Buried SWA cable fault finding

danielz000 · Sep 10, 2021

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

danielz000 · Sep 10, 2021

littlespark said:
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

Thanks mate, I'll look about for a professional but a brief search last night didn't pick anything up. I know the exact root and depth of the cable.

littlespark · Sep 10, 2021

danielz000 said:
Thanks! Swapping cores isn't something I thought about. I believe my earth and neutral are in good condition so that might work!

As my further post said…. Get the cable checked by a professional with the right test equipment. There may be further damage to the cable that you’re multimeter isn’t detecting.

Make sure you identify the cables properly at each end… not just a little flag of tape that could fall off.
Might surprise a future electrician finding a brown core on the earth bar

DPG · Sep 10, 2021

danielz000 said:
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.

You definitely need to IR test the cable and see what the damage is. The other cores may be damaged also.

danielz000 · Sep 10, 2021

Thanks everyone.

It's a long story, but in regards to the ducting, these builders were absolutely ripping the arse out of it on day rate. In the end I got so frustrated I just had to settle with a number of things I wasn't happy with including the lack of ducting.

In regards to a second hand Megger I'll look in to that if I can't find an electrician smart enough to do it.

Out of curiosity though, how would an insulation resistance test work and would it help me locate the break? Or just confirm the damage exists?

Thanks again little spark, swapping the cores isn't ideal and a repair was my favoured option but swapping cores might be a good temporary fix!

DPG · Sep 10, 2021

danielz000 said:
Thanks everyone.

It's a long story, but in regards to the ducting, these builders were absolutely ripping the arse out of it on day rate. In the end I got so frustrated I just had to settle with a number of things I wasn't happy with including the lack of ducting.

In regards to a second hand Megger I'll look in to that if I can't find an electrician smart enough to do it.

Out of curiosity though, how would an insulation resistance test work and would it help me locate the break? Or just confirm the damage exists?

Thanks again little spark, swapping the cores isn't ideal and a repair was my favoured option but swapping cores might be a good temporary fix!

I'd be getting the builders back and getting some of the original bill refunded. . The repair to this cable is going to cost you money - it will almost certainly need an electrician.

timhoward · Sep 10, 2021

danielz000 said:
Out of curiosity though, how would an insulation resistance test work and would it help me locate the break? Or just confirm the damage exists?

It's exactly the same concept as a resistance test with a multi-meter, it just uses 500v dc not (probably) 9v. It can be thought of as a pressure test to confirm the condition of the cable and effectiveness of the insulation between the cores / armour. Any sparks worth their salt would do this test before considering re-energising the circuit. (you obviously aren't anywhere near that stage yet)
As Tel said if you were indeed measuring the actual fault with a multi-meter then it's a very low resistance fault indeed. An IR+continuity test device (most Megger BM devices do both) can measure resistance very accurately (to roughly hundredth of an ohm), much more accurately than most low-mid range multimeters where the good ones do +- 0.2 ohms.
So it would help confirm the cable's integrity and (best case) gain an idea where the issue is.
I do expect you to need a sparks at some point. It would be nice if you can find where the issue is though.

marconi · Sep 10, 2021

Could I confirm/ask some things please?

1. 3 core swa in one length?

2a. As far as you can tell with your multimeter the only continuity/contact is between one core and the steel wire armouring?

2b. There is continuity end-to-end on each core and for the SWA?

3. Please send me a photo of your multimeter.

4. Please draw me a plan of the garden, garage, CUs, cable route and tell me whether you can put your car in the garage - I'd need access to the car's 12V battery.

What is the first half of your post code?

Lucien Nunes · Sep 10, 2021

Testing with a multimeter and/or an insulation tester cannot easily locate the damage. The plan of measuring resistance to the fault from each end doesn't always work because the end-end resistance of the cable is usually so much lower than that of the fault. It could be like trying to locate a pinhole in the channel tunnel by filling the tunnel full of water first from the English end and then from the French end, and comparing how fast the water flows in.

There are two standard methods to locate a fault. One does use the resistance of the conductor from each end to the fault, but in a balanced bridge configuration that is very sensitive and largely independent of the resistance at the fault. This type of Murray or Varley loop test can be done with a dedicated test instrument, or one can rig up a method using a reasonably high current source, some resistors and a multimeter. I think this is what Marconi is hinting at.

The other method is Time Domain Reflectometry, which injects a pulse into the cable and measures the time taken for it to be reflected by any features along its length (including the fault) that alter its impedance. Again it is normally done with a dedicated test instrument.

timhoward · Sep 10, 2021

I’m learning a lot tonight. So it seems the principle of both Murray and Varley loop tests are that when the middle of a Wheatstone bridge measures zero volts you know the ratios of the two pairs of resistors are equal, so if three of the values are known the fourth can be calculated.

It would seem that whichever of the two tests are used one would need a voltage source, two known resistance values and a variable resistor to balance the bridge. I’m particularly fascinated how @marconi was going to improvise the variable resistor

pc1966 · Sep 10, 2021

I think this is the method that @marconi and @Lucien Nunes are discussing:

[ElectriciansForums.net] Buried SWA cable fault finding

Here the 3 conductors are joined at one end. One of the "good" conductors is used to feed a high-ish test current (say 5A from car battery & 60W lamp), the 2nd is used as a Kelvin connection to sense the voltage at the far end of the faulty 3rd conductor, and then the two voltages are measured:

V1 = drop from both R1 and R2 at I test
V2 = voltage across R1 due to the test seen by the fault to armour

You need enough current to get a measurable voltage, and you want it to be above the mV sort of point to avoid thermoelectric offsets.

Then the distance ratio = resistance ratio allows you to guess the R1 length as a fraction of total length (R1 + R2) from V2/V1

pc1966 · Sep 10, 2021

Probably should have though this through and made the resistor numbers and voltage numbers a little more consistent!

timhoward · Sep 10, 2021

Well that’s certainly a bit easier than figuring this out…!

I’d got as far as A and B being equal,
y adjusted so G is zero, meaning….
A/B=R/(X+y)
A/B is 1 so R=X+y or X=R-y
y is known.
That leaves half of a pair of simultaneous equations and I gave up and opened the 1664 at that point. I’ll return to this one day…

Markyd · Sep 10, 2021

Brilliant vid

danielz000 · Sep 11, 2021

Hi guys,

Thanks again for all the responses! I'll watch that you tube video and read through them diagrams shortly.

Marconi, thanks for your offer. In short, last night my brother randomly decided he knew where the damage was and so pulled up a load of blocks and exposed the cable (it wasn't where he thought obviously).

Today we agreed to just chop the cable in half where it was exposed and test the two halves (removing half the block paving is a lot easier than all of it right...)

Turns out the problem is at the house end.

I'm totally curious as to how someone with the know how and tools would solve the riddle so have answered your questions below. Out of curiosity, can you see my location? Are you local to Romford/Essex?

1. Yes, it WAS 3 core and 1 complete length (about 25 meters)

2a. Yes, there was only continuity between live and the armour.

2b. Yes, I believe so for 2 reasons, 1, the circuit was working for 2 weeks and no digging occurred in that time and, 2, we connected different pairs up at one end and checked continuity along them at the other (the leads don't reach across the garden so that was the only was to do it).

3. LAP AC/DC Digital Multimeter 600V - https://www.screwfix.com/p/lap-ac-dc-digital-multimeter-600v/161fg that's the multimeter I got but when I tried to measure resistance across the live and armour it just read "ERR". Nothing in the manual about how or why that can happen...

4. The car battery could get directly on top of the wire at the house end and within about 6 foot of the CU at the garage end. I've attached an image of my garden before all the work was carried out. It's basically 2 meters of exposed swa cable at both ends with the garage end running under 5m of concrete slab and then under about 15m of block paving before before rising above ground on the outside wall of where the CU is inside the house.

Thanks again all!

Dan

pc1966 · Sep 11, 2021

timhoward said:
Well that’s certainly a bit easier than figuring this out…!

I would prefer to to have any significant current going via the fault as we don't know its exact resistance, other than "low".

However, that is something else to check to make sure the resistance of good and faulty conductors is more or less the same, in case there is a chunk of copper blown off it where the fault originally tripped. In my drawing you could also measure a 3rd voltage from conductors 1 & 2 at the sending end - that should be more or less identical to the voltage V1 between 2 & 3 as they should be the same R for #1 (good) and #3 (faulty) under normal circumstances.

Buried SWA cable fault finding

danielz000

danielz000

littlespark

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DPG

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danielz000

DPG

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timhoward

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marconi

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Lucien Nunes

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timhoward

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pc1966

pc1966

timhoward

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Markyd

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danielz000

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