Hi,

Just put in a new CU, small house with only one ring circuit.

Continuity of ring:
N-N = 0.63ohm
cpc - cpc = 1.1 ohm
L-L = 640 ohm!

Thought it may be a loose socket screw giving high resistance so taken off sockets and did indeed find some loose terminals but tightening did not cure the problem. Then when looking at sockets in an upstairs bedroom I left the ring (live - live) open circuit and the resistance at the CU went up to 900 ohms????? Closed the ring and 640 ohm appeared again, repeated the test with second socket in bedroom and same happened! Breaking the ring in a downstairs socket didn't break the ring!?!?
It's been a long day (bitch pulling 25mm tails through cavity ) so thought I'd come home and think about it but I'm stumped.

One point, It looks as if there has been a conservatory wired off the upstairs bedroom somewhere maybe someone has done something naughty with junction boxes but how would it explain that fault? New floor in upstairs bedroom so will have to do some chopping to see.

I checked the Zs at a downstairs socket and I'm sure it was 0.7 although I didn't write it down yet. RCD works fine.

Any ideas anyone, I'm back at job in morning.

take all the sockets off and by means of continuity testing, work out the run through the house and isolate the locality of the problem to one length of cable and work on that.
sledgehammer to crack a nut, but will save you time in the long run

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Lack of planning on your part doesn't make it an emergency on mine....

Every man has to know his limitations- Dirty Harry

Hi, I had a job with almost the same readings just two weeks ago!

In my situation I found that the ring final conductors had been nailed through three times in a kitchen, burried in the walls and plastered over. Fault was repeated on 3 separate drops. Once we found them and resolved with ne legs it came down a hell of a lot.

I would have a look around for wall units, curtain poles that might have once been fixed through etc.........

Good luck. Let us know if you find out what it is:-)

Yup, take off every socket and split the circuit into individual segments. Measure r1+r2 on each section should find you a duff bit.

Sounds like a nasty junction box hidden away.Don't rule out rodent damage though. I once had a partly cremated mouse carcase wrapped round a badly chewed T&E that measured a few hundred Ohms!

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"I learned very early the difference between knowing
the name of something and knowing something". - Richard P. Feynman

Cool thanks, off now will let ya know later

Did insulation tests with very low results so split circuit kind of mid way and powered up good side of leg until I found an ashley maintenance free jb imbedded in wall behind freezer, looks blood awful!!! but complies I guess. From this was a spur and from that another spur through the wall behind a bookcase. Moved out bookcase and found a RCD socket.....that was the problem! Replaced with standard skt as new board and all is hunky dorey.

Conservatory jb and spur was fine btw and no sign of mice luckily!

Thanks for your help guy's

Are you saying the maintenance free JB on the ring was the culprit , which sounds plausible or that the spur on spur RCd socket was the problem (which does not sound plausible)?

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Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik

You're right ebee, it initially seemed like the RCD socket to me as that was the cause of the low IR. Once I linked out that leg the ring tested fine. I was so happy to find it that I didn't think to work out why.

The RCD socket was the cause of the low insulation resistance test, L-N = 0.09ohm L or N to E = 0.1 ohm across the socket. Why should that be? Looking at a RCD circuit the only connection across L and N is the test button via a resistor which won't be in circuit until the button is pressed and there should be nothing from L - E....guess it's faulty, any ideas or explanation? It's a Timeguard Passive 30mA 2G Socket. A control circuit maybe?

I'm going back to finish off and will have another look.

Don't you mean 0.09 and 0.10 megohms?

Dave.

Your OP never mentioned a poor IR reading only a poor r1 of 640 ohms.
An RCD socket as a spur would not account for this but the JB on the ring might do.
Now it seems there were two faults.
IR should be taken without RCD (including an RCD socket) in circuit

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Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik

Yes Dave, sorry should have been megohms.

With you ebee, the poor r1 could well have been the jb as I disconnected it all when testing. Remaking it may well have cured the problem.

Shoudn't you also be sorting out the spur off the spur !.
Rgds.

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Helping others today enables
others to help you tomorrow.

Yep, done that..only one spur now.

Had a re-check and all was good with readings so I'm happy the installation is fine and safe.

Just to close this, holding my hand up all I can think is that the Fluke MFT 1553 that I am borrowing holds resistance readings on the screen...unlike my other tester....which I didn't realise :"> it was a pretty long day so I guess the ring not breaking was down to me reading it wrong