Maddening MCB Tripping fault

[vwes59]

Hate these. Been called out to troubleshoot a tripping MCB. It's a house I've been to before and I know the wiring is a real dogs diner, but anyway.

Got an MCB that's tripping out with no appliances plugged in. The sockets themselves are not in a great state, however continuity and insulation resistance tests across the circuit showed no problems (Did that at every socket on the radial). Turned on the breaker and within 5 minutes or so it tripped out. Not seeing any singeing on the conductors or sockets.

Now I have to say, last year I attended this house and replaced a burnt out MCB (Different circuit) - surely this couldn't be a second one. The board is only maybe 10 or so years old (Hager).

In my experience these rarely turn out to be cable faults as it would show up in the insulation resistance test, so I'm left with one or more dodgy sockets (6 total on this radial), loose wiring or a broken MCB.

Any other thoughts ?

 

[buzzlightyear]

If no faults found and minum gear plugged in, possibly a faulty mcb.

 

[davesparks]

Put clamp meter on it and see what current is flowing when turned on and when it trips. 5 minutes suggests it may be a small overload situation.

 

[vwes59]

Ok, little bit further forward. I've narrowed it down a bit now. I thought I had a ring with a spur, but it turns out I have 3 independent radials all in one MCB, which is ok, messy, but not against the regs. So, I've isolated the faulty radial now which has 3 sockets on it, 2 in hall way and 1 in the lounge. At least it's got their kitchen and combi boiler up and going - so I just need to find out what's wrong with that radial. The sockets are in an awful state, so I might just clean up the conductors and back boxes and replace the socket fronts. I redid the continuity and resistance on that circuit and they came back fine (0.4 at furthest point, 0.3 nearest the board and >2000Mohms on the IR) - Perhaps you could argue the resistance values might be a little high for a radial that's perhaps 10m long but it's definitely not a cable fault, and it's not an MCB issue either now as everything else is fine. As an aside I found the boiler hadn't been earthed! So sorted that out.

 

[buzzlightyear]

What size is the MCB in the board for the radial.

 

[timhoward]

Assuming it's a correctly functioning B16 breaker, a trip after 5 minutes (300 seconds) suggests roughly 28A of current.
With equipment connected, it might be worth trying a L-N continuity test on that branch and see if you happen to get about 8-10 ohms. Worth a shot! It's unlikely there will be a purely permanent resistive load but it doesn't take long to try.

It could be a faulty socket / plugged in equipment on that branch.
It could also be that something hungry on that branch is simply bringing the total draw up considerably and the whole circuit is overloaded.

The earlier suggestion of a clamp meter is really the way to go.
It sounds like you are getting there!

 

[vwes59]

Thought I'd found the issue. Found one socket with loose line and neutral conductors, so got those sorted, then I found a socket that was rusty, so sorted that, powered up and nothing tripped out....until a few minutes ago when I got a call. So the tripping has gone out from once every few minutes to once every couple of hours. Client has discovered a socket hidden behind a cupboard that they never knew was there, so going back to have a look.

 

[davesparks]

Thought I'd found the issue. Found one socket with loose line and neutral conductors, so got those sorted, then I found a socket that was rusty, so sorted that, powered up and nothing tripped out....until a few minutes ago when I got a call. So the tripping has gone out from once every few minutes to once every couple of hours. Client has discovered a socket hidden behind a cupboard that they never knew was there, so going back to have a look.

Have you measured the current flowing through the circuit yet?

How do you think the loose connection or rusty socket could have been causing the MCB to trip?

 

[littlespark]

If the cables check out, then just maybe a faulty socket itself.
But looks like you’re looking in that direction already.

 

[timhoward]

If you don't have one, a very basic AC clamp meter can be had for £30 at screwfix and would be of immense help with this fault.
No point chasing your tail if it's simply too many things on the same circuit!

So, I've isolated the faulty radial now

At the moment I've read nothing to conclusively prove that this branch is faulty. Are you sure it's faulty?
If you connect this branch again and can reliably get trips every 5 minutes, you could try swapping each socket for 3 wagos.

If there's a fridge freezer (with defrost element that periodically operates) or anything with a timer and a heating element plugged in I'd also be suspecting that, and a bit of PAT testing might help.