Insulation resistance issue.

[wsoppitt]

This is a new 1 on me.
Testing a ring final circuit within a timber outbuilding.
All clear on insulation resistance with socket fronts connected but not screwed into metal back box lug. As soon as I connect any of the plates to the back box I'm getting around 16megohm between n and cpc. When all socket fronts connected getting around 2.2megohm. All I can conclude is that the neutral has been caught and is conducting through the timber frame (damp) to the backbox. What are your thoughts please.

 

[snowhead]

Have you split the ring to try to isolate the fault?

 

[Lucien Nunes]

Your analysis seems about right, that the neutral is leaking to part of the structure and hence to the back boxkes, but not to the earth terminal in the CU. As Snowhead says a normal 'divide and conquer' approach should find it.

There was a very similar thread (albeit involving the line conductor and breeze block walls) here: Previous low IR to back boxes thread

 

[wsoppitt]

Have you split the ring to try to isolate the fault?

Snowhead,
The problem with that is I'm getting a reading irrespective of which socket is connected to backbox.
[automerge]1592602354[/automerge]

Your analysis seems about right, that the neutral is leaking to part of the structure and hence to the back boxkes, but not to the earth terminal in the CU. As Snowhead says a normal 'divide and conquer' approach should find it.

There was a very similar thread (albeit involving the line conductor and breeze block walls) here: Previous low IR to back boxes thread

Lucien, just read that previous thread you referenced , exactly the same issue. So what I need to do is disconnect all neutrals and individually test between each leg and screwed back front plate until I find the pairing ends. Got it

 

[davesparks]

Why not test directly between the back box and neutral at each point to see if you can find something?

 

[Midwest]

I had something similar on am existing installation, in this case a tw&e used as a pair of strappers for 2 way lighting, had just one of the cores nicked by a nail, into a plastered wall. If the back boxes do not have an earth fly lead, as suggested, screwing the back boxes back would make the fault connection.

In my case the actual fault was easy to find, it was nail fixing a dado rail directly below a landing switch. The dado rail had been installed some years prior.

 

[wsoppitt]

Why not test directly between the back box and neutral at each point to see if you can find something?

Dave that would probably be quicker?