Fault on lighting circuit

Pretty Mouth · Dec 13, 2024

Welsh_sparky_21 said:
I had the CPC disconnected at the consumer unit when I was testing the cable

This is your first mistake. Keep the CPC continuous and connected to the MET throughout the fault-finding exercise.

Welsh_sparky_21 · Dec 13, 2024

Pretty Mouth said:
This is your first mistake. Keep the CPC continuous and connected to the MET throughout the fault-finding exercise.

ah okay I thought disconnecting it would have given me a truer reading. Not the most experienced fault finding tbh

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

Is there 2 faults here or if the cpc between db and first switch was clear between l,n,e and not damaged and earth continuity verified then cpc further down the line might be restored .or have I misunderstood your wiring description

Welsh_sparky_21 · Dec 13, 2024

Jock from the north said:
Is there 2 faults here or if the cpc between db and first switch was clear between l,n,e and not damaged then cpc further down the line might be restored .or have I misunderstood your wiring description

So there is 2 faults but I don’t know if they are linked. Basically losing the CPC between switch 1 and 2. But not sure if this is what’s causing the tripping as the cable must be damaged

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 will try this when I’m next on the job, thanks for the help.

Jock from the north · Dec 13, 2024

Welsh_sparky_21 said:
So there is 2 faults but I don’t know if they are linked. Basically losing the CPC between switch 1 and 2. But not sure if this is what’s causing the tripping as the cable must be

Welsh_sparky_21 said:
I will try this when I’m next on the job, thanks for the help.

Good luck

HappyHippyDad · Dec 13, 2024

Welsh_sparky_21 said:
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

Like I said, don't take offense.
I'm just speaking factually.
You said 'I think' I got, therefore I'm just asking for accurate figures.
It would be very surprising to get a reading almost exactly 100 times smaller than the IR reading. It's more likely that you got 10000 ohms.

Welsh_sparky_21 · Dec 13, 2024

HappyHippyDad said:
Like I said, don't take offense.
I'm just speaking factually.
You said 'I think' I got, therefore I'm just asking for accurate figures.
It would be very surprising to get a reading almost exactly 100 times smaller than the IR reading. It's more likely that you got 10000 ohms.

I definitely didn’t get 10000 ohms

HappyHippyDad · Dec 13, 2024

Welsh_sparky_21 said:
I definitely didn’t get 10000 ohms

Cool.
Let us know the outcome.
It'll be interesting to know what it is.

Welsh_sparky_21 · Dec 14, 2024

HappyHippyDad said:
Cool.
Let us know the outcome.
It'll be interesting to know what it is.

Will do, if I ever find the fault that is :/ lol

littlespark · Dec 14, 2024

Any electronics in circuit that might be skewing the readings?
Smokies. Or smart switches?

Might have been covered already, but what about shared neutrals from another circuit still connected to earth at source