Discuss RCD Trip - insulation problem from too much load? in the Electrical Forum area at ElectriciansForums.net

Newbie101

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Hi guys, first time poster, be easy!

I'm caring for a friend and last night while sleeping on the sofa-bed an emergency system that had lost power was sending out a warning signal. Startling to say the least. The RCD had tripped and after some internet sleuthing I managed to isolate the problem to the ceiling lights. It was raining at the time, so there was a chance that water had somehow affected the earthing(?).

In the morning I've looked at around all 29(!) ceiling spots to see if there was obvious water damage - there isn't any. So we called my friends neighbour who is a handy man and electrician.

All 29 bulbs are on the same ring. Handyman said this is crazy. It's impossible to isolate where the problem is. For 29 bulbs there are 8 dimmer switches and 1 regular switch. Two of those dimmers have two dimmers on the plate so technically 10 dimmers in total. Some are old, some are new. The old ones don't work so well and the lights can flicker a lot (they're not LEDs), but if you find the sweet spot it's not a problem.

Anyway, handyman took a few of the spots out of the enclosures, but they seem to be wired within the ceiling, each having a thick grey wire from bulb to beyond where he can access. In an attempt to isolate the problem further he disconnected the live wires from all the dimmer switches, individually. We tested the MCB switch each time but even after all were disconnected it still tripped. He said this was odd and not good!

He tested the wires going into the fuse box with his multimeter and it seems it's reading at a high number when it should be zero. He said the brown live was too thin for the load, it's 1.5mm, and that the insulation has gone (somewhere) and touching the earth wire. He says the only way to fix this is to replace the wire entirely, with a thicker one. This will involve a few holes drilled into the ceiling to feed the new wire through.

I guess my question is, has he checked everything? Is this really necessary? It's not a small job, and will involve me moving my terminally ill friend back to mine for a few days, not to mention the unsightly interventions to the ceiling...and perhaps walls.
Sorry for the long query, but any advice would be most welcome. Thanks
 
TL;DR
Is this live wire too thin for the load and is it possible that over 15 years later the insulation finally gave up and shorted against the earth somewhere on the ring?

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Mike Johnson

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Welcome to the forum, I am sure one of us can help, where about's in London is this problem, someone may be local and be able to help.

You say it could be Water damage, therefore a couple of questions.

Is this a top floor flat?

Are there any external lights or sockets?

29 ceiling spots at what wattage, even if they are all 100watts that's only 12.6amps, a 1.5 supply can take a maximum of 17amps so below the threshold, but its not a good idea to have all the lights on one circuit due to one fault and all the lights are off.
 
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Newbie101

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Welcome to the forum, I am sure one of us can help, where about's in London is this problem, someone may be local and be able to help.

You say it could be Water damage, therefore a couple of questions.

Is this a top floor flat?

Are there any external lights or sockets?

29 ceiling spots at what wattage, even if they are all 100watts that's only 12.6amps, a 1.5 supply can take a maximum of 17amps so below the threshold, but its not a good idea to have all the lights on one circuit due to one fault and all the lights are off.
Hi Mike,

Thanks for quick reply. Edgware Road area - but it's tricky because it's his neighbour who is helping so it's a tricky situation to say no thanks I want to get someone else to check. Though, if I can point him in the direction of trying something else first, that may work!

It's a top floor flat and water has been a some concern in the past. Though never re electrics. There is some water damage in the flat from about 9 months ago, but since then no new signs of it - and I've checked all the existing ares and potential new areas, and they all seem ok.

There are two external light fixtures, covered up as you'd expect and these are on the same ring (? line/circuit breaker) as the other 27 lights.

The rain was rather light last night, so surprising that it could have such an affect, but then again, water is water and if it gets in the wrong place that's it. This handyman is convinced, because of the reading at the fuse box, that it can't be any of the individual lights. Is there something exploratory you could suggest he should do on the external lights before he does this massive job?

Thanks
 

westward10

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As already mentioned the cable is not undersized so that is not an issue. The circuit needs insulation resistance testing and if there is a fault the circuit needs systematically breaking down to locate the fault. It sounds like the neighbour is not up to this task.
 

Mike Johnson

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Yes disconnect the external lights from the circuit and see if the problem persists, that would be the first investigation a competent electrician would do, they would than use a MFT (multi function tester) to test insulation and continuity of the whole circuit, did the handyman have a MFT or just a multimeter, MFT's cost upwards of £500.00 they are a bit more complex that a multimeter.

Typed at the same time WW.
 

Wilko

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Hi - sorry to hear about the light circuit tripping the RCD.

As per MikeJ, the cable size of 1.5mm2 is fine for a lighting circuit and it has a 6A MCB (from your photos) to protect it.

Yes the problem is likely to be an unwanted connection to E somewhere in the circuit. An Electrician would work to find this fault with their test gear. It’s just not warranted to re wire the circuit until all other possibilities have been examined.

Someone here will be nearby and will help you if you ask for it.
 
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Newbie101

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This is disturbing, and so difficult to manage. He's gone off to buy new wire and new dimmer switches.

To answer one question, the external lights were disconnected from the circuit by removing the live wire from the dimmer switches. The box still tripped after doing that. Is that sufficient to rule out the external lights as being the problem?

The only multimeter he has was a longish yellow one, it wasn't as sophisticated as I've seen on youtube videos, but that's a layman's view.

How would you test the line elsewhere to determine where the problem is? When he took the spots down from the holes, there were grey insulated cables, nothing on show etc. Would he need to take them apart and use an MFT?
 

westward10

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You have ended up in a tricky situation here trying to do the right thing. The cable may or not need replacing but needs confirming by testing using an insulation resistance tester, it doesn't have to be a Multi Function Tester as long as the tester can carry out the correct test. Replacing parts and hoping isn't really the logical way forward.
 

Newbie101

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You have ended up in a tricky situation here trying to do the right thing. The cable may or not need replacing but needs confirming by testing using an insulation resistance tester, it doesn't have to be a Multi Function Tester as long as the tester can carry out the correct test. Replacing parts and hoping isn't really the logical way forward.
Exactly that, and exactly my concern. This is not a small job, and there is no piping so replacing the wire will involve digging out plaster and potentially concrete, or the alternative, unsightly trunking.

If one of you were here, how would you be checking systematically for the fault?
 

westward10

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I know the neighbour is trying to help but they do not know the correct way to proceed with this. You test the entire circuit using an insulation resistance test and if a fault is indicated you break the circuit into parts to locate the fault.
 

Mike Johnson

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Insulation resistance test on the circuit at the DB and then breaking down the circuit into individual bits and testing at each light.

WW you have done it again, you are just that bit quicker. 😜
 

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