Oven tripping RCD

Assume you mean it's the over heater supply RCD tripping.

It could be the thermostat, but could also be a heater element. If the oven light is run off a spur from that supply, it could also be that circuit. What make/model, and is it possible to set the heating to separate use for top or bottom elements of the 40 cm oven? If so, and it trips with either one, then it's not an element, so could be the thermostat or something else (oven internal wiring insulation faulty?). If it trips with just top or bottom, then it's whichever one does the tripping, or its associated wiring.

Most likely is a leak from one of the elements to earth that is OK whilst the element is live at one end and grounded at the other but can leak enough current to trip and RCD when the whole thing floats high.

It might also be that the insulation is OK when cold and dodgy when warm.

You need to measure the resistance between earth and live and earth and neutral to see where the current path is. You might need to get it hot enough to trigger the thermostat and then take it off power and make the measurements with the elements hot and thermostat open. Be careful!

I have seen similar things with water boiler and immersion elements that would intermittently trip RCDs and then self heal for weeks at a time (at least in a hard water area).

Sounds like a leak from ground to neutral or something. One assumes if you turn the thermostat down while its on that the same thing happens no matter what the temperature? Brian

I don't think you can assume the thermostat... Generally if a RCD is "sensitised" (i.e. running very close to its trip threshold due to high leakage) then any transient (like that from switching off a large load) can be enough to push it over the edge. So it could be that the small oven element is "leaky" but not quite enough to trip it outright.

You may find that a few repeated heatings will actually make the problem go away.

If you have access to test equipment, a ramp test[1] using a RCD tester on the circuit might show how much trip "headroom" is left. Or an insulation resistance check on the small oven element would also let you eliminator (or otherwise) that as a possible cause.

Most RCD testers can do a test that ramps up the leakage and then records the (added) leakage current at which the trip occurred. It can give an indirect indication of how much leakage is on a circuit. Another approach is to use a high sensitivity clamp meter on the earth conductor of the appliance in question - that will also show a direct measurement of the leakage to earth. (or used round the L&N will give a direct reading of the imbalance)

In general oven elements have a bit of a reputation for problems like this - it only takes a small crack in the casing of the element to allow moisture into the magnesium oxide insulating material. That then changes it to magnesium hydroxide which is electrically more conductive. Heating it can drive the water out though.

You may find some extra information here:

I also cannot see how it could be the thermostat.

That's a good theory John.

Only other plausible stuff I have met is when a DP switch switched the neutral just before the live. I doubt the thermostat is dp switched but there could be a LN reversal at the oven.

Thanks for ideas.

To answer some questions....

It?s a Bertazzoni MAS100.

Yes, it trips when turning down the thermostat below the current actual temp - in fact that it is all it has done so far as I found the issue when I was turning down the dial to hear the ?click? to work out what was the temperature of the oven to see how near it was to temp (the 40 cm oven does not have a temperature gauge).

I can run different elements separately so that?s a v good idea to test. Not right now as household will not appreciate late night power cut!

I had in mind thermostat as it trips on power off rather than power on, so I thought maybe a nasty dirty contact (lack of use) causing a transient. Element presumably fixable from front, so hopefully that and I?ll save having to pull it out

Don't bank on it, my oven has a rear fan element, a top grill element and a bottom element, I essentially only ever use the fan, yet the bottom element was the one that needed replacing, whole oven needed taking out of housing and flipping over to get at it from below ...

Right. That does not mean the stat is at fault though. If the stat is in the neutral path, a hot leaky element that just lost its neutral might well trip

I emphasise that whatever you think it is, element leakage is so common compared with anything else you might as well replace it anyway,

I think that your reasoning is not sufficiently detailed. Also, it could be something else on the circuit leaking and tripping on a transient.

a neutral earth shirt somewhere could cause this.

My advice:

whip out the element or at least disconnect it and look for earth leakage.

If the element is at all suspect, replace it (cheap as chips) and try again.

Else look for another fault on your circuit. As I said a neutral earth fault anywhere casn cause all sorts of stuff to trip an RCD.

Method there is to go into CU, switch it off, and disconnect neutrals one MCB at a time and look for shorts.

is that a black & green or a blue, yellow and green shirt? where can I get one! :-)

It's not full length - it's a cutty sark.

One thing you said in your first post is that the oven that is tripping has probably only done 5 hours in a couple of years.

Elements have an inner coating of Magnesium Hydroxide that absorbs water. So it is possible that that there is some damp in the element causing a LE leakage.