In some cases it could be (and is in fact what I did on my own CU to move the kitchen off the 30mA trip RCD and give it its own). However with the setup I am guessing is here it might be tricky to get the RCBO in there so that its not still being fed from one of the existing RCDs.
ISTM that a check on the oven circuit polarity, and insulation resistance test on it might be a good start before looking further. Also testing that RCD with a "ramp test" would be worthwhile.
And a 7880s RCD is a bog standard AC type.
A ramp test would be interesting.
Seems to be an echo in here this morning John:-)
Older ovens used to have lazily designed electronics power supplies using a dropper capacitor from the mains. If this is the case, on switching off a transient from discharging the DC supply at the bottom of the capacitor to earth could pass through the capacitor via the (now disconnected) live circuitry of the oven which may be fairly low resistance to neutral, and via the intact neutral oven connection through the RCD to incoming neutral. This probably should not be enough to trip if the RCD is not already carrying a leakage current, but would be a design fault. It is true that an RCD insensitive to unipolar currents (?? type A) would prevent this tripping, but you shouldn't have to change it!
An annoying workaround would be to always turn the oven off at the isolating switch on the wall.
Why do you need a main switch when all the cirtuits are RCD protected? Is it just to isolate the live busbar instead of using the main supply switch, if any?
There need not be a main switch as part of the CU but AFAIK you do need one.
Not true of course.
Here's the rest of the CU:
I should add that the red lights are a result of the flash on the camera, and they are not actually illuminated.
It does assume the oven has a mains switch in the live supply which, come to think of it, my last two cookers didn't have. If it does, it
*could* be true.If the OP is just talking about switching the oven heating off, with the electronics and switches still live, then it is clearly a leaky element, which could easily be checked with a Megger or similar with a bit of dismantling. My old grill did precisely this, tripped on turning off one of the elements. I also didn't think if through and changed the one that *wasn't* being switched off first!
From the picture he has a type AC RCD, and they are insensitive to unipolar currents. So switching to any of the other types would make that kind of trip more likely.
Ok, sorry, I got type A and type AC the wrong way round. I'll go with everyone else that it is a defective component, very probably a heater element, especially given that it happens on switching off.
Switching on and tripping would suggest a defective heating element.
Absolutely, but having recently investigated a switching *off* and tripping problem in my cooker I know that a leaky element can do this! I can't explain it convincingly, but perhaps the leak was so near the neutral end the leakage was equal in both wires when switched on. The observation, and the cure with the new element, was completely consistent though. The measured leakage was quite low, about 50kohms at
100v when cold. Which really means nothing much about when it was hot at 240v except that it wasn't a dead short.
It can't possibly be true. Capacitor power supplies don't run live to earth, but live to neutral. And no new appliance passes enough i to E to trip an RCD.
And there's nothing lazy about capacitor PSUs.
NT
Yes. So does tripping on switch off. I'll leave it to the electrician to explain how.
NT
Dodgy heater elements are well known for RCD trips, however I would expect it to do it either at switch on, or while running. Not necessarily at switch off. So while a dodgy heater element is possible, there are other possible causes.
Turn off all the other MCBs on the left side of the CU, then see if turning the oven on/off still reliably trips the left RCD, will tell you if some other circuit is taking the RCD to the brink, or whether the oven alone can trip the RCD ... might save finger pointing between John Lewis and the sparky
I'm curious to see whether Adam understands how/why it can happen.
leaving the 'oven' MCB switched on
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