Ring circuit problem

As mentioned in my "soundproofing and AV cabinet in the chimney" thread I'm relocating the mains and aerial boxes from the party wall, and while doing so adding some extra sockets to the ring.
At the weekend I was ready to remove the final old socket and insert my loop of extra sockets, so I'd isolated the CU, did a check L->L, N->N and E->E around my new loop which was OK, then I did a similar check around the existing ring at the point I would join into it ... live and neutral OK, but oh dear, no continuity round the earth!
Spent bloody hours over the weekend (in bursts of a few hours so the fridge/freezer wasn't off for too long at a time) with temporary shorts between E/L or E/N at various points to trace where the circuit went, and whether the fault was up or downstream of those points, checking junction boxes under under floorboards which I knew were there, finding one I didn't know existed, remembering sockets that were behind wardrobes that haven't been used for years etc, what a ball-ache.
Finally today traced it to the cable on the ring that's between one socket in the back bedroom and another socket in the front bedroom on the same wall, cable goes down wall, under floor, back up wall, it's plastered over, can't see if there's metal capping over it, but I imagine so, similar cables in other original parts of the house are, I've never done any work along the route of the cable, and can't see where any previous owners have either.
There's a convenient loose floorboard (I presume from when the central heating was added) about midway between the sockets, where I can poke a torch and a mirror on a stick down, no damage to the cable where it's visible, so I conclude it has probably been like it since the house was built in ~1976.
Getting access to the cable at either end is a bit of a problem at the moment, back bedroom is chock-full of stuff while I'm working on many other parts of the house at once, front bedroom there's a large wardrobe in front of the socket which I can barely move, and there's nowhere to move it to at the moment.
So for /now/ I have put it all back as it was, knowing that every socket is earthed, some from one direction, some from the other. There's nothing which is a "high powered" appliance on this ring (I put a second ring in just for the kitchen and garage a few years ago) in fact it's probably only the fridge and boiler which aren't double insulated appliances anyway.
I suppose I could deliberately split the ring at one of the junctions, and replace the 32A MCB with a 20A MCB and call it a radial instead of a ring (that just happens to have the CU at its centre).
Not having a trained monkey to assist with going round with the multimeter while adding/removing shorts made it a slow job, in fact I thought it might be worthwhile knocking up a battery powered box to toggle a set of relays in a sequence, so you could wire it in and it would give e.g. one brief short from L/E followed by two longer shorts from L/N to cut down the number of trips up and down the stairs and squeezing behind furniture etc, does a similar testing device exist?
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Andy Burns wrote:

Turn off the power. Join up L+E for the circuit at the CU. Go to the predicted midpoint of the circuit, disconnect the ring at that point. Check which side has no continuity between L+E at that point. Then go round the 'no continuity' half to find where the continuity re-starts between L+E. You use an adapted plug socket for this to test continuity (or the proper thing for £10ish), to save taking the socket outlet off the wall. You then know the rough area where the break is.
Sometimes you will not be able to find a break, so the only option is to either put in a new cable where the break is, or disconnect the broken length of cable, and put the circuit on a 20A CB. You are unlikley to trip a 20A CB in a domestic property for socket outlet use.
--
Alan
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A.Lee wrote:

Thanks, yes I'd done that, didn't help that I was unaware of the actual wiring routes in the house, but I do now and have tracked the CPC discontinuity to a single cable between two sockets.

Yep, I have one of the Kewcheck R2 plugs with 3x4mm banana sockets, used with shrouded leads to multimeter.

I intend to do that "later", by which I mean in the next 2-3 months when the downstairs is done and I have some breathing space upstairs. The joists run the right way so that all I need to do is break into the plaster for 6" under each of the two sockets and use fish rods to poke a replacement cable between them.

I doubt if the total load that currently exists on the ring in question would trip a 6A MCB, all the heavy stuff is on a separate ring, pretty sure I have a spare 20A one in a box somewhere, will chuck that in until I replace the cable and return it to a ring ...
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On 27/05/14 14:16, Andy Burns wrote:

That would be of course OK.

As you know NN and LL are fine in the ring end-end continuity test, short one E to it's L (same cable) and test every socket for L-E continuity would have been the standard way to find the last working socket on the ring. Repeat with the other L-E shorted and the first one removed and you should end up with a pair of sockets either side of the fault.
The faults will be in one or the other socket[1] or in the cable or any mysterious junction boxes between.
[1] or one socket "back" if you happened to hit a spurred socket as the last tested working one. An obvious spur (single cable into socket) can then be discounted. You might of course still fall victim to non standard wiring, spur off spur and whacky wiring faults but you'll be in the right area physically.
Now what I would do is to take your 2 "end of part broken ring" sockets, take them off the wall, disconnect and seperate all cables into both and test the cable that links them.
And repeat the LE test on both halves of the ring to the free disconnected cables - should get a reasonable confirmation that your understanding of the layout is correct or not.
If you find a run of cable with the fault, are both ends accessible under the floor enough to fish a fresh piece through and joint in without having to disturb the bit that goes into the sockets?
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It's most likely the break is at an actual socket so a repair shouldn't be too difficult. Apart from accessing that socket if it's behind furniture, of course.
An easy way to test an earth continuity on a ring is to disconnect one end at the CU and make up a lead with say a 100 watt bulb wired from line to earth in a 13 amp plug. The current that takes prevents the false reading you can get with a high impedance DVM. Go round all the sockets until you find the break - perhaps marking them all as good or bad. It's not always obvious the route a ring takes.
But do this test with no-one else in the house, and nothing else plugged into the ring. As anything which is plugged in after the break and earthed will become 'live' when you use this type of test.
--
*Marathon runners with bad footwear suffer the agony of defeat*

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Dave Plowman (News) wrote:

Of the two sockets in the region of the break, one was where I'd taken a spur to feed the distribution amp in the loft and the other was where a previous occupant had replaced a flush single with a surface mounted double, so both were under suspicion for a disturbed earth connection inside.
But as it turned out, the connections were all fine (original with green sleeving rather than green/yellow) and it is a cable fault, or another hidden junction box somewhere under the airing cupboard/host water tank, but the mirror and torch didn't show anything.

Yes, one end behind wardrobe, other end tight behind vanity panel of desk with a huge laser printer and a mountain of tat on top of it, under it and around it - typical plus I'm not build for squeezing into small gaps!

Didn't really have much issue with false readings, other than once when I left fridge switched on at the thermostat (it's difficult to unplug it) and when it warmed up sufficiently it kicked it and presented a mystery 19 ohm load while I was trying to locate my deliberate dead short between L&N.

Oh yes, if wouldn't have wanted *anyone* else in the house while I've been opening the ring at various points, and as you say shorting L/E or L/N at various points to test continuity and occasionally re-energising half the ring to track cable routes with a volt stick knowing there were bare wires sticking out!
A pocket full of wago connectors has been very handy for making/breaking/remaking connections without chewing up cable ends by constant screwing of connections.
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