Cable tracing ring main

[thirdtimelucky]

I have a socket on a ring main and I need to find the the next sockets on the ring in order to change one of the cable that has been cut and joined with a choc box in the wall (previous householder). Is there a way to find these next in line sockets without disconnecting all sockets and finding them with a loop test?

 

[Adam_151]

Is there a way to find these next in line sockets without disconnecting all sockets and finding them with a loop test?

There is a way to find them with a loop test without disconnecting all the sockets (just one leg in the board). That any good to you?

 

[ericmark]

You will likely find the next socket with a loop impedance test however I would select prospective short circuit current as there could be a odd extra earth connection but less likely to have a odd extra line or neutral connection so select the sockets near to one with problem and measure the PSSC and you should see different readings for each socket so the reading closest to damaged socket reading will be the next in line.

However most DIY people don't own a loop impedance tester. If you don't have one then it gets harder and you have to decide if it is worth hiring some one with the meter or not.

 

[thirdtimelucky]

I only have a multimeter and a megger but are the cable lengths between sockets long enough anyway to discern a difference in impedance?
Adam. Can you expand on that ?

 

[theprinceofdarkness]

You could try a cable tracer, sort of mini mine detector. Use "Post its" to mark where you think the cable seems to be going. Then after a few samples you will get the general run, even if a few hits are wrong.
Frank

 

[Taylortwocities]

are the cable lengths between sockets long enough anyway to discern a difference in impedance?

You would need a low-ohm resistance test meter. The type used by every electrician.
Standard multimeters do not have the ability to measure resistance difference in the hundreths of an ohm.

You can find out where a ring (or other) circuit goes by just using a bell, or a standard multimeter on the ohms setting, but it can take a long time..


Ps, you should be making the measurements without the circuit energised, so you would be measuring resistance, not impedance.

 

[deadshort]

are the cable lengths between sockets long enough anyway to discern a difference in impedance?

You would need a low-ohm resistance test meter. The type used by every electrician.
Standard multimeters do not have the ability to measure resistance difference in the hundreths of an ohm.

You can find out where a ring (or other) circuit goes by just using a bell, or a standard multimeter on the ohms setting, but it can take a long time..

if you split the ring at the connector block and disconnect the sockets nearest to the joint and use the "BELLING OUT METHOD". TTC , you cheer me up ! no long winded explinations , just practical advice

DS

 

[Taylortwocities]

K.I.S.S.

 

[thirdtimelucky]

Finding where the cable goes won't be difficult if I diss all of the sockets, then I just have to find two with the m/m and a loop but I was hoping to avoid doing that.
If I can find the two sockets either side of the socket that has a damaged feed cable, I can connect those two together with a new cable to continue the ring and disconnect the faulty cable and leave that socket as a spur with just the one cable. It only feeds a tv and hi fi so doesn't pull much power.

 

[flameport]

one of the cable that has been cut and joined with a choc box in the wall (previous householder)

How do you know of the existence of this joint, yet do not know where it is?

 

[Adam_151]

FP, I Read it as that he has discovered the joint and knows therefore exactly where it is, but what he doesn't know, is which two sockets on the ring are either side of it, in order to re-wire a new leg between them that doesn't include the joint

 

[thirdtimelucky]

Adam, that is correct. I want to bypass the faulty cable and leave that particular socket as a spur on the remaining good cable. The ring doesn't seem to go to the obvious sockets and if I 'take a chance' I could end up with disconnected sockets. I think I'm going to have to disconnect all of the sockets and then find the two that buzz out to the faulty socket. I was trying to avoid that for various reasons.

 

[Adam_151]

The way I would do it is with loop tester.

Disconnect one end of the ring (all three conductors) at the board, place in connector blocks. Liven up the circuit, go round the sockets and get loop readings, get a loop reading at the joint you want to get rid off. The readings will start off low at the first socket the still connected leg goes to, and get gradually higher as you go around the ring. The closest ones above and below the reading at the connector block are the sockets either side of it. Note that as Eric says, any spurs might throw you off track, so after you have found the sockets you think it is, you need to check that neither is a spur.

If on on RCD, apply loop test between Live and neutral instead of live and earth, the non trip mode can often give fruit machine numbers


However you said in a later post that you do not have access to a loop tester, so the plan is de-railed!

 

[SimonH2]

There is a way to do it with just a multimeter .. plus a battery and light bulb. If you use (eg) a car battery and a spare headlight bulb to limit the current, you can put a few amps through one wire - I would pick the earth for the simple reason that it has the smaller core size and so will have higher volt drops, but you can use the L or N depending on what's most convenient.

So knock the power off, and make sure it can't be turned back on - ideally lock it off but you probably don't have the means to do that. You can do this bit in the main board, or at any convenient socket. Separate the earth wires so you have access to both ends of the loop, but leave the L & N connected together. Link one of the earth ends to L&N and bring a tail out & connect to one side of the battery. Link the other earth wire via the headlight bulb to the other side of the battery. You will now have something in the order of 4A going round the earth wire.
You can now use your multimeter to measure the voltage between N (your reference line) and E at each socket - you'll see the voltage progressively go up as you go round the ring and it should be easy to work out the order of sockets (and the JB) on the ring. You can also estimate the relative length of each section - and if you have a reasonable measure of the current flowing, the actual lengths.

 

[thirdtimelucky]

Thanks for the advice. I've now solved the problem but in so doing have unearthed 2 more problems.
I'm asking for advice on them in the forum