Damaged Ring Main Cable

[shirley83]

Hi.

My 3in1 detector failed to warm me about a ring main cable in the wall. When I put the screw in the raw plug the point of the screw pierced the cable and tripped the RCD.

The cable runs up the wall and into the loft space which I have access too. So running a new cable from the socket and joining it in the loft space is not too much of an issue.

What is an acceptable method of joining the cable? My house was built in 2000 in red and black and is currently up to the regs of that era, so I dont want to bodge it and void that status. Plus I want it safe. I understand that a ring main has a breaker of 32amp based on the ring using two cables. But any single point of the ring main must support 20 amps. So the joining method must be rated for 20amps or above. Is this correct?

Secondly. Am I allowed to do this or is it a job for a sparky? I'm a pretty confident DIY person.

Many thanks.

 

[Taylortwocities]

If you are joining it in the loft, and the junction will be accessible then a standard junction box will be fine. I would look for a 30A box, very common and that gives you planty of space for the two cables to join.
one of these eg 30 Amp 3 Terminal Junction Box - Brown

If the join will ot be accessible then you must use a 'maintenance free' junction box like this Ashley J803 Maintenance Free Junction Box 32A 3 Terminal

Edit: some points in a ring final may exceed 20A, so err on the side of caution and use a 30/32A box.

PS, throw that detector away, they are unreliable, as you have found.
The cable should be in a recognised zone for mains wiring, was it?

And yes, you can do this work, safely....

 

[terryplumb]

a 30 amp junction box would be used. although making the connections and running the new cable length is very straight forward , but would you be able to test ??

 

[shirley83]

I able to test with my socket tester which does correct wiring and earth impedance. Is this sufficient?

http://www.screwfix.com/p/kewtech-l...gclsrc=aw.ds&dclid=CNnOw6GiptQCFbUg0wodDpgEPA

 

[Taylortwocities]

No. That tester will give you an OK indication, Even if you have a ring circuit with a complete break in the middle (ie not a ring any more).

After completing your repair, you should use a continuity tester at the consumer unit. Remove the cables from the consumer unit and test continuity of the ring for L, N and E conductors.
After a cable damage incident I would also carry out an Insulation Resistance test (needs specialist equipment).

 

[terryplumb]

obviously that tester will only work on a live circuit . tests should be done on the ring final before energising the circuit.

 

[shirley83]

I have multi meter so I could test continuing but that is all. I also want a new consumer unit adding in the garage. Which i know a sparky must do. Might be worth getting one in do repair, test and add the new unit.

 

[ericmark]

1) Are you sure it is a ring? you can get both fused and unfused spurs from a ring, and it is not normal to run the ring through the loft.
2) Methods of repair already covered so I will not repeat.
3) Testing for a ring can be done from any socket which is part of the ring, you do not "Need" to go into the consumer unit (CU).
4) Using a proper earth loop impedance tester a good electrician can actually have a good idea if the ring is intact assuming the test results are available. If a ring has already been tested and we know the earth loop is say 1.15Ω and neutral loop is 0.93Ω for example from the central socket of the ring which would need to be identified, then we could repeat the test on the same socket and we would know if the ring was intact as there would be a wildly different reading if broken. However the normal test for a ring involves moving wires for the test, to get the loop reading, so in practice you just will not have the data to be able to do a live test. So it needs the experience of the electrician who can look at the house and say what would be the likely reading if the system is good. Although the electrician may be able to be 95% sure it is OK or not, he would likely not want to take the 5% chance of being wrong, so he would test dead. One of the places I worked we had designated test sockets for the ring, and we would test the loop impedance and prospective short circuit current every two months so we would know if there was damage. But this is unusual.

But we must start at number 1) are you sure it is a ring? What does it feed in the loft? if it only feeds one socket, how many cables go into the socket, and also is there any fused connection unit

which may be on the feed of that cable.

 

[Taylortwocities]

I have multi meter so I could test continuing but that is all. I also want a new consumer unit adding in the garage. Which i know a sparky must do. Might be worth getting one in do repair, test and add the new unit.

Definitely the better plan

 

[EFLImpudence]

Surely it would be better to test the circuit at the socket - if a ring - than to open the CU and disconnect the conductors.

 

[terryplumb]

testing within the socket would require the conductors to be removed ,checked then replaced and the socket front put back .so you then do not know ,for instance , if a conductor had broken or come out a terminal whilst putting socket back. so testing at the cu gives you visibility of the re connected conductors.

 

[EFLImpudence]

testing within the socket would require the conductors to be removed ,checked then replaced and the socket front put back .so you then do not know ,for instance , if a conductor had broken or come out a terminal whilst putting socket back. so testing at the cu gives you visibility of the re connected conductors.

Shirley is replacing one of them.

 

[terryplumb]

got that EFL, what's your point though ,am I missing something here ? dont see the relevance .regards T.

 

[shirley83]

I'm very confident it is a ring main cable. There are two cables running down the wall to the socket below (slightly to right of the two wires, which is the reason I believed the tester I used) which has two wires connected to it. I've attached a diagram. I've removed some plaster to accesses the damage, the reason I know there is two wires. To to be honest, it looks like the screw head has only nicked the insulation and shorted two wires together but still, damage is damage.

 

[shirley83]

Diagram.