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.

 

[vwes59]

Ok, made a lot of progress. Have now isolated the fault to the lounge circuit, which is only two double sockets - everything else now working perfectly. Even put this circuit on its own MCB, so if it trips, only losing two sockets. I've replaced both socket fronts, and have done continuity and IR testing again, everything fine - so it's now a waiting game to see what (if anything) happens. If it trips now, then it's going to take some serious pondering.

 

[James]

Ok, made a lot of progress. Have now isolated the fault to the lounge circuit, which is only two double sockets - everything else now working perfectly. Even put this circuit on its own MCB, so if it trips, only losing two sockets. I've replaced both socket fronts, and have done continuity and IR testing again, everything fine - so it's now a waiting game to see what (if anything) happens. If it trips now, then it's going to take some serious pondering.

Ah, the old try this and see if it works approach.

Not the best way to show the customer that you know what you are doing and have tested everything found a fault and rectified it.

 

[davesparks]

Ok, made a lot of progress. Have now isolated the fault to the lounge circuit, which is only two double sockets - everything else now working perfectly. Even put this circuit on its own MCB, so if it trips, only losing two sockets. I've replaced both socket fronts, and have done continuity and IR testing again, everything fine - so it's now a waiting game to see what (if anything) happens. If it trips now, then it's going to take some serious pondering.

And who is footing the bill for your merry game of parts darts?

 

[vwes59]

And who is footing the bill for your merry game of parts darts?

Client. But we've finally located the fault, and it was one of those "Hang on - what's this?" moments. I won't go into details but I suddenly realised that even though all my tests were coming up ok, the conductors in the sockets were not the same. That's to say in one socket it was 2.5mm T&E, but at the next socket along it was (I kid you not) 6mm T&E ! and appeared to go back to the CU which was 2.5mm T&E in the MCB. So somewhere either in the walls or under the floor is a junction box. Once I'd sussed that and made an educated guess where to put my leads, hey presto, some really wacky continuity and IR readings.

PLEASE for the love of all things holy, don't do DIY electrics. It's very obvious this was a "It works, that'll do"

Anyway, I'm going to totally decommission that radial circuit and remove it from the CU - there's not a hope in hell I'm going to find where that junction box is without either putting holes in walls or ripping up the floor, which as you can imagine the client doesn't want me to do. So they have requested me to put in a new circuit with one socket in the lounge, which, as far as possible, and within regs, to be surface mounted. I do have a couple of options, which I don't know why this wasn't done by the person who put in the radial in the lounge in the first place. I can extend one of the two kitchen radials into the lounge, or I can extend the radial in the hallway into the lounge. Both circuits have ample capacity and work fine.

 

[yorkieone]

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.

great find on the boiler earth could of being a neutral to earth fault causing the trip when the boiler fired up