IR short, only when cable connected?

[ManchesterKirchoff]

So was on a call out today and cooker circuit kept tripping the RCD. Disconnected at the board and at the cooker switch and IR’d the cable and received the following readings:

L-E: 200
L-N: 999
N-E: 999

Everything’s good, reconnected at the board and re-tested the cable going back to the board at the switch and then I got the following readings:

L-E: 0.00
L-N: 0.04
N-E: 0.04

I have one theory but I don’t know if it actually makes sense- The cable ran through “boxing-in” which also contains the water stop tap which is bonded. When I connected the cables into the board and IR, I’m getting a dead short because the cable may have been damaged and touching the water pipe, so with both the bonding and cooker CPC being connected at the earth terminal I’m then getting my faulty readings, by disconnecting the cpc of the cooker out of the earth bar it’s no longer reading through the water pipe.

I replaced the cable as I didn’t have this idea at the time and the circuit now works as intended, I just wanted to see if my theory does make sense and this could have been the reason for the fault.

 

[Pretty Mouth]

Disconnected at the board and at the cooker switch

what conductors did you disconnect? all 3 L, N, CPC?

 

[ManchesterKirchoff]

what conductors did you disconnect? all 3 L, N, CPC?

Yeah, but I wish I tried with and without the CPC only connected into the earth bar just to see if that conductor alone was causing the readings I was getting

 

[Pretty Mouth]

Yeah, but I wish I tried with and without the CPC only connected into the earth bar just to see if that conductor alone was causing the readings I was getting

yeah, if you were testing between live conductors and CPC disconnected from the MET, then your IR testing wasn't compliant. CPC needs to be connected to the MET.

When you say you reconnected at the CU, what did you connect to where?

 

[ManchesterKirchoff]

yeah, if you were testing between live conductors and CPC disconnected from the MET, then your IR testing wasn't compliant. CPC needs to be connected to the MET.

When you say you reconnected at the CU, what did you connect to where?

Personally I would have kept the cpc connected but the bloke I’m working with was adamant on disconnecting the cpc from the board along with L and N (I’m still an apprentice so I didn’t want to question his judgement).

When I reconnected the CPC was in the earth bar, N in the neutral bar and L in the breaker. I double checked beforehand to make sure the neutral was connected into the right bar

 

[buzzlightyear]

yeah, if you were testing between live conductors and CPC disconnected from the MET, then your IR testing wasn't compliant. CPC needs to be connected to the MET.

Not nessasary, two ways if your circuit has a fault, disconnect both lines and cpc
You need to test which side of the cable is damaged.

 

[foresite123]

Have you got the main switch on ?

If so then you need to disconnect the neutral from the neutral bar.

 

[Pretty Mouth]

Once reconnected, N was in the neutral bar, along with several other circuit's Ns, yes? So your IR test between N and E included all those other Ns.

Also: it is likely that equipment was in circuit on some of those other circuits, effectively connecting those Ns to their respective Ls. So your N to E IR test was in fact N+L to E test for several circuits. The low IR reading between live conductors and earth may have been from any of those circuits.

Also: any test between L and N would be across said installed equipment, which would give low IR.

Also: if the main switch was closed, then the N bar would be connected to E via the incoming N conductor, giving a short.

 

[westward10]

I concur with the above advice.
Confirm when you reconnected whether the cooker was in circuit with the switch on. The most obvious thing here causing the fault is the cooker and this is what the majority would test first.

 

[westward10]

Personally I would have kept the cpc connected but the bloke I’m working with was adamant on disconnecting the cpc from the board along with L and N (I’m still an apprentice so I didn’t want to question his judgement).

When I reconnected the CPC was in the earth bar, N in the neutral bar and L in the breaker. I double checked beforehand to make sure the neutral was connected into the right bar

It is actually specified in BS7671 that the cpcs must be connected to all other sources of earth.