Breeze block voltage conundrum.

[Bobble23]

Hi all,

Was called to a job today with intermittent RCD tripping from the garage. Standard domestic TNS with three core SWA from the house to an (inaccessible) junction box down the side of the garage where it joins to 10mm T&E which then runs on surface plastic trunking to a metal-clad CU on an internal breeze wall (two rooms to the garage) about four metres away.

Previous chap has disconnected the circuit and told owners that the problem is probably damage to the buried SWA somewhere in the garden. I started by disconnecting the supply from the garage CU and doing an IR test from the house, and got perfect readings. However, once the supply was reconnected to the garage CU the LE reading was 0.59 MOhm (LN was still over 200 MOhm). This was the same even after taking out all of the downstream cables from the CU.

After a bit of head scratching I ended up taking the board off the breeze wall and found that, with the board dangling by the supply cable without being screwed to the wall, the readings were fine again.

The only thing I can think is that something funny is going on in the inaccessible junction box which is somehow giving the wall a potential. I didn't use a meter but the wall doesn't feel damp at all. Is that possible at all?

Any ideas gratefully received!

Thanks.

 

[*-jester-*]

...The only thing I can think is that something funny is going on in the inaccessible junction box which is somehow giving the wall a potential. I didn't use a meter but the wall doesn't feel damp at all. Is that possible at all?

Yes! Anything is possible. A screw or nail through the wall but just touching the line conductor could do it.
Breeze block walls won't need much dampness to become potentially conductive.

 

[DPG]

You may have to find that junction box!

 

[snowhead]

So the reading is fine when the C.U is off the wall?
But low I.R when it's on the wall, with the circuits out but Feed in?
It's got to be something with the metal C.U then.
Try I.R in the C.U with no cables connected.

Failing that it's knocking bricks out the wall from inside the garage to reach the junction box.

 

[Deleted member 26818]

Sounds like the live might be touching the breez block wall and that is in contact with earth through the metal CU.
I would suspect the T&E to be the culprit.

 

[ddelectrical]

Could it be IR L-E fault in CU conducting back to ground through wall? Check earth continuity to CU case. It may be high.

 

[Bobble23]

Thanks all. Looks like I'm knocking down garden fences to get to that junction box. I'm due back next week so I'll let you know what I find.

 

[NDG Elecs]

I'm curious as to why you decided on the course of action you did by taking the board off the wall hanging loose. Not having a go at all or anything, but I don't think that would have come to my mind. Were you getting voltage from the screws or similar.