Todays fault finding

[HappyHippyDad]

I had a bit of a head scratcher today, I think I've got to the bottom of it but thought I would run it past you guys.

0.15MΩ between N and CPC on a ring circuit.

I started breaking down the circuit and when I disconnected one of the sockets the fault cleared. I now got 15MΩ. Nothing obvious to see, but it cleared so I was happy.
I then started putting the sockets back on. One of the sockets was a spur and it was feeding another spur. Before I put this socket back on I IR tested both cables, to eliminate the possibility of the fault being on the cable leading to the (2nd) spur. It tested fine, however, the cable feeding the first spur came up with an IR reading of 0.0MΩ. I then did a continuity test and got 0.1Ω (N-E)

I then retested the whole ring, and still got 15MΩ, even with the spur testing at 0.1Ω (N-E).

My plug in tester had indicated that all was ok when I was initially determining which sockets were on the circuit (i.e 3 lights showing on the tester).

I then did a R1 + R2 which showed me the CPC was broken.

I then calmed down and realised that the CPC must be completely broken, but something was causing continuity between the N-E.

I'm now thinking a screw through the cable, but how likely is it that a screw would completely severe the CPC and then make contact with the neutral, but actually only be in contact with one part of the now broken CPC?

Another thought is that the screw has been removed, somehow leaving the neutral still touching one part of the broken CPC?

Another thought is that the CPC is actually connected to the neutral in the JB under the floorboards?

Just thought I'd check if I'm missing anything obvious before I start chopping in to the wall or taking up floorboards?

 

[littlespark]

Faulty sockets perhaps?

Or earthed faceplate screws pressing into neutral wire?

Another (hidden) socket or joint box?
A spur from a spur? Has there been a lot of bad alterations?

 

[HappyHippyDad]

Faulty sockets perhaps?

Or earthed faceplate screws pressing into neutral wire?

Another (hidden) socket or joint box?
A spur from a spur? Has there been a lot of bad alterations?

The socket was disconnected when i did the tests.
If the face plate screw was pressing into neutral wire then this would show up in an IT test at the CU
Yes, there could be a JB as per OP.
Yes, quite a few bad DIY bits

 

[happysteve]

I think you're going to have to draw us a picture, HHD. Gut feeling says there are 2 faults, though - broken cpc, and a short somewhere.

 

[HappyHippyDad]

I think you're going to have to draw us a picture, HHD. Gut feeling says there are 2 faults, though - broken cpc, and a short somewhere.

I'll try and get around to it

 

[HappyHippyDad]

I think you're going to have to draw us a picture, HHD. Gut feeling says there are 2 faults, though - broken cpc, and a short somewhere.

Here we go

With both option 1 and option 2 in the picture below I would still get satisfactory N-E IR results when testing on an actual part of the ring (not at the spur), but at the end of the spur I would get continuity between N-E.

Also, when testing R1+R2 at the spur I would get an open circuit, as in both options the E does not come in contact with the earth of the ring.

Does this seem right?
Are these my only options?

 

[Pretty Mouth]

Looks about right. So the spur originates from a hidden junction, damn. Have you worked out which two points on the ring it sits between?

 

[westward10]

Pull the cpc at the spurred socket see if it actually pulls out.

 

[HappyHippyDad]

Looks about right. So the spur originates from a hidden junction, damn. Have you worked out which two points on the ring it sits between?

No.. it was time to go home, I'm had enough! Good reminder to do so though, thanks.

 

[HappyHippyDad]

Pull the cpc at the spurred socket see if it actually pulls out.

Very technical