Locating an SWA fault?

[SJD]

A customer previously (earlier this year) had a new 65m length of 10mm SWA installed but not connected – from a utility room housing the CU, to the far end of the garden, intended to power lights and sockets in a shed, done at the same time as an extension to the house.

Testing the SWA, sadly it is damaged. There is a reliable / stable short between Brown & Black, measures 0.03 Ohm from the CU end, 0.21 Ohm from the shed end. This puts the short at approx. 8m from the origin of the cable by the CU, but the trouble is with such low value readings using a KT64 MFT is I don’t know how accurate they are. The 8m point is approx. where the cable exits the recently built extension - it is concreted into the extension floor, then exits via the wall to the patio.

There is also a less reliable short between these cores and the sheath – it varies but typically under 1 Ohm – and so not very helpful in locating the fault. I’m guessing it is could be something like a screw or nail into the cable and firmly resting against the two cores, and just touching the sheath?

My question: is there any easy way to determine the position of the fault more accurately?

 

[Octopus]

Shame they didn't get the cable tested immediately after it was installed.

Could be interesting running a replacement!

How many cores does the SWA have?

 

[SJD]

Hi Murdoch, Yes, it is a shame they didn’t, but the cable has sat unused and untested for a while. It is 3 core SWA (the grey core appears OK).

 

[Silly Sausage]

Ooooh...I think there is a Wheatstone bridge method of locating the fault.
I'm sure Tony has mentioned this before.

 

[HandySparks]

If you can get your hands on a Time Domain Reflectometer, that might locate the fault more accurately:

Megger TDR500 Cable Fault Locator - Megger QualiFyr TDR500 - Time Domain Reflectometer

Sounds like you need to cut the cable where it exits the house, allowing enough accessible length to re-join as required, depending which part of the cable you find the fault in.

 

[SJD]

I looked up Wheatstone bridges, I vaguely remember using one in school physics some decades ago, seems no reason why I couldn't make one myself if I had the right value accurate resistors (which I think I have, so might try at home). The TDR500 looks nice, but I don't have access to one.

The trouble is there is about 30cm of easily accessible cable where it exits the house, before it disappears under brick flower beds all planted up. If I have to cut it there, I’d want to get the cut in the right place!

 

[Octopus]

I looked up Wheatstone bridges, I vaguely remember using one in school physics some decades ago, seems no reason why I couldn't make one myself if I had the right value accurate resistors (which I think I have, so might try at home). The TDR500 looks nice, but I don't have access to one.

The trouble is there is about 30cm of easily accessible cable where it exits the house, before it disappears under brick flower beds all planted up. If I have to cut it there, I’d want to get the cut in the right place!

In which case you only have 1 option and that is to cut it in the middle of the accessible strip!

 

[Guitarist]

Even if you locate and rectify the faulty section, you have no idea of how the rest was installed. It could be laying on sharp rubble, with the sheathing split and the armouring rusting away.
As far as I'm concerned, the only course of action here is to dig up the entire length, repair and check it all, then re-lay it properly to correct depth/protection or sand for sheathing/marker tape etc.

 

[Darkwood]

From experience forget it, the time to sort it and try re-se is is not worth it.... new cable different route and if you can cut the cable from outside to prove fault is under extension then you may be able to re-coup costs from builders....sounds like its been crushed to me ....not light things them extensions :rockstar:

 

[SJD]

I'd agree there could be other problems, and be inclined to say a known good cable needs re-laying the entire length. However, the owner wants to know where the damage is, because I think if it is inside, he plans to ask the builder who put in that part of the cable while doing the extension to pay for the replacement.

 

[Octopus]

I'd agree there could be other problems, and be inclined to say a known good cable needs re-laying the entire length. However, the owner wants to know where the damage is, because I think if it is inside, he plans to ask the builder who put in that part of the cable while doing the extension to pay for the replacement.

In which case go and do the necessary and bill the client!

 

[Engineer54]

Alas, you're going to be an extremely lucky pilgrim to locate this fault in an area that is easily accessible to be repaired. I bet if you did a sheath test you'll also find the sheath has been compromised, and probably not in just one location either. By the sound of it this SWA cable has just been thrown in and trampled over while waiting for the concrete to be poured. Probably the buried run depth in the garden, is nowhere near deep enough to boot!!

Don't know how difficult it's going to be for you, but i can quite honestly see that a new cable is going to be required/needed here!!

 

[Guitarist]

Drives me mad, how builders constantly say to me "it's armoured so can't be damaged". Sometimes it amazes me how stupid some people can be!

 

[Engineer54]

Drives me mad, how builders constantly say to me "it's armoured so can't be damaged". Sometimes it amazes me how stupid some people can be!

Yeah, But what do you expect from your typical builder?? But in any circumstances you don't expect the cable to be damaged even before it's put in service. They wouldn't realise that any PVC sheath damage will render the wire armouring to ongoing corrosion, which can be devastating, if the armouring is being used as the circuits CPC!!

 

[Guitarist]

Yeah, But what do you expect from your typical builder?? But in any circumstances you don't expect the cable to be damaged even before it's put in service. They wouldn't realise that any PVC sheath damage will render the wire armouring to ongoing corrosion, which can be devastating, if the armouring is being used as the circuits CPC!!

Funny how many builders actually lay the swa, then build over it without any testing. As I say, they seem to think that it's indestructible.