Buried SWA cable fault finding

[danielz000]

Hi all,

My first post here, thought I'd see if it was possible to get some advice or tips...

Basically, I had a large amount of block paving and large concrete base laid at the far end (25m) of my garden a few months ago. Under this I had 3 core 16mm SWA laid.

(I most definitely can't vouch for the quality of the work done by any of these guys)

More recently me and my brother built a large garage on the concrete base and had an electrician add a new CU in the garage and connect the other end of the SWA to an MCB in the CU in the house.

Today, after about 2 weeks of not having any problems, the RCD in the house tripped out and wouldn't switch back on unless I turned off the MCB to the garage. It's worth noting, it rained fairly heavily last night.

After trying a few different things, I've eventually disconnected the SWA from the CUs at both ends.

I've tested for continuity across all cable/ armour combinations and noticed there's continuity (beeping) when I touch the live and armour.

Is my assumption that someone has damaged the cable, breaking the live wire insulation and water has entered this and created a circuit between the live and the armour a reasonable one? Are there any other ways this continuity could occur given both ends are now fully disconnected?

Assuming this assumption is correct, I'm curious if there's a simple way for me to locate this fault without digging up 30m of block paving?

For example, I was wondering if, by using a multimeter and testing the resistance across the live and armour at both ends I might be able to work out how far down the wire the break is?

So maybe if I got a resistance 10 times higher at one end than the other I can assume the damage to the 25m long cable is 10% (2.5m) from the end of the cable with lower reading?

I guess I don't need to be super accurate and that there are industrial tools that could accurately locate the fault, all I really want to know is roughly were to start digging in the hope of keeping the damage to the paving to a minimum.

Thanks in advance for any help or suggestions.

Dan

 

[James]

Yes, a difference in resistance measured from each end should give you a rough place to start digging

 

[littlespark]

What were all the readings between cores and between cores and armour?
and end to end continuity of each core?

as it’s a 3 core, you could swap the colours and use the shorting out core as earth.

That’s only possible if only one core is damaged. And it’s not a long term solution as any moisture will eventually rot the armour away.

 

[telectrix]

if the resistance is low enough to cause a multimeter to beep, then it's not water ingress. youneed accurate readings between each core and armour.luckily itmayonly be 1 core affected so littlespark's idea of a temp. fix could be a winner.

 

[littlespark]

Even if you work out how far from each end it is, and unless you know for sure it’s in a straight line and not wiggling all over the place…. You might still end up digging a 6ft wide hole looking for it.

Advise getting a professional with proper test equipment to check it. Your multimeter may not be picking up slight damage to a second core which would scupper my suggestion above.

multimeters give out roughly 9v…. Professional gear- up to 1000v

 

[DPG]

What were all the readings between cores and between cores and armour?
and end to end continuity of each core?

as it’s a 3 core, you could swap the colours and use the shorting out core as earth.

That’s only possible if only one core is damaged. And it’s not a long term solution as any moisture will eventually rot the armour away.

If the cable has been damaged enough to short a core to the armour then surely it needs digging up and a repair made to it.

 

[telectrix]

a vote in favour of putting it in ducting, methinks.

 

[Intoelectrics]

This is one of those situations where I'd have installed the cable in a duct. So many times I've come across this where the cable is installed under ground that has been landscaped or an hard standing added.

a vote in favour of putting it in ducting, methinks.

I was just writing a post stating exactly this! :-

In my experience a cable laid underground that will become inaccessible (under hard finished surfaces) should be ducted. This solves a couple of problems (provide the duct doesn't get damaged and is installed correctly) 1. if the cable gets damaged it can be drawn out and replaced. 2. If the building that is being supplied requires a supply upgrade either an additional cable can be added, or the original drawn and replaced.

 

[7029 dave]

You need to do an insulation resistance test on the SWA, with an IR tester.

 

[timhoward]

The OP has the right idea about measuring resistances from each end. Most of the expense is finding these faults. A left of field suggestion would be to acquire an insulation resistance (IR) tester, eBay always has lots of them as sparks ditch their separate testers and get all in one units. A megger bm403 or similar would be fairly cheap and very helpful in this situation.

 

[ruston]

As the posts above , get the people in with the gear to find it. The cost will be compensated by the time you waste and end up having to get help anyway.

 

[timhoward]

Quick further thought - if recently installed it will be metal boxes both ends. To ask an obvious question, are you sure the live isn't simply resting on the box the end you aren't testing, with the SWA still glanded to the case and providing the continuity you are seeing? If so this might be a red herring.
I'd be looking first and foremost where the cable leaves the house and enters the garage - are there any joint boxes there that have water ingress.
Obviously if you completely removed the SWA, including glands both ends then this isn't a valid thought.

 

[Intoelectrics]

Quick further thought - if recently installed it will be metal boxes both ends. To ask an obvious question, are you sure the live isn't simply resting on the box the end you aren't testing, with the SWA still glanded to the case and providing the continuity you are seeing? If so this might be a red herring.
I'd be looking first and foremost where the cable leaves the house and enters the garage - are there any joint boxes there that have water ingress.
Obviously if you completely removed the SWA, including glands both ends then this isn't a valid thought.

Absolutely, you have got to completely rule out all the obvious and exposed parts first, else it could end up an unnecessary expensive repair.

 

[danielz000]

What were all the readings between cores and between cores and armour?
and end to end continuity of each core?

as it’s a 3 core, you could swap the colours and use the shorting out core as earth.

That’s only possible if only one core is damaged. And it’s not a long term solution as any moisture will eventually rot the armour away.

Thanks! Swapping cores isn't something I thought about. I believe my earth and neutral are in good condition so that might work!

Edit: I haven't checked continuity end to end as the multimeter won't reach the far. I can try connecting the other ends and doubling back though. I haven't tried resistance yet but will do asap.

 

[danielz000]

Guys loads of replies. Thank you so much. I'm going to read through them all now.