Ring Main Insulation Resistance Query

29 Jul 2008
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Mid Glamorgan
United Kingdom

Don't know if you can help with a couple of queries / issues:

I have added a few extra sockets into an existing ring main, and when I come to the testing of the insulation resistance, I have acceptable results on Line - Neutral and Line - CPC, but I get a reading of 0 ohms on Neutral - CPC. All other parts of the ring main tests are satisfactory, ie Earth Fault loop Impedance, continuity of conductors, RCD etc. Any ideas what would be causing this please??

Also, I have another query: I am doing the testing at the sockets themselves as the cabling in the consumer unit is a bit of dogs dinner so I don't want to go pulling cables around etc in the CU. Is it acceptable to measure IR from ther front of the socket using a 'socket and see' type device or do I have to disconnect the cables from the socket and test from the cables themselves. The reason I ask is that I get slightly better readings using the 'socket and see' plug than I do when connecting my meter to the cables themseves.

Any help and advice on these issues / queries would be appreciated as i have my first ever NICEIC assessment next week and this is down as one of my assessment sites!

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Did you swithch off the CU main switch before doing your IR test?

Is there an RCD in the circuit which you have been IR testing?

If you leave the N & E wires connected in the CU you will end up testing all the N & E wiring for the installation as they will be commoned together at the CU.
As it sounds like you are doing the test with the neutral and earth connected in the consumer unit (probably with main switch closed too) you will get a low IR between N & E, think of how a TN system works.
Either that or you have a short somewhere.
I'd also think that you could fail an assessment on the grounds that you have energized a circuit which you have proved not to have a sound insulation resistance.
Thanks for your replies.

You're right, I didn't switch off the main cu switch or pull the main fuse, I only turned off the mcb supplying the circuit in question. The cu main switch is a rccb.

I was hoping to be able to do all of the testing at the sockets themselves as I didn't want to go pulling cables in the cu as it's all a mass jumble and I can't identify the relevant without disconnecting loads of others first.

Is it possible / acceptable to do the testing at the sockets or do I have no choice but to do it at the cu? Also, if testing at the sockets, is it acceptable to do the testing from the front of the socket using a 'socket and see' device or do I have to do the testing at the back of the socket?

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How hard can it be to discon the live cores of the circuit at the CU and tug them to reveal the neutrals and the earths ?
It's pretty difficult, it's a big house with a lot of cables at the cu, all of which are tangled / tight and a lot have been extended within the cu, so there are also a lot of connecting blocks to contend with. Also, as all I've done is added a couple of extra sockets, I don't want to be held responsible for somebody elses mess at the cu.

I'll have another look later see what can be done.

Also, as all I've done is added a couple of extra sockets, I don't want to be held responsible for somebody elses mess at the cu.

You still have to ensure that the circuit you have added to is safe.

I'm worried that you don't seem to want to do the job properly on a job you will be using for your NICEIC (I presume domestic installer) assessment. It isn't going to look favourable for your assessment.

Have you been on a formal testing course? You don't seem to be displaying a working knowledge of testing practices.

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