MCB tripping after a couple of minutes

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GeX

GeX

GeX

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The MCB for some of my downstairs lights keeps tripping. If I flick it back on, it will stay on for a couple of minutes and then pop again. Doesn't seem related to any lights being on or off.

This has happened since I lifted a floorboard that's been squeaking on the landing upstairs. It's a rats nest under there, and I'm sure it's because I've disturbed a cable (that must be damaged).

https://photos.app.goo.gl/GkuUAhpD9swjJ2xA6

The wires near the red tape are for the lights below. There are 3 wires squished into a notch next to the water pipes. Nothing looks obviously damaged, burnt or otherwise.

How can I start debugging this, and what kind of failure mode causes the breaker to trip after a few minutes and not right away?
 
Yep, I can do that. It just seems too coincidential with me having lifted that board.
 
That install is a load of ****

It's unusual for a MCB to trip out after a few minutes other than in overload situations. It is an MCB and not an RCBO?

I'd defo swap the breaker first and see if it still happens.

If so then I'd look at trying to replace and re-route those cables first.
 
Yes, it's an awful mess. There was new carpet due to go down on Friday but I've delayed that now as as well as the electrical issue there is a central heating manifold close by that has a very slight leak.

It's a Chint NB1-63 B6; https://photos.app.goo.gl/iPKGocCnsTPEeoks7 which as far as I can tell is just an MCB and not an RCBO.
 
jpaul said:
... do you have dimmers in the circuits (leaking?) .. or did you isolate them ?
Click to expand...

There is actually. One dimmer, that is broken. It only allows 100% or off. So it's really just a switch with a knob!

I didn't isolate that, but I'll try that also.

I've also realised that the is a porch light on that same circuit, controlled by a PIR. I did not isolate that either.
 
What lighting do you have? Any LEDs? Any recent changes other than the floorboards?

MCBs may be susceptible to overcurrent faults tripping them out, but there is many ways to produce an overcurrent fault than simply overloading it.
The cables that are wedged into the notches, what do they look like? Any possibilities that they have crushed and damaged the internal insulation that you are now getting a line-neutral fault when the conductors are starting to get warmer with the current passing through. Ref; insulation resistance test.

The dimmer is a good shout if it’s of an inductive type, the same with the older inductive transformers for LED lighting. Newer LED lighting can suffer from high inrush current which can trip a standard type B MCB (of course that only applies if you have recently changed lighting)
 
I had similar once and it was due to the new flooring striking a nail in one of the ring mains :X.

Can you hear it possibly when it turns off?
 
BigBoy said:
My money is on either the PIR or the dimmer switch
Click to expand...

10 internet points to BigBoy.. actually, you only get 5 - you can't bet on PIR and dimmer :p

I picked up a new breaker at lunch to fit when I got home (they're £2.50 and a short walk to screwfix). Thought I'd test it one last time before swapping in the new one and it's now been on for 6 hours.

What is not working anymore however, is the porch light / PIR. Had some heavy wind and rain blowing on the front of the house the past couple of days - guess some water found its way in. So glad it's not the underfloor wiring!
 
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Plot twist, the lights on the dimmer don't work either. Hmm...

Shall investigate later.
 
Replaced the dimmer as I has one to hand. Still nothing on dining room or porch lights. Though the under stairs and hallway work. I guess it must be a cable fault, I'll look for what those two sets of lights have in common.
 
I moved some wires around and it's back to tripping straight away. I guess that makes it easier to find :D
 
I've given up.

There's so much of the old wiring in there (house was 'rewired' at some point in the recent past by the looks of it), a lot of the old stuff is left in situ - just snipped.

I can't see anything obviously wrong, and there are so many red herrings - going to get a professional in to take a look.
 
disappointing - so what kit/technique could a pro use ? and can you do anything further ...
will a sensitive clasp type device determine which wire has the current flow, causing the trip ?
 
After having a good look about and tracing the circuit and accessories out, they will start to pull the offending circuit apart and IR test separate cables to track which cable (if any) is causing the issue.
 
What I think is the feed for dining room light is showing ~22v between live and neutral. I'm not sure why that'd be. I'm perfectly at home debugging electronics / PCBs with my trusty multimeter..

It's back to not tripping the MCB, but the dining room and porch lights not working.
 
Best advice would be to get an electrician in...playing with electrics is not safe etc

Seems most likely its got to be related to taking up or putting the floor boards back down, so chances are the fault is right at that spot, crushed cable or disturbed connections shorting etc

If you feel confident then make sure power is off and check for any cables that appear damaged or crushed and check any jbs for issues - because the fault seems to have changed each time the boards have been up makes me think of a crushed cable which when moved is blowing quicker or slower depending on its position

Where the cables have been run over the joists the joists might not be notched deep enough and have been crushed by the floor boards etc

Not a recommended method but...if its going bang pretty quickly now and making a big enough bang chances are you will be able to see it flash or hear the bang - if its near the floor boards that have been up you can take them up again reset mcb and watch for flash - this can work in dark lofts etc as a short cut to trace faults

Obviously not recommended method
 
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