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Went to a job today where an RCD was tripping after heavy rain and taking two MCBs with it. The two MCBs are both for ring circuits - one apparently just for a microwave, and the other for most of the downstairs sockets and outside lights.

IR testing was a nightmare due to sockets in inaccessible places and PIRs / night sensors on the outside lights, but I'm fairly sure the problem was a JB for an outside light with a gland missing, allowing water to get in.

However, what is confusing me is why TWO MCBs would have been tripping. Also, he said it took a few hours before they could both be reset.

Why would an MCB on a different circuit not reset? Does this mean there must be an unrelated fault on that circuit? Or just a dodgy breaker (the CU's only 2 years old)? Or something else I'm missing?

Any thoughts greatly appreciated.
 
Is it possible that the two circuits are linked in some way?
I've come across instances where one leg of a RFC is on one MCB, and the other leg on another MCB.
 
Right, its just one of them jobs you're going to have to be patient with OP.

Isit a dual board and if so are the 2 MCB's that are tripping the only 2 on said RCD? What size are the MCB's?

Do not for one minute believe anything the client tells you...in regards to one circuit only being for the microwave. Disconnect the microwave and IR, the readings will let you know if anything else is in circuit...if there is then trace(outside socket spur with water in ect), if not PAT the microwave.

You could have found the fault with the light outside and water getting in, and the other MCB to the microwave is just weak...try replace it and see if it still goes. Or the 2 circuits could be linked, with 1 leg of the ring in 1 breaker and the other leg in the other breaker with the lights...unlikley, but it sounds like a DIY so anything possible.
 
I think I'll go along with what has been stated, probably two faults conspiring together

1/ Faulty Microwave
2/ Ingress of water into both the outside light and a ground floor socket/s somewhere
3/ One rfc across two MCBs
4/ Delay in resetting MCBs is usually caused by overloading but can also be caused by intermittent tripping of RCDs due to ingress of water at the terminations of an accessory
 
Is it possible that the two circuits are linked in some way?
I've come across instances where one leg of a RFC is on one MCB, and the other leg on another MCB.

I did a house rewire and the QS did the testing. He crossed the legs over at the cu and did not now until a fault appeared some time afterwards. Sounds like the OP's story, cu 2 years old, water getting into an o/s light.

Do an end to end test of the 2 mcb's and make sure they are not crossed.
 
I had something like this and switch off the SFCU for the outside light the customer was cock a hoop as I never even got the tester out she asked how did I know and I said well we have had constant heavy rain then your main RCD trips and stays tripped so its not rocket science to figure out what is vunrable to the rain
 
Also the op has answered his own question why the mcb wont reset.

Customer says "Also, he said it took a few hours before they could both be reset."

First place I would be looking as well.

 
Thanks for responses. All very helpful.

Murdoch - yes, I did disconnect the suspect outside light, and I said I would ring the customer after the next heavy rain to see whether that had stopped it tripping.

I did end to end tests on both circuits, and the results were as expected, so I don't think it can be crossed legs. On the "microwave" circuit I got 0.25ohm on l and n, and 0.41ohm on e, which suggests there is more than just the microwave on the circuit (since the microwave is only about 2 metres from the CU). However, I couldn't unplug the microwave to IR test that circuit properly as I would have had to dismantling the kitchen units to get to the socket (I know that sounds ridiculous but its true!).

Mark C - The 2 circuits in question are on 32A breakers. There are four other circuits on that RCD. I think he said they all trip when the RCD goes, but then he is able to switch them all back on, apart from the two ring circuits that I've been taking about which take a few hours before he can switch back on.

I think its looking like, as Ackbhar says, two faults conspiring together.

Paul M - don't quite follow your last post, could you clarify? Ta
 
Also the op has answered his own question why the mcb wont reset.

Customer says "Also, he said it took a few hours before they could both be reset."

First place I would be looking as well.


Yes mate I was referring to post 7 by Oldtimer regarding water in the o/s light fitting or jb.

You need to find out what is on these two circuits, I would switch off everything else in the house and go to each skt and see which ones in the house are being fed for each mcb.
 
Thanks for responses. All very helpful.

Murdoch - yes, I did disconnect the suspect outside light, and I said I would ring the customer after the next heavy rain to see whether that had stopped it tripping.

I did end to end tests on both circuits, and the results were as expected, so I don't think it can be crossed legs. On the "microwave" circuit I got 0.25ohm on l and n, and 0.41ohm on e, which suggests there is more than just the microwave on the circuit (since the microwave is only about 2 metres from the CU). However, I couldn't unplug the microwave to IR test that circuit properly as I would have had to dismantling the kitchen units to get to the socket (I know that sounds ridiculous but its true!).

Mark C - The 2 circuits in question are on 32A breakers. There are four other circuits on that RCD. I think he said they all trip when the RCD goes, but then he is able to switch them all back on, apart from the two ring circuits that I've been taking about which take a few hours before he can switch back on.

I think its looking like, as Ackbhar says, two faults conspiring together.

Paul M - don't quite follow your last post, could you clarify? Ta

Might be me misinterpreting this, but are you saying all 4 mcbs trip at the same time the rcd trips? Daz
 
Mind unless the sockets and spurs are double pole the RCD can still trip on a neutral to earth fault I had this on a job and reaslised it 2 hours later
 
I had a similar problem in a garage in witch someone had stole a neutral from a different cct in a jb under some upstairs floor boards. Bus as previous ppl state try disconnecting light 1st.
 
Mark C - The 2 circuits in question are on 32A breakers. There are four other circuits on that RCD. I think he said they all trip when the RCD goes, but then he is able to switch them all back on, apart from the two ring circuits that I've been taking about which take a few hours before he can switch back on.



Are you saying that the MCB's actually trip, or do you mean it's only the RCD tripping, taking out of service to the 4 MCB's??
 

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