Fault on lighting circuit

Welsh_sparky_21 · Dec 13, 2024

Bear with me because this is long. So got an intermittent fault on the upstairs lights. Trips the mcb after a few hours, getting a reading of 0.01 mega ohms between live and earth at the board, tested cable going to db from first switch and it’s clear. Tested the cable going from first switch to rest of switches and getting a reading of 5.2 megaohms so can’t understand where the 0.01 is coming from as I wired it so know there isn’t any joins. Also the cable going from first switch to rest of switches loses its earth, so I think the cable is obviously damaged but can’t understand why this isn’t showing when I test and why it’s tripping intermittently. Maybe I’m missing something completely obvious

DPG · Dec 13, 2024

What does L-E measure at the board when using a low Ohms test rather than an IR test?

Jock from the north · Dec 13, 2024

Welsh_sparky_21 said:
Bear with me because this is long. So got an intermittent fault on the upstairs lights. Trips the mcb after a few hours, getting a reading of 0.01 mega ohms between live and earth at the board, tested cable going to db from first switch and it’s clear. Tested the cable going from first switch to rest of switches and getting a reading of 5.2 megaohms so can’t understand where the 0.01 is coming from as I wired it so know there isn’t any joins. Also the cable going from first switch to rest of switches loses its earth, so I think the cable is obviously damaged but can’t understand why this isn’t showing when I test and why it’s tripping intermittently. Maybe I’m missing something completely obvious

Have you meggered both sides of first leg leaving db going to first switch.could be damaged on one side maybe does your first feed connect onto any strappers if its up stairs lights

HappyHippyDad · Dec 13, 2024

Welsh_sparky_21 said:
Bear with me because this is long. So got an intermittent fault on the upstairs lights. Trips the mcb after a few hours, getting a reading of 0.01 mega ohms between live and earth at the board, tested cable going to db from first switch and it’s clear. Tested the cable going from first switch to rest of switches and getting a reading of 5.2 megaohms so can’t understand where the 0.01 is coming from as I wired it so know there isn’t any joins. Also the cable going from first switch to rest of switches loses its earth, so I think the cable is obviously damaged but can’t understand why this isn’t showing when I test and why it’s tripping intermittently. Maybe I’m missing something completely obvious

I always have a diagram written out of how I wired the lighting circuit. If you have one, pop it up. It'll make it easier to see where you are testing in the circuit.
Are your conductors removed from the mcb AND neutral bar (in the CU) when you are testing?
Is it definitely an MCB not an RCBO?

I think you must be missing something simple, as the 0.01 cannot disappear. You will find it, and think.. 'oh, that's so obvious'.

It will be something to do with the break in the earth. The initial test (0.01Mohms) is including the fault.

The next 2 tests somehow bypass the fault. Are you sure the fault is not on another circuit, again this could be getting picked up through the L, if you have left the neutral connected in the neutral bar. There could be a neutral earth fault on another circuit (or L-E). This may explain the poor result, but not the tripping.

Good luck chap. Go back the next day, in the morning. I have found so many faults within a few minutes the following morning after looking for hours the previous afternoon, as I have a fresh head. Plus a bit of help from here most likely.

Welsh_sparky_21 · Dec 13, 2024

DPG said:
What does L-E measure at the board when using a low Ohms test rather than an IR test?

1056 ohms I got im sure

Welsh_sparky_21 · Dec 13, 2024

Jock from the north said:
Have you meggered both sides of first leg leaving db going to first switch.could be damaged on one side maybe does your first feed connect onto any strappers if its up stairs lights

Disconnected feeds at first switch, fault disappears when I do this. But when I test the outgoing feed I get 5.2 megaohms or something. When I reconnect this out going feed the 0.01 reading re appears at the consumer unit again

Jock from the north · Dec 13, 2024

Welsh_sparky_21 said:
Bear with me because this is long. So got an intermittent fault on the upstairs lights. Trips the mcb after a few hours, getting a reading of 0.01 mega ohms between live and earth at the board, tested cable going to db from first switch and it’s clear. Tested the cable going from first switch to rest of switches and getting a reading of 5.2 megaohms so can’t understand where the 0.01 is coming from as I wired it so know there isn’t any joins. Also the cable going from first switch to rest of switches loses its earth, so I think the cable is obviously damaged but can’t understand why this isn’t showing when I test and why it’s tripping intermittently. Maybe I’m missing something

Welsh_sparky_21 said:
Bear with me because this is long. So got an intermittent fault on the upstairs lights. Trips the mcb after a few hours, getting a reading of 0.01 mega ohms between live and earth at the board, tested cable going to db from first switch and it’s clear. Tested the cable going from first switch to rest of switches and getting a reading of 5.2 megaohms so can’t understand where the 0.01 is coming from as I wired it so know there isn’t any joins. Also the cable going from first switch to rest of switches loses its earth, so I think the cable is obviously damaged but can’t understand why this isn’t showing when I test and why it’s tripping intermittently. Maybe I’m missing something completely obvious

You could perform your continuity tests again on lighting legs again but individually

Welsh_sparky_21 · Dec 13, 2024

HappyHippyDad said:
I always have a diagram written out of how I wired the lighting circuit. If you have one, pop it up. It'll make it easier to see where you are testing in the circuit.
Are your conductors removed from the mcb AND neutral bar (in the CU) when you are testing?
Is it definitely an MCB not an RCBO?

I think you must be missing something simple, as the 0.01 cannot disappear. You will find it, and think.. 'oh, that's so obvious'.

It will be something to do with the break in the earth. The initial test (0.01Mohms) is including the fault.

The next 2 tests somehow bypass the fault. Are you sure the fault is not on another circuit, again this could be getting picked up through the L, if you have left the neutral connected in the neutral bar. There could be a neutral earth fault on another circuit (or L-E). This may explain the poor result, but not the tripping.

Good luck chap. Go back the next day, in the morning. I have found so many faults within a few minutes the following morning after looking for hours the previous afternoon, as I have a fresh head. Plus a bit of help from here most likely.

So i basically go from the board to upstairs hall. From the hall to bed room 1, from there to the bathroom then finishes at bedroom 2. Between hall and bedroom 1 I lose the earth in the cable somehow, possible compression or joiner screw through it. But lights all work fine so live and neutral haven’t been broken. 0.01 ohms when I test between live and earth at db, this reading disappears when I disconnect feeds at hall switch. But when I test outgoing feed at hall switch I don’t get a reading of 0.01 megaohms I get 5.2. Tested strappers and all clear. It’s a head scratcher lol

Welsh_sparky_21 · Dec 13, 2024

Jock from the north said:
You could perform your continuity tests again on lighting legs again but individually

Was thinking this, disconnect whole upstairs and bell it all out

HappyHippyDad · Dec 13, 2024

Welsh_sparky_21 said:
1056 ohms I got im sure

That would be 0.001Mohms not 0.01Mohms. Give us some accurate figures and don't guess. If you don't know, just say you don't know. This is said because facts are required, no offence is meant.

Jock from the north · Dec 13, 2024

Welsh_sparky_21 said:
Was thinking this, disconnect whole upstairs and bell it all out

Yeh it's time consuming the last thing you want at this time of year

HappyHippyDad · Dec 13, 2024

Welsh_sparky_21 said:
So i basically go from the board to upstairs hall. From the hall to bed room 1, from there to the bathroom then finishes at bedroom 2. Between hall and bedroom 1 I lose the earth in the cable somehow, possible compression or joiner screw through it. But lights all work fine so live and neutral haven’t been broken. 0.01 ohms when I test between live and earth at db, this reading disappears when I disconnect feeds at hall switch. But when I test outgoing feed at hall switch I don’t get a reading of 0.01 megaohms I get 5.2. Tested strappers and all clear. It’s a head scratcher lol

A diagram speaks a thousand words. We can't tell if you're looping at the switch, or the lights or both, or if there are 2 way switches, or more than one light per switch.

Hopefully you'll find it tomorrow, but if not then take the time to draw out the circuit and pop it on here.

Pretty Mouth · Dec 13, 2024

Welsh_sparky_21 said:
getting a reading of 0.01 mega ohms between live and earth at the board, tested cable going to db from first switch and it’s clear. Tested the cable going from first switch to rest of switches and getting a reading of 5.2 megaohms so can’t understand where the 0.01 is coming from as I wired it so know there isn’t any joins. Also the cable going from first switch to rest of switches loses its earth

Possibly:

Your IR tests carried out downstream of the first switch are between line and the floating CPC, that is not connected to the earthing system.

However, the fault may not be between line and that floating CPC. It may actually be between line and some other part of the building that does have a tenuous connection to the earthing system. Hence why you get low IR L-E tested at the board (where the CPC is connected to the MET), but not downstream of that first switch.

Try: Connect your wander lead to the MET, and take that with you to a point downstream of the first switch, and do your IR test between L and the wander lead. I suspect you will now get a low IR reading.

Had a similar fault yesterday. Low IR 0.08 N-E, but the CPC had become disconnected somewhere. Once the CPC was reconnected, it was a dead short of a fraction of an ohm.

Welsh_sparky_21 · Dec 13, 2024

HappyHippyDad said:
That would be 0.001Mohms not 0.01Mohms. Give us some accurate figures and don't guess. If you don't know, just say you don't know. This is said because facts are required, no offence is meant.

No it’s not. He asked what reading I got when I tested on low resistance ohms so wasn’t megaohms. So maybe it’s yourself that needs to know the facts beforehand

Welsh_sparky_21 · Dec 13, 2024

Pretty Mouth said:
Possibly:

Your IR tests carried out downstream of the first switch are between line and the floating CPC, that is not connected to the earthing system.

However, the fault may not be between line and that floating CPC. It may actually be between line and some other part of the building that does have a tenuous connection to the earthing system. Hence why you get low IR L-E tested at the board (where the CPC is connected to the MET), but not downstream of that first switch.

Try: Connect your wander lead to the MET, and take that with you to a point downstream of the first switch, and do your IR test between L and the wander lead. I suspect you will now get a low IR reading.

Had a similar fault yesterday. Low IR 0.08 N-E, but the CPC had become disconnected somewhere. Once the CPC was reconnected, it was a dead short of a fraction of an ohm.

I had the CPC disconnected at the consumer unit when I was testing the cable

Fault on lighting circuit

Welsh_sparky_21

DPG

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Jock from the north

HappyHippyDad

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Welsh_sparky_21

Welsh_sparky_21

Jock from the north

Welsh_sparky_21

Welsh_sparky_21

HappyHippyDad

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Jock from the north

HappyHippyDad

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Pretty Mouth

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Welsh_sparky_21

Welsh_sparky_21

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