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

 

[Marvo]

It sounds like you're beginning to run around in disorganised circles.

As this is your first job I'm guessing you might not be completely familiar with your test equipment. You're misinterpreting the readings, if there was a L-N short then it would have been blowing the fuse.

Go back and start with the primary fault again (no earth). Find the missing earth and reconnect it. Don't start complicating the issue with other tests until that's done. Once you've fixed that you should be able to power up the installation as long as you haven't altered anything else.

Could the N-E low resistance be coming from the TNCS arrangement at the point of supply entry?

 

[bf2k]

Thanks for the reply

I have re-connected the earth and done the appropriate test, all good. I have disconnected the Live, Neutral & Earth for the ring from the rest of the supply (plus I'm not getting a low resistance from N+E). As part of the minor works an IR reading must be taken, which is where I've found the fault. I don't really want to juice it up to be honest until I'm 100%.

 

[Marvo]

plus I'm not getting a low resistance from N+E

I'm possibly misunderstanding your tests, I got the impression you found low IR N-E when you said;
I also did a test from earth and neutral to a back box and got continuity, which again has puzzled me

 

[ExArmy]

so what was plugged in when you measured your L-N 53ohms!?
just go through the test sequence for a ring main in order and then it will make more sense.

 

[spark 68]

Hi Marvo,

I think you on on the money the first time, if it is TNC-S and only the fuse was pulled he would get N-E continuity, this reading would then disappear when the cable was disconnected at the source

 

[bf2k]

This was done with a continuity tester (Fluke T100) between the back box and Live, back box and Neutral & back box and earth.

It was also that last test I did before light gave up on me.

The thing which is puzzling me is that the fault(s) is between these 4 sockets which aren't consecutive. Would you be powering up the circuit after re-connecting the earth even with the poor IR readings?

To reply to other posts:
Nothing is plugged in. I've unplugged all appliances and disconnected any spurs that are run off sockets (although this isn't saying there isn't anything under the floor)

 

[valleybilly]

Your fitting a cu on a minor cert to a shed ? . Next time someone tells you there is another problem then tell the client directly , Your cuoriosity is comendable but it dont pay the bills tick tock mate lol ..

 

[Strima]

TBH it might be worth lift a couple of boards to see if any DIY additions have been made, choc blocks etc can easily be hidden.

 

[Strima]

One of the 30 amp circuits is a radial with a 4mm cable, but I don't know where it is running to.

Cooker/shower? Have you been in the loft? May have also been a radial for the old forced air heating systems so a socket/blanking plate near the airing cupboard may be a good place to look.

The first thing I do after a nice cuppa from the client is to identify all circuits, this can actually turn out to be an advantage and save you pulling cables through when there's already one there.

 

[spark 68]

The first thing I would do is disconnect the entire circuit from the CU (including the N+ E ), then do the tests in sequence, and see where you go from there.

 

[bf2k]

valleybilly - at what point did I say I was fitting a CU on a minor works? I think you'll find I stated it was a separate job and, quite correctly, I was doing the socket on a minor works.

Is it always this "hostile" on here or is it the time of the night?

 

[Strima]

Don't worry about the locals mate, it's Friday night and and no one has pulled... :lol:

 

[bf2k]

Thanks Strima

I've been in the loft and, although extremely full, only a light run off an upstairs light is up there. The shower was on its own CU, but has since been removed. The oven is run off a plug top to a socket on the ring main. The own of the house has been there since it was built and said she'd had storage heater before central heating, so I'll go and have a look under the floor boards near the boiler for a capped off supply. Lifting other boards in the house maybe tricky though. Its not a minimalists house if you get my drift.

 

[Marvo]

Lol, I think you've got some food for thought at the very least. It's difficult to give specifics because a lot of the tests you did are inconclusive and some of your results such as there 'a short between Live & Neutral' are contradictory if the fuses weren't blowing.

I would go back and as suggested completely disconnect any suspect circuits to eliminate any parallel paths and conduct all the tests again from scratch. If your tester gives any confusing or inconclusive results try the same tests again with an analogue tester.