How to deal with issue in underground wiring?

[twoscoops]

Hi,

I've got an external extension that was installed before I own the house. It runs to an outside socket, and then underground for 50 feet or so to an outbuilding.

Over this winter the RCD for this circuit has started tripping. With a bit of DIY disconnecting of things, I've determined the fault is in the buried cable and not at either end,

Would a professional be able to isolate where in the cable the fault lies to save digging up the entire garden? Or would it be simpler to just install a new run of cable? Either way, any idea for a ballpark on how much that might cost?

I'm in the UK.

 

[Strima]

What sort of cable is it? How has it been run, buried direct or in conduit?

Pictures would help.

 

[twoscoops]

What sort of cable is it? How has it been run, buried direct or in conduit?

Pictures would help.

It looks like fairly standard SWA cable, and it's buried in soil. From what I can tell, without any conduit ... I can only see the ends where it disappears off into the ground.

 

[timhoward]

A professional could prove beyond doubt the fault is with the cable.
It would be reasonably rare for an SWA cable to break down underground.
When you did your DIY disconnecting, did you disconnect (and safely terminate) both Live and Neutral the far end or just Live?

 

[twoscoops]

A professional could prove beyond doubt the fault is with the cable.
It would be reasonably rare for an SWA cable to break down underground.
When you did your DIY disconnecting, did you disconnect (and safely terminate) both Live and Neutral the far end or just Live?

Yep - it was tripping consistently before I did anything. I then removed all three wires from the far end, still tripping.
Reconnected them, and removed all 3 wires from the near end (in the external socket) - stopped tripping.
Reconnected them - started tripping again.

At this point I'm 95% certain it's a problem with the cable somewhere.

 

[James]

If it was installed before you moved in, it is possible that the cable has a joint or terminal box somewhere that you don’t know about.

 

[twoscoops]

If it was installed before you moved in, it is possible that the cable has a joint or terminal box somewhere that you don’t know about.

Interesting thought - it would have to be buried though. Is there any way to find such a thing without digging?

 

[telectrix]

depending on how low the resistance of the fault is, it maybe possible to measure and calculate the distance from 1 or both ends. this would involve rather expensive test equipment so an electricain visit is my advice.

 

[timhoward]

I think the last easy thing you can do is to check it looks exactly the same both ends, size, colours, number of cores. It's possible to trace the route without digging up, but this requires decent equipment. Identifying a join without digging is hard, unless it is in fact a tee off to something else.

If the fault is a dead short then it's possible to calculate the approximate location of fault from one end. If (more likely) it's broken/damaged insulation that's damp or full of water it becomes guesswork.
If the fault can be found, and the cable is otherwise testing out ok then it can be repaired.

It sounds as though it's a 3 core SWA cable. Within 15 minutes a decent sparks can IR test it and see which cores are faulty. If there are two cores that are still 100% sound it might be possible to change the earthing arrangements and continue to use the cable. I think it's worth paying a sparks for an hour at this point as to do much more you unfortunately need fairly expensive equipment.

 

[twoscoops]

I think the last easy thing you can do is to check it looks exactly the same both ends, size, colours, number of cores. It's possible to trace the route without digging up, but this requires decent equipment. Identifying a join without digging is hard, unless it is in fact a tee off to something else.

If the fault is a dead short then it's possible to calculate the approximate location of fault from one end. If (more likely) it's broken/damaged insulation that's damp or full of water it becomes guesswork.
If the fault can be found, and the cable is otherwise testing out ok then it can be repaired.

It sounds as though it's a 3 core SWA cable. Within 15 minutes a decent sparks can IR test it and see which cores are faulty. If there are two cores that are still 100% sound it might be possible to change the earthing arrangements and continue to use the cable. I think it's worth paying a sparks for an hour at this point as to do much more you unfortunately need fairly expensive equipment.

Thanks all. I did (with the cable disconnected both ends!) put an ohmmeter across neutral and earth wires and got a high but non-zero resistance. But I'm at the limit of my skill there. It did start having a problem after some heavy rain, so water damage is a good possibility.