Maddening MCB Tripping fault

[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

 

[Lucien Nunes]

could of being a neutral to earth fault causing the trip

No, this could not have been the problem as it was an MCB tripping, not an RCD.

 

[Intoelectrics]

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

As Lucien says^^^^ As the electrician you should already have a clear understanding of protection device's functionalities and what type of fault you are expecting to be looking for. Hence the suggestions previously mentioned regarding checking the current flowing through the device.

 

[Intoelectrics]

As Lucien says^^^^ As the electrician you should already have a clear understanding of protection device's functionalities and what type of fault you are expecting to be looking for. Hence the suggestions previously mentioned regarding checking the current flowing through the device.

I forgot to clarify, for an mcb you would be looking for the total load current to see if or when it is exceeding the rating of the device. For an RCD you would be looking for the total current imbalance to see if or when that is exceeding the rating of the device. For a combined rcd and mcb (rcbo/rccb) you would be looking at both.

 

[cliffed]

There’s nothing worst trying to find faults when there are non accessible junction boxes.
The Client’s not interested in your efforts & all the blame comes to you…I hate it

 

[Midwest]

It wasn’t the OP reopening the thread.