Cooker MCB/RCD

On class 1 devices, mains RFI input filters are typically so called 5 terminal devices, so live and neutral in and out plus an earth connection. The earth connection normally acting as a centre point between a pair of caps[1].

So they leak a very small amount at 50Hz (< 1mA), but that can rise at higher frequencies. So a switching transient which has more HF content may cause a brief spike in earth leakage.

Also worth noting that the filter does not actually need to be present in the device being switched to cause a RCD trip - elsewhere on the circuit in another appliance, can be enough.

Most of that is as unambiguous as mud.

NT

If I knew why a dodgy element would only cause a problem when switching off the oven I would say so.

So why don't you tell us why it does?

If the leakage is at or near the neutral end of the element, when on there's very little leakage because there's very little voltage difference between leaky spot & earth. If the switch is 2 pole, sometimes the neutral opens before the live. Then the neutral end of the element sees 240v briefly, and enough current flows to earth to trip the RCD.

NT

OP here: Indeed that would be a good test. Turning off the oven does NOT reliably trip the RCD. When the NEFF engineer was present he saw that it tripped sometimes, same with the sparky. When I've had a go, it has not tripped after several attempts.

Is that just turning off the oven heating, or has it got a switch that turns it entirely off including the display, or are you referring to a separate (from the oven) isolating switch?

Just turning off the oven.

I can say with certainty that a leaky element *can* do that, though obviously I don't know if that applies to your oven. The element(s) can probably easily be removed, and tested with an insulation tester. If faulty if could easily be replaced without replacing the whole oven. Assuming they have any good ones at all, anyway. No common insulation tester (up to, say, 5Gigaohms) should show any leakage at all at 500V.

OK, I'll bite --- why is type A missing?

And why this odd set of types?

Presumably because they thought there was too much scope for confusion with the current rating... e.g. a Type A 32A MCB.

With B32, C6 etc , there is no scope for misunderstandings.

I suppose I can see some logic for the F since that is the one sensitive to high frequency waveform leakage (as you may get on a VFD drive feeding a machine tool).

Not sure what the logic was for A and B though. (perhaps R for rectified DC, and S for smoothed DC would have made more sense?)

I agree with the MCBs, but there is already a S type RCD (the time delayed one).

So S was already used. S meaning selective.

as for A and B maybe it was just a lack of imagination? Why not start at Z and work down the alphabet?

Or a hangover from times past, like C-drive on computers. I vaguely recall in the days before desk-top computers had HDD's, there were two slots in the front, one for the system 5 1/4" floppy, the other for the programme 5 1/4" floppy, A and B (or the other way around). I think the one I used to use was a Rair.

Indeed... good point.

(Although I note that MK quite often like to stick a "s" suffix on all their RCDs just to confuse you!)

I would have used T for time delayed:-)

Are you considering the NE fault as high resistance ?

In case you're interested I'll describe my trip-on-turning-off fault to you. It was high resistance, but the mechanism was not the double pole switching one that tabbypurr postulates.

There were two grill elements I'll call A and B. Only the live was switched and the switch positions were off, A on and A+B on. The fault first manifested as intermitten trips on turning the switch from A+B to A. It then became reproducible trips every time on switching from A+B to A. Later, (when I had to fix it!) it became trips every time A was furned on from the off position. I never tested turning the isolating switch on with the switch in the A+B position, that might have been instructive. Or not.

I didn't work out what was wrong and initially changed element A. This had no effect and the removed element A seemed intact (>2Gohm at 100V, I somehow acquried a comms type Megger that provides 10-100V, a rather specialised instrument!).

When I changed element B it completely cured the fault. Element B had a resisance to earth of about 40kohms at 100V as I recollect.

My best theory is that the leak was very near the neutral end, and with ilive switched on B the leackage from neutral and live was about equal, but from neutral only when B live was switched off. And as it progressed the leakage got greater even when B was not hot and the rise in neutral voltage on turning element A on was sufficient to trip. Had it progressed further, it would presumably have tripped even with element A off, but I didn't leave it to happen. I could have checked the element potentiometrically to see where the low insulation resistance was along the element if I had a very high impedance voltmeter, but I'm afraid I didn't.

I am quite prepared to be contradicted, if you have a better theory to explain the fault.

What the N-E resistance needs to be for this scenario to play out depends on where it is on the element, ie what the voltage difference is between fault & earth. If there were eg 10v between the faulty patch & earth, to get under 20mA rms flowing while powered would require at least 12k? of resistance.

When the neutral feed is disconnected, but live still connected, that 12k would then pass... 480mA rms.

NT

None of which makes sense when it is only the live that is been switched.

It only applies to 2 pole switching, or 1 pole if some genius put the switch in the neutral. I guess in principle it might also happen with a dodgy neutral connection. And also for elements - there must be a formal name for them - where a switch puts sections of them in series & parallel to get different powers.

NT