Cooker circuit occasionally knocking out MCB

[alan333]

Hi,
My cooker circuit occasionally trips - usually once every few months, but occasionally more often. I have a standard 45A cooker switch with a combined 13A socket in the kitchen, where the microwave is plugged into. I have a range cooker with an electric grill and two electric ovens (and a clock and a sparker for the gas hobs). Sometimes it trips when using only the microwave, sometimes with only the grill, or with only the oven. Normally you can have all three on with no problems. We don't use the second oven except Xmas etc, but it didn't trip then either (ie when we were probably using all four). It has never tripped with nothing being used, although the clock is always on.

It has a Crabtree B32 MCB on the RCD side of the consumer unit. The RCD is 80a 30mA, and the other MCB's are shower, downstairs sockets and upstairs sockets.

I've taken the faceplate off the cooker switch and all is good there, and I have done the same with the connection plate on the back of the cooker. Is there anything I can check relating to the MCB, or any other pointers?

TIA

 

[EFLImpudence]

What is tripping - the MCB or the RCD?

I realise you wrote MCB but wanted to make sure.

 

[riveralt]

Could be one of your grill or oven elements is on the way out.

 

[Iggifer]

Could be one of your grill or oven elements is on the way out.

Do elements not usually take out the RCD? He also said it can happen with just one of the 3 items on it energised. I appreciated there is the possibility of N-E continuity without the elements being energised, but if it truly is the MCB, can't see it being an issue

 

[JohnW2]

Could be one of your grill or oven elements is on the way out.

Possibly - but that wouldn't explain tripping (of whatever) by the microwave alone.

If we are talking about the MCB, then none of the loads which the OP says can, alone, result in a trip would draw anything like enough to trip a correctly-functioning B32 - so, in that case, I would suspect the MCB itself. If it's an RCD that's tripping, that's obviously very different.

Kind Regards, John

 

[alan333]

Hi and thanks for the replies. The OH just told me a few mins ago that any time it's tripped over the last while it's been with the microwave, so I'll monitor it with a view to binning the microwave. Could she not have told me this ffs??

There's a bit more to the story, to explain my original question. About 2 years ago the grill switch failed and a new one was fitted. It was around here that the circuit was tripping a couple of times per week. The cooker is wired by spade terminals, mostly un-insulated. I insulated all the spades which reduced the tripping to once every few months. I have now come to the conclusion that I had two problems: one which I cured by insulating the spades in the cooker, and another concerning the microwave.

Sorry for wasting your time folks, and thanks for your help. BTW are there any tests I can do on the microwave now?

Edit..it seems it was the RCD that was tripping, cos apparently the last time it also took out the tv and Sky (even tho we would both swear blind that it was only the cooker MCB switch that we needed to reset).

 

[Iggifer]

Not really any tests you can do yourself on the microwave no. If you wanted, you could get an electrician to carry out an earth leakage test, but it would probably be better putting the money towards a new microwave!

For now I would just monitor it, make sure you find out exactly what trips next time, make a note of what you were using when it happened, and after a few trips, if there's a pattern, that's your problem.

 

[jj4091]

Have you tried your microwave in a different socket? If it still trips the RCD then it's definitely faulty.

 

[JohnW2]

Have you tried your microwave in a different socket? If it still trips the RCD then it's definitely faulty.

What 'it' - the microwave or the RCD? If the 'different socket' were protected by the same RCD, it could theoretically be either (or a combination of the two ... high-ish leakage in microwave + somewhat trigger-happy RCD).

Kind Regards, John.

 

[333rocky333]

The cooker is wired by spade terminals, mostly un-insulated. I insulated all the spades .

Were they meant to be uninsulated due to the temperature

 

[alan333]

Thanks for the further replies
I haven't tried the microwave in another socket, there isn't one in range. I could move it but it would probably end up in a silly place for a few months before it tripped.

There wasn't any pattern to the insulated spades - some had the plastic cover, some didn't. It wasn't just the live's that were insulated for example, or just the ones under the hobs etc.

I'll monitor it with the new info ie it's probably the microwave that's had it's day. The cooker is probably towards 15 years old BTW, we considered replacing it but it suits our needs for now.

Thanks again

 

[jj4091]

Have you tried your microwave in a different socket? If it still trips the RCD then it's definitely faulty.

What 'it' - the microwave or the RCD? If the 'different socket' were protected by the same RCD, it could theoretically be either (or a combination of the two ... high-ish leakage in microwave + somewhat trigger-happy RCD).

Kind Regards, John.

Theoretically , yes. But it will rule out the cooker circuit & give more support to the microwave being faulty if it does trip on a different circuit when no other appliance does. Other of his appliances with heating elements are more likely to be causing an earth leakage problem than the microwave, so I would think it is the easiest way to rule it in or out.

 

[JohnW2]

What 'it' - the microwave or the RCD? If the 'different socket' were protected by the same RCD, it could theoretically be either (or a combination of the two ... high-ish leakage in microwave + somewhat trigger-happy RCD).

Theoretically , yes. But it will rule out the cooker circuit & give more support to the microwave being faulty if it does trip on a different circuit when no other appliance does. Other of his appliances with heating elements are more likely to be causing an earth leakage problem than the microwave, so I would think it is the easiest way to rule it in or out.

I am not sure that I fully understand. IF it is an RCD which is tripping, then I don't see how continued tripping when the microwave were moved to a different circuit (on the same RCD) would, in itself, "rule out the cooker circuit", since one would not which circuit had resulted in a trip. If the microwave could be moved to a circuit on a different (or even no) RCD, that would obviously be a different matter. As below, in terms of appliances/loads protected by the same RCD, what matters (at least, in terms of L-E leaks) is which of those appliances is being used when the trips occur, regardless of which circuit (on the same RCD) they are connected to.

If the issue were due, partially or completely, to N-E leaks, then it obviously gets more complicated. If any of the appliances in question are usually left with a live feed (any socket switches or 'cooker unit' switches being left 'on') and if their internal switch is SP, then any N-E leak they have will still be affecting the circuit even when the leaky appliance is not being used, and the very occasional trips might then occur at times when the total load (on all circuits) was particularly high.

In general, the main problem with any such experiments is that with the trips only happening "once every few months", it would be a very long time (very many months) after one had moved the microwave to a different socket (with no subsequent trips) before one could conclude that it was unlikely that the microwave was to blame.

There also seems to be uncertainty as to whether or not these (very infrequent) trips do always occur when the microwave is being used - so, as has been said, whether it is left on the same circuit or moved to a different one, 'monitoring' what is actually 'on' when the trips occur (again, a very long-term exercise!) is clearly an important part of any diagnostic exercise.

Finally, there even seems to be some uncertainty as to whether it is the RCD or the cooker circuit MCB that is tripping - so that certainly also needs to be 'monitored'. As I said before, given what we were told, if it were the MCB, my greatest suspicions would relate to the MCB itself.

Kind Regards, John

 

[JohnW2]

I haven't tried the microwave in another socket, there isn't one in range. I could move it but it would probably end up in a silly place for a few months before it tripped.

You could use an appropriate extension lead, without physically moving the microwave, but if it's likely to be months before the next trip, you probably wouldn't want to do that, either!

However, as I've just written, moving the microwave to a socket would probably only have limited diagnostic value, anyway. Far more important is to keep records of exactly what appliances are 'on' (all appliances/loads which are 'on') when the trips occur, and whether it is an MCB or RCD (or both!) which actually trips.

Kind Regards, John

 

[alan333]

I've given things a bit more thought now that I have all your input, and with the OH informing me it's only the microwave that occasionally trips since I 'fixed the cooker'. I've come to the logical conclusion that it must be the RCD that trips.

Back when it tripped irrespective of which 'thing' I was using, I now remember trying to figure out what was tripping, and I switched off all the MCB's, reset the RCD, and switched the MCB's back on one at a time. The cooker one wouldn't go back on, which is probably why I thought it was the MCB, and my only excuse for not remembering this is that it's been a while since I personally have reset it.

I'll (she'll?) pay attention more closely next time, but I'm now mostly convinced it's the microwave. BTW the microwave also has a built in conventional oven (which we don't use) if that would make any better sense for it to leak to earth. And also, if relevant, we don't switch things off at the wall, ie the appliances are always live.

Thanks again for all your help, I'll certainly update this thread if/when it trips again.
Regards, Alan