Another Strange Fault

[securespark]

Time for another odd fault. I went to the aid of a colleague on a lighting fault-find. The circuit was whole-house lighting.

Breaker tripping between 1-30s after energising. Now, IMO, that's down to an LE / LN (or both!) short circuit, either caused by water ingress, or cable damage by rodent or burn-out for example.

We IR'd the circuit having split the loop in several places and it was close to zero between LE and NE.

We eventually happened across an external floodlight fed from a standard junction box screwed to the external brickwork...

So my colleague went up the ladder & disconnected it. The T & E going into the JB disappeared into the brickwork. Enough was visible to see that it was clearly damaged and shorting out, but he was unable to clear the fault by separating the cores. We did find the switch for the flood and turned it off.

Having isolated this fault we then returned to the CU to do more IR tests.

However, with the faulty circuit cable disconnected from the CU & an adjacent MCB switched on, there was a voltage present at the cable of 150V.

Turning off this MCB killed the voltage.

Any ideas??

 

[ColJack]

2 lightig circuits interlinked.. / borrowed neutral and capacitive coupling on the 2 way strappers..

 

[jj4091]

Or similarly to one I had recently, a plasterboard nail used instead of clip on the cables in a switch drop. It took about 5 years to burn/corrode through & s/c the cores.

 

[securespark]

Cheers, guys. I'm just off to look at it now.

I forgot to say that once we had switched off the faulty feed to the flood, the IR jumped up to 1.5 Meg.

Not brill, but with everything in circuit that's not a bad reading.