Oven trips RCD when turning it off

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I have a Logik oven - probably about 10 years old.
If the RCD is on, switch on the wall is on and I switch the oven OFF - it trips the electric.
If I switch the oven off at the wall, it doesn’t matter which direction the on/off switch is facing, it doesn’t trip.

Any initial thoughts before I get someone out - I’m not very good at DIY
 
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Quite possibly a failing element. I had exactly this with a previous oven.
Ditto.
FIL's oven did this and guy he called out disconnected the element as the fix. It took a while to work out why the grill Hi/Lo switch made no difference.
 
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Neutral earth fault ....
Quite possibly a failing element. I had exactly this with a previous oven.
We do hear reports of this phenomenon and have discussed it at length in the past, but I don't think anyone has really come up with a particularly convincing (or even nece4ssarily particularly credible!) explanation of the mechanism whereby 'switching something off' seemingly can cause an RCD to trip.

As for eric's suggestion, I don't see why (whatever the mechanism) it has to be due to an N-E fault, and as for slippyr4's suggestion, whilst failing elements can certainly trip RCDs, I've yet to see any reasonable explanation as to why that should happen only when it is switched from on to off.

Kind Regards, John
 
I have to agree with @JohnW2 there is no reason I can think of why any RCD should trip when items turned off with a single phase supply.

However neither can I see any reason why turning MCB's on one at a time does not trip the RCD, but turning all on together i.e. using isolator the RCD trips, but I know this does happen.

I blamed spikes on the supply, for many of my RCD trips, as could not find any leakage testing with an insulation tester, I do intend to buy one of these 1661496980560.png which on the 2 amp AC range will show 1 mA so can actually measure the leakage, but even this cheap model costs around £40, the same with one of these 1661497188840.pngat £32 even the cheap testers cost a lot of money, lucky I have a couple of insulation testers, but without them the DIY guy is unlikely to find the fault. And even with an insulation tester I failed to find out why my RCD would every so often trip, so on moving I went all RCBO's. Not sure that helps, as when they do trip no idea if due to overload or earth leakage.

I know you have said your RCD's have only tripped with a known fault, but mine would trip maybe 5 times in two weeks, then without doing anything to cure the problem, would go for 2 years without tripping, when my daughter got rid of a boy friend some 20 years ago, the number of trips reduced, seems likely some thing to do with his PC's.

But my RCD's were fitted in the 90's, they were 4 module wide, and before we used electronics in them. No marking for type AC, A, F, or B, we had only just got rid of the ELCB-v type, all it says is 30 mA. It passed with a ½ leakage test, but two identical units, one would allow the use of my loop impedance tester (Non RCD trip type) but other would always trip.

I did consider changing the RCD these were allowed in domestic
at the time, auto resetting, darn expensive, but did consider fitting one. Now not permitted in domestic, but the X-pole RCD was claimed to only trip 80 - 100% rather than 50 - 100% and was claimed less likely to nuisance trip, and when we first started fitting 30 mA rather than 100 mA RCD's nuisance tripping was a problem.

But RCD's have got better over the years, also things like filtered sockets and power supplies no longer leak to earth, early days of PAT testing many items failed on the earth leakage test.

We tend to consider our own situation, in my case a TN-C-S supply, so neutral it tied to earth potential, with a TT supply likely different as to what faults will trip a RCD, and of course how important the RCD is, with a TN-C-S supply the RCD is secondary protection and I am not worried that all mine at type AC, but if I was on a TT supply would want to change them.

On another site some one posted this 1661499278717.png set of instructions for a boiler, first time I have seen type B RCD recommended for a boiler, seems it is a "Veissman, vitodens 100-w" but with my make of consumer unit I could not fit a type B even if I wanted to, can't get single module width RCBO's type B although they did say type B on the packet, but turned out to be curve B type AC.

I am not convinced that type AC will fail to trip, and that 6 mA detectors for DC are really required, however it is clear the RCD's on the market today are very different to those of 1992 when I first fitted them, and we have no idea of the age of the RCD in question, or what faults if any the oven has.

However since test gear costs so much, and using it can cause danger, likely the best option is get some one with the gear to test it, and see what the fault is.
 
Definitely talking absolute rubbish :)
But with an oven filament (probably dodgy, poor insulation resistance etc, i.e. existing earth leakage), could there be enough back emf generated (from the element), to create an additional unbalance in the circuit (and RCD trip), if only the live is switched?
...and thus wouldn't trip if the double pole wall switch is used instead.
 
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My theory. Switching off, with an aging switch, can cause arcing at the switch contacts. That arcing will have high frequency components. Any suppression capacitor(s) in the load circuit would have a lower impedance at those frequencies than at normal mains frequency. This might allow sufficient current to pass to ground via the cap(s) to trip an RCD.
 
Or how about a fault with leakage to earth when hot (Grease?). In use the imbalance is not enough to trip the RCD.
One pole of the isolator breaks sufficiently slower than the other and the increased imbalance is enough to trip RCD.

Yes barrels and scraping springs to mind but we have to be looking somewhere but I've known less likely faults.
 
Or how about a fault with leakage to earth when hot (Grease?). In use the imbalance is not enough to trip the RCD.
Interesting idea :)
A different non-domestic situation, but we have set up a special PA Test on a couple of troublesome ovens, where they are run up to temp for a couple of minutes on an earth leakage test, before performing an IR test (which it would marginally fail when cold).
Running at temp evaporates off some of the moisture that has been absorbed by the hygroscopic insulation.
So my experience is that hot ovens have better IR than cold ones :)
 
Definitely talking absolute rubbish :) ... But with an oven filament (probably dodgy, poor insulation resistance etc, i.e. existing earth leakage), could there be enough back emf generated (from the element), to create an additional unbalance in the circuit (and RCD trip), if only the live is switched?
...and thus wouldn't trip if the double pole wall switch is used instead.
My theory. Switching off, with an aging switch, can cause arcing at the switch contacts. That arcing will have high frequency components. Any suppression capacitor(s) in the load circuit would have a lower impedance at those frequencies than at normal mains frequency. This might allow sufficient current to pass to ground via the cap(s) to trip an RCD.
Or how about a fault with leakage to earth when hot (Grease?). In use the imbalance is not enough to trip the RCD.
One pole of the isolator breaks sufficiently slower than the other and the increased imbalance is enough to trip RCD. ... Yes barrels and scraping springs to mind but we have to be looking somewhere but I've known less likely faults.

The above are all 'not impossible' theories/speculations but, as I said, we just don't know. As I wrote ...
.... I don't think anyone has really come up with a particularly convincing (or even nece4ssarily particularly credible!) explanation of the mechanism whereby 'switching something off' seemingly can cause an RCD to trip.
One possibility that has not (recently) been mentioned is that it may be nothing to do with an L-N current imbalance ('residual current') through the RCD but, rather the result of transients (e.g. from arcing, per Alec_t) interfering with the electronics inevitably in RCDs these days, thereby causing it to trip despite there being no 'L-N imbalance' in currents passing through it.

Kind Regards, John
 
Sorry I missed the continuation of this thread.

I suspect that the reason for the trip to occur at switch off is that the mechanical design of the main oven switch (which is double pole) in practice switches the neutral out of the circuit first, fractionally before the phase.

In that moment, the entire element is at line potential and a fault to earth (the element casing) towards the neutral end of the element suddenly carries more current to earth than it did when the element is loaded. Remember, the element wasn’t open circuit.
 
I suspect that the reason for the trip to occur at switch off is that the mechanical design of the main oven switch (which is double pole) in practice switches the neutral out of the circuit first, fractionally before the phase. ... In that moment, the entire element is at line potential and a fault to earth (the element casing) towards the neutral end of the element suddenly carries more current to earth than it did when the element is loaded. Remember, the element wasn’t open circuit.
That's an interesting theory, which I haven't heard before - but it makes total sense. We've heard of a few cases in which switching off a DP switch has resulted in RCD operation (but that that can be 'cured' by changing to SP switching) - and what you suggest could explain that.

Kind Regards, John
 

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