roger said. We both would expect almost infinity at 500V with an insulation tester.

[Andy Burns]

I wouldn't mind if I could see that it was corroded or pitted or had burnt spiders between the terminals and the outer jacket, but it looks perfect, it's underneath the main oven cavity so no fat/steam would get anywhere near it. Replacement is on its way.

[ARW]

So is my new cooker. Bottom oven (fan assisted) element packed in a few weeks ago. The grill element packed in a few days ago.

Actually both are available at a reasonable price and look easy to replace. However when fetching in the new settee yesterday (tight space) the glass on the main oven got smashed.

[The Natural Philosopher]

Those elements simply fail internally, often due to thermal cycling opening up a pinhole and then moisture gets in.

I've replaced more than one. They always look just perfect to me. But a meter shows otherwise.