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.

 

[Lucien Nunes]

If there is damage that allows water to penetrate to the conductors, the armour wire is also involved and might already be badly corroded. Therefore be prepared to find that even if the localised damage could be repaired with a resin joint, the rest of the cable is compromised and not worth retaining.

 

[cliffed]

Had a swa go down while ago we just renewed it,new trench new cable .

 

[pc1966]

SWA is waterproof by design, so if the water has got in it means there is some significant damage, not just to the outer sheath, but it has managed to penetrate the armour and compromise the cable's conductor insulation.

The option of "rotating conductors" to try and keep the worst of that off the L is really not a good solution for any sort of long-term.

Is it mostly soft ground for the route? Your best solution would be to get someone with a small digger to cut a trench and put in some duct. That way you can pull through a new cable and have very little change of it being damaged/faulty. And if it is you can pull through another cable in the future!

Here is an example of duct:

40mm Twinwall Ducting Coil Black Flexi "Electric" x 50m | Speedy Plastics

This Underground 40mm Twinwall Ducting System is UK Manufactured With A Robust Corrugated Design Both Inside And Outside Made From A High Density Polyethylene Making It Light Weight And Highly Portable Cutting Installation Times. Its Is Easy To Cut And Join For Placing Through Draw Through. Used...

www.speedy-plastics.co.uk

If you feel able, digging a trench yourself would make it easier and cheaper for any electrician to install a new cable. Unlike public roads, etc, for private land there is no specific requirement on cable depth, other than it has to be unlikely to be damaged. Typically 0.5m deep is enough to escape shovels, etc.

 

[littlespark]

It’s likely been a diy job in the first place. Lucky if it’s 6 inches down.

If there’s no sand around it, a simple stone can pierce the sheath and let water in.

 

[R-fur]

Do you know the exact route of the cable if so, has there been any ground disturbance that may have damaged the cable? If you don't know the exact route hire or borrow a CAT (cable avoidance tool) this should tell you the route.
If you are happy with joints, dig down on the cable half way along, cut the cable and get a spark to test it. Then you only need to dig out 12 feet.
Best solution, dig it out and install a new cable through a duct.

 

[Soi disant]

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.

Without knowing anything about how it has gone bad?

Mmmmm. ?

 

[timhoward]

Without knowing anything about how it has gone bad?

Mmmmm. ?

Yes agree. Not one of my better ideas.

 

[Percyprod]

Working for DNO we regularly had to find faults on underground cables. Usual equipment was a pulse echo, which sends a signal down the cable and measures the time it takes to return, a 'sniffer', which detects gases given off on a short circuit, and a cat and genny. So yes, a professional would be able to find the fault, but even with the right equipment, not always easy or accurate! One method if an underground cable had a short on it and was blowing fuses was to 'bang it', ie put a portable circuit breaker on the circuit in the substation, close the breaker and listen for the bang in the ground! Crude but effective.

 

[littlespark]

Two pieces of fence wire, about 1ft long, with a right angle bend in the middle.
Hold in each hand loosely with an end pointing forward.
Walk the suspected route until the loose ends turn toward each other.....


No sorry.... that’s water devining

?

 

[DPG]

Two pieces of fence wire, about 1ft long, with a right angle bend in the middle.
Hold in each hand loosely with an end pointing forward.
Walk the suspected route until the loose ends turn toward each other.....


No sorry.... that’s water devining

?

Sounds like knitting.