UK MCB/RCD tripping randomly

[harrysewell83@]

Hi there,
Apologies if this is the wrong place to post but as a complete novice I was hoping for some advice on the next steps when engaging with a professional electrician.

We recently purchased a house and after a couple of months being here the MCB for the kitchen circuit and the associated RCD keeps randomly tripping (once or twice a day, usually late at night 11pm-1am or in the morning 8am-11am). Both MCB/RCD can be switched back on immediately without being retriggered.

It first happened after heavy heavy rain, so we suspected some water had made its way into a socket or appliance, but after checking the roof we couldn't see any obvious points of ingress (kitchen extension Inc wiring was done in 2017). It then proceeded to persist in dry spells too.

So we set about eliminating the appliances on the circuit. During this process we found out that the cooker and extractor are situated on the main kitchen circuit, I assume this is okay as it is a single oven and has a max power input of 2.4kw, as i understand everything over 3kw has to be on its own circuit?

After leaving the oven/extractor off at the isolator (except when using it) we found that we had no more random tripping....... Until today when my other half heard a "pop" from the general direction of the washing machine (not on at the time) or a wall socket with a coffee machine (previously eliminated). The washing machine wasn't previously eliminated as it was such an arse to move and is also brand new. I've now plugged that into a different circuit to be sure.

So my question is; if it isn't the washing machine, what could be causing the fault and what would be the next steps for an electrician in order to try and trace what is going on?

Many thanks in advance.
H

 

[radiohead]

An electrician will carry out an insulation resistance test. If both the MCB and RCD are operating there is the possibility of a serious intermittent fault. An RCD will trip at very low earth leakage current, whereas an MCB will only operate instantaneously at very high fault current. You should get this investigated without delay.

 

[harrysewell83@]

Many thanks for the reply, just scoping out some local electricians now. For the moment we are leaving all appliances off and unplugged and only reinstating them for the duration that they are needed.

 

[harrysewell83@]

While waiting for Electricians to get back to me, I got a kewtech 107 as an interim peace of mind.

All sockets check out fine except one....

Alarmingly I get a warning condition on the neutral wire when the socket is not turned on.

If the kewtech is plugged in when the socket is on, the tests all pass and there are no errors.

Is it safer to leave the socket:
With the switch on and nothing plugged in
With the switch off and nothing plugged in
With the switch on and something plugged in arbitrarily on standby.

These are the fault conditions that correlate with the tester being plugged in and the socket switch off....

[automerge]1593859514[/automerge]

While waiting for Electricians to get back to me, I got a kewtech 107 as an interim peace of mind.

All sockets check out fine except one....

Alarmingly I get a warning condition on the neutral wire when the socket is not turned on.

If the kewtech is plugged in when the socket is on, the tests all pass and there are no errors.

Is it safer to leave the socket:
With the switch on and nothing plugged in
With the switch off and nothing plugged in
With the switch on and something plugged in arbitrarily on standby.

These are the fault conditions that correlate with the tester being plugged in and the socket switch off....

Worth noting that the socket with the fault is a double socket and only one side exhibits the neutral fault when switched off

 

[buzzlightyear]

The only way is disconnection on mcb and investigating it.

 

[Lucien Nunes]

I got a kewtech 107 as an interim peace of mind

Anything that causes an MCB to trip without load is a short-circuit, which a socket-tester does not detect. It will tell you (sometimes) whether the wires have been put in the right holes, but not whether they are arcing and sparking and about to catch fire. It could be a damaged cable or fitting, or even an appliance, as it is possible to trip a B32 MCB even while a 13A fuse remains intact, although I would expect the 13A fuse to have gone by now after a couple of trips.

Alarmingly I get a warning condition on the neutral wire when the socket is not turned on.

As it only happens on one socket of a double, it is unlikely to be reversed polarity at the connections. If the socket has a double-pole switch, it could be that the line pole is welded shut but the neutral is still working. Therefore, when you switch it off, the tester detects it as a missing neutral. Replace this socket while fixing the short, avoid using in the meantime as anything connected might still be live while seeming to be off, or the switch may arc if it is partially jammed.

 

[harrysewell83@]

Anything that causes an MCB to trip without load is a short-circuit, which a socket-tester does not detect. It will tell you (sometimes) whether the wires have been put in the right holes, but not whether they are arcing and sparking and about to catch fire. It could be a damaged cable or fitting, or even an appliance, as it is possible to trip a B32 MCB even while a 13A fuse remains intact, although I would expect the 13A fuse to have gone by now after a couple of trips.



As it only happens on one socket of a double, it is unlikely to be reversed polarity at the connections. If the socket has a double-pole switch, it could be that the line pole is welded shut but the neutral is still working. Therefore, when you switch it off, the tester detects it as a missing neutral. Replace this socket while fixing the short, avoid using in the meantime as anything connected might still be live while seeming to be off, or the switch may arc if it is partially jammed.

Many thanks for taking the time to read and reply. I got the kew tech in an effort to highlight anything obvious knowing it would only highlight a limited scope of issues, I always have intended to get in a professional (as clearly I know nothing about electrics) but as a software developer by trade, not knowing or understanding why something is happening drives me round the bend!

The socket in question was in the general direction of where my other half heard a 'pop' when the MCB/RCD last tripped. Equally the switch on this particular switch stuck in the 'on' position the other day and it took me about five attempts to click it to off. The appliance plugged into this switch was a coffee machine which were most suspicious of as a cause as it sometime behaves erratically (auto timer not cutting off/random numbers on the display).

We aren't touching the socket for now, the coffee machine is being binned and appliances are only being used on the circuit as and when they are actually in use. The RCD/MCB hasn't gone since Tuesday which is when we started only using appliances when needed.

I'm eagerly awaiting a few call backs from electricians before I select one, in the mean time I obsess about my inability to fix it myself!

cheers
H

 

[Lucien Nunes]

Now this is starting to make sense, ironically with the aid of the socket tester.

A likely cause is an intermittent but direct L-E fault to earth in the coffee machine that had been making occasional light contact, but when the pop occurred, the fault current was high enough to damage the switch on the socket while the MCB tripped. The fuse in the plug might have survived earlier momentary fault contacts partly due to the RCD also tripping and reducing the let-through, but has probably opened this time. When you freed the switch off mechanically, only the neutral pole actually started to open and close; the line pole is still welded closed hence the socket tester indication.

If you still have the coffee machine, you might like to open it up; the fault will probably have left visible evidence as unless the functional switching is in the neutral, it must lie between the flex inlet and the controls and is unlikely to be hidden within the element. It would also be interesting to see whether the plug fuse has indeed opened, and whether it is genuine or counterfeit/substandard.

 

[pc1966]

Equally the switch on this particular switch stuck in the 'on' position the other day and it took me about five attempts to click it to off.

That is 4 more than the "needs to be replaced" test!

 

[harrysewell83@]

Now this is starting to make sense, ironically with the aid of the socket tester.

A likely cause is an intermittent but direct L-E fault to earth in the coffee machine that had been making occasional light contact, but when the pop occurred, the fault current was high enough to damage the switch on the socket while the MCB tripped. The fuse in the plug might have survived earlier momentary fault contacts partly due to the RCD also tripping and reducing the let-through, but has probably opened this time. When you freed the switch off mechanically, only the neutral pole actually started to open and close; the line pole is still welded closed hence the socket tester indication.

If you still have the coffee machine, you might like to open it up; the fault will probably have left visible evidence as unless the functional switching is in the neutral, it must lie between the flex inlet and the controls and is unlikely to be hidden within the element. It would also be interesting to see whether the plug fuse has indeed opened, and whether it is genuine or counterfeit/substandard.

Thanks so much for the reply!
Had to pretty much destroy the coffee machine to get it apart (cheap moulded thing). The main PCB has a some significant scorching around a bunch of components, equally the chamber that houses the inlet from the flex/element and the main PCB has clearly had some degree of water and coffee ingress over it's lifespan.

 

[James]

Remember to get that socket changed.
And well done for going to the effort of proving @Lucien Nunes theory correct.

 

[harrysewell83@]

Socket was replaced by a local elctrician; it was indeed passing current when switched off. Thanks for all your advice, hypotheses and explanations.