Insulation resistance testing issues

[riveralt]

I'm in the final stages of a partial rewire and am in the process of preparing the current kitchen socket radial circuit for connection to the Consumer Unit.

Continuity Testing gave R1+R2 = 0.25ohms. However, the Line/Neutral to earth IR test gave 1.56Mohms.
I broke the circuit down and tested each individual leg and got 200+Mohm readings.
I then connected each individual leg of the circuit and tested each time. Everything was fine until I connected one socket to the backbox and took the reading - it was 1.49Mohms.
This socket has a surface mounted metal backbox. The IR reading is in excess of 200Mohns until I connect the socket to the backbox - remove the screws and everything is fine.

Question is, will replacing the metal (surface mounted) backbox with a plastic version cure the problem or does the problem lie with the socket itself?
Any clues or help would be appreciated.

 

[JohnD]

I'd swap the socket first, yours might be damaged or wet. If no help, examine the cables for damage e.g when stripping they might be nicked.

You have got a grommet in the metal box, haven't you?

It is possible to use heatshrink sleeving over a nick in insulation, but in your case especially, preferable to replace damaged cable with new.

 

[Taylortwocities]

Has the place/area been recently plastered?

I have sometimes had rubbish IR readings until the plaster has dried out.

There may, of course, be a nail / plasterer's trowel something through one of your nice new cables

 

[JohnD]

p.s. could the fixing screws for this backbox have penetrated a cable?

 

[DetlefSchmitz]

Also, why not test the socket on its own?

 

[MrTinker]

I would say it's got to be the socket, assuming no wires are getting traped/crushed as it gets screwed down, the back box is deep enough isn't it?

 

[JohnD]

I'd swap the socket first, yours might be damaged or wet. ... .

 

[MrTinker]

sorry mate my computer is on a go slow today, nobody had posted a reply when i started typing.

 

[riveralt]

Thanks for the replies.
JohnD - Grommets used - well I have put them in anyway.
Tested cable between each leg of the circuit and got good readings. Was planning to swop/change the socket but ran out of time/steam!!!!


TTC. The house is a very damp - I think this could be the problem. I tested the circuit with the socket connected to the cable but hanging loose from the backbox - good reading. But as soon as I touched the socket screw to the metal back box I got the poor reading.

I think I will replace both backbox (for a plastic one) and socket tomorrow.

Thanks for your help.

 

[bernardgreen]

I tested the circuit with the socket connected to the cable but hanging loose from the backbox -

To me that suggests there is a path from the back box when NOT connected to the CPC ( earth wire ) to live or neutral.

That suggests there is another fault somewhere where live or neutral are in contact with something conductive ( damp plaster ) that is also in contact with the back box. The resistance of that something conductive being the 1.56 Meg when the back box is connected.

If it is damaged insulation allowing the live wire to contact damp plaster then any time in the future that area of plaster becomes damp it will also become live to the touch.

 

[riveralt]

I tested the circuit with the socket connected to the cable but hanging loose from the backbox -

To me that suggests there is a path from the back box when NOT connected to the CPC ( earth wire ) to live or neutral.

That may well be the case given that so far I have found over 40 jb's dotted about the place including two buried in plaster.

That suggests there is another fault somewhere where live or neutral are in contact with something conductive ( damp plaster ) that is also in contact with the back box. The resistance of that something conductive being the 1.56 Meg when the back box is connected.

But wouldn't that be ruled out for this circuit given the fact that I got good continuity and good insulation resistance readings for each individual leg of the circuit.
The only circuits currently live are the upstairs & downstairs lights - and they have been totally rewired.

 

[Spark123]

Is it a bog standard socket, i.e. not one with a neon, SRCD or mains filter?

 

[riveralt]

Is it a bog standard socket, i.e. not one with a neon, SRCD or mains filter?

No - it is a bog standard double switched socket - apart from the fact that it is red in colour and not white!!

 

[Spark123]

No hidden microphones/cameras?

 

[RF Lighting]