Identifying cable break with meter

[bilbo*baggins]

While screwing home the faceplate screws on a ringmain twin socket outlet after easing them for painting, I had a 'flash-bang' moment, for one of the screws had trapped - then penetrated - the live cable. Doh! After I'd isolated the whole shooting-match and replaced the fuse, I discovered that the spur socket, fed from the original ringmain socket, was dead. So I disconnected the spur cable completely, isolating the ends properly.

Now I want to identify the point of breach of the ~6 metre spur circuit cable, and repair it. However I do this, there will be some plaster to cut away, somewhere, and make good. This, I want to minimise.

I have a Newlec multimeter and simplistic instructions ( appropriate! ), and expect I'm going to do a 'Continuity test' at intervals along the cable, but I'm uncertain about what I'm looking to see, to identify the point of breach.

Can someone kindly advise?

[stevelup]

It's fairly unlikely that the cable has been damaged - are you positive this is the case?

[tinnews]

Well unless something *very* wierd has happened the break is going to be in the box behind the twin socket where the 'flash-bang' occurred.

I suppose you *might* have pulled the cable to the spur socket so hard that you've parted it somewhere under the plaster but there's still a great likelihood that you've broken the wire (maybe inside the insulation) very close to the end that is/was in the back box of the socket you were working on.

My suggestion would be to remove the double socket from its box again and unwrap the ends of the wires that go to the spurred socket and give the wires (not the insulation) a tug with a pair of pliers, if one is broken near the end it will pull out.

If it gets to this you'll be checking between the point where you have broken in to the cable and the end at the ring main socket, i.e. the red wire should be continuous and the black wire the same. However if there really is a break in the wire it might be less messy to give up and replace it completely. (For 'Red' and 'Black' read 'Brown' and 'Blue' if this is modern installation). The damage you will almost inevitably do to the cable while checking continuity will mean that it needs replacing anyway!

[bilbo*baggins]

Stevelup,

Having belatedly made safe, unscrewed, inspected and then replaced the spur wiring most carefully - then reinstated power - the ringmain twin outlets worked fine, but the spur outlet didn't. There was 'blackening' on the brickwork 'chase' adjacent to the spur cable entry, spur live cable end, and adjacent metal box. The spur live cable sheathing was visibly pierced, presumably by the faceplate screw, for that was what I was tightening when the event occurred. I'm guessing the 'flash-bang' damaged the spur live cable locally to the ringmain box.

Bilbo

The spur socket outlet, some 6 metres away, seemed undamaged on visual inspection.

[John Rumm]

Chances are you had a poor connection at the spur socket, and it is that which failed rather than the cable.

What sort of fuse do you have - re-wireable or cartridge?

What size is the cable on the circuit - and in particular is it old T&E with a 1mm earth?

First make sure your cable under test really is isolated at both ends.

Set your meter to its lowest resistance range. Then measure the resistance with the test leads shorted together, so you can factor this out of your results.

If you twist all three wires together at one end and then measure resistance between all combinations of them at the other end, you should get some useful information.

The Live to earth resistance, and Neutral to earth resistance should be equal. The Live - Neutral resistance should be lower. See the table here for expected values:

for a modern 2.5mm^2 cable with 1.5mm^2 earth at six meters, you would expect to measure values of:

6 x 14.82 = 0.088 ohms L to N. and 6 x 19.51 = 0.117 ohms L or N to E.

(your meter will need to be reasonably decent to measure to sufficient accuracy here)

See more of the above article for details on tests.

[stevelup]

I really cannot think of any possible way that a short circuit within your twin socket box could damage the spur cable downstream of that location.

The only possible place that damage could have occurred is within the twin socket box.

Aside from that, the fault current would have had to be both huge and sustained to damage 2.5mm T&E. I really think you should focus your diagnostic energy into the double socket area. Did you also have the spur off the wall for decorating? Are you positive the connections are sound at the other end.

Steve

[mick]

Examine the cable leading to the spur, at the ringmain end. As the screw penetrated the wire it put an earth on the live conductor, which will have blown a hole in the copper conductor at this end. You can test this.

First, MAKE SURE THAT THE SPUR IS REALLY DEAD - don't trust the obvious disconnected wire! It sounds stupid, but mistakes can kill you. If you don't, your meter will be damaged at best.

You then need to get a connection from the spur (probably using an extension lead and a plug with 3 wires on it. Use the Ohms range (or continuity buzzer if it has one) on your meter to verify L-L, N-N and E-E between the end of the cable in the ringmain socket and the spur. Also check for short circuits between the conductors.

Fixing it is more of a problem... If there is enough spare cable to remake the connection you will be very lucky.

[meow2222]

Its ever the weakpoints in a circuit that go o/c during a heavy fault current, ie connections & switches.

NT

[<me9]

Or in the case of my originalupstairs lighting circuit, where the chippy forced the floorboard into the cable, severing the earth, and reducing the cross section of the live. How it passed its initial inspection I don't know, but it must have, cos it had a label saying "Installed by ...., Nic/eic approved contractor.

I must get round to put the wylex plug in MCBs from my old cu on Ebay.