Puzzling IR & Continuity results on an old Ring Main

[bf2k]

Hi, I'm relatively new to this forum and have not long since qualified.

As one of my first job, I recently installed a new CU to feed a large shed. I got the clients provider out to install a service switch so I could safely tail up the new CU. Whilst the engineer was out he plugged his tester into one of the house sockets and found there was no earth. After I'd finished the shed feed I looked at this problem and have uncovered a much bigger problem.

The existing CU is the original from the 70's, standard 3 way re-wirable Wylex box with a 5 AMP and 2x 30 AMP fuses. The lighting circuit is working fine. One of the 30 amp circuits is a radial with a 4mm cable, but I don't know where it is running to. But the 30 Amp circuit I have the problem with is for the house socket ring main (which had no earth). After removing the socket (which to begin with I thought it was a fault with the socket as the rest of the circuit was fine) I found that the earth connection was loose. However, I did the standard IR test on the circuit to issue the minor works cert where I found that there was a problem. My meter (fluke 1653) wouldn't even give me a reading. I've tried doing the test on both 500 VDC and 250 VDC, where both gave me a 0.00mohms reading @ 0.8VDC (which has puzzled me). So I did an R1+Rn gave 53.4ohms (R1+R2 and R2+Rn give >2000ohms). So from this I thought they must be a short between Live & Neutral and proceeded to fault find.

I split the circuit, found faults on both sides. So I split each side and have now managed to narrow this down to 4 sockets (the second in the line, one at the top of the stairs and 2 in the kitchen). What I'm puzzled at is no work has been done recently and the sockets which have faults don't seem to be consecutive sockets in the circuit. Plus before I changed the socket in the hall (which was the first in the line) the system worked. I also did a test from earth and neutral to a back box and got continuity, which again has puzzled me. I didn't do any tests other than the plug in tester to confirm what the engineer found before I changed the socket. They seems to be no logic to which socket follow the last and there are a few spurs running in a few places, but they seem to be fine.

This is the first fault finding mission I've had to do on my own since qualifying and I think I've been dealt a real one. Its a good job I know the client. I guess they can't prepare you for things like this at college.

Does anyone have any ideas, or come across something similar before?

Thanks
Andy

 

[bf2k]

I've suggested that but she doesn't want to redecorate. Plus that would then be notifiable work and would need to comply, which it wouldn't because there is no rcd.

I've tried to find the E+N fault at each point and can't locate it. It is also in the lighting circuit. She doesn't want me ruining her house basically .

It's 2.5mm^2 cable as well.

 

[Ses2010]

It does, especially when you flick the switch and plug your tester in and you don't hear snap crackle pop and get the results you want

I thought it would do but thought I'd look into the idea. I've told her that she really needs a new CU, which in turn will mean she needs a rewire (mainly because of the N+E fault and the IR between L+N was 8mohms. Must be puzzling for her though because the system has worked for 40 years. I've told her to go and get a second opinion if she needs but at least give me a chance to quote . She has taken me up on the suggestion of recplacing the re-wirables with plugin mcb's.

Just out of interest why does a l-n ir reading of 8meg ohms make you think it needs a rewire?
Its still well within the allowable although low it still is acceptable.
Would it not be better to suggest finding the n-e fault and just a fuse board upgrade

 

[Geoffsd]

I've tried doing the test on both 500 VDC and 250 VDC, where both gave me a 0.00mohms reading @ 0.8VDC (which has puzzled me).

Just for information -

This is because the meter has detected the very low reading ( 54Ω in this case ) and stopped the test at 0.8V rather than go up to 250V or 500V which may damage equipment.



Also, you haven't mentioned end to end continuity testing of the conductors.

This should be the first test on a Ring and would have told you right away that it wasn't a ring - because it never was or that it was broken.

 

[bf2k]

Just for information -

This is because the meter has detected the very low reading ( 54Ω in this case ) and stopped the test at 0.8V rather than go up to 250V or 500V which may damage equipment.



Also, you haven't mentioned end to end continuity testing of the conductors.

This should be the first test on a Ring and would have told you right away that it wasn't a ring - because it never was or that it was broken.

Thank you for the info. Like I said, lessons obviously learnt and all down to experience.

 

[ayjay]

bf2k--
take all of the sockets off of the faulty radial, and part the conductors--(its an old 70,s built, so there can,t be many sockets),
in the existing d/b, disonnect the l/n/e of that radial circuit, then test each leg,starting from the outgoing 2.5 twe then each loop to the last one, the final socket with only one cable
if you find some kind of fault on a leg trace it