Fault in SWA cable

[sosa]

A friend of mine recently asked me to go to a job he was working on as he had broken his hand and was unable to work. When i arrived i found out that there was a fault on a 100m lenght of SWA, which was buried underground, after speaking to the customer it became apparent that he had fallen out with his neighbour and his neighbour had already put a screw through the underground water mains so we assumed he had done the same with the SWA.

So here i was tasked with finding the fault after speaking with my electrician friend we decided the only way to find the fault would unfortunatly be to dig up the cable in the centre of the run and then test which side of the cable the fault was on and then repeating this another four times until we found a length of cable which had to be dug up until we found the fault, which also happened to be a screw.

Now after finding the fault and leaving it in the customers hands and not hearing off him for a while i've just decided to give him a ring and he tells me he has got another electrician there who questioned why it had been cut so many times, as he says there is a machine that can pin point where exactly the fault is on the SWA.

I've never heard of this myself and was just wondering if any of you had any experience of it and whether i had gone about it in the right way bearing in mind he wanted the piece of cable with the screw in to use as evidence as he has gone to the police.

 

[westie101]

Yes the machine is a TDR (Time Domain Reflectometer) we've been using them for years.

http://uk.rs-online.com/web/c/test-measurement/electrical-test/tdr-cable-fault-locators/

However they are not cheap (the one I use cost £3500 some years ago but there are far less expensive versions) Even if you get one you will need specialist training and will need to use it on a regular basis to get "good" at interpreting the results and justify the cost.
Though, we have a much advanced HV version that could be used on LV which will give you a distance to the fault - trouble is they are in the £13,000 range.

As for the cut and test method, it works and has been done for years.

 

[EFLImpudence]

dig up the cable in the centre of the run and then test which side of the cable the fault was on

How did you do that?

 

[sosa]

dig up the cable in the centre of the run and then test which side of the cable the fault was on

How did you do that?

There was a mini digger and driver on site.

 

[EFLImpudence]

cut and test method,

That's what I was thinking.

What kind of fault had you detected?

 

[Sherbertlion]

dig up the cable in the centre of the run and then test which side of the cable the fault was on

How did you do that?

There was a mini digger and driver on site.

Must have been a very long screw, or the neighbor is very good at gardening / ground works to hide the spot where they had dug it up to put the screw in!

 

[EFLImpudence]

There was a mini digger and driver on site.

Not quite what I meant.

 

[sosa]

There was a mini digger and driver on site.

Not quite what I meant.

Sorry meant dug up the cable and then seperated the ends and did an insulation resistance test to eliminate the good side and faulty side.

 

[sosa]

cut and test method,

That's what I was thinking.

What kind of fault had you detected?

A reading of 0.01 mega ohms on the brown cable and the armouring.

 

[chivers67]

We used to use TDR's all the time in the Cable TV industry, a very accurate tool and excellent for locating faults on Coax. I've never used one of electrical cable though.

 

[Lectrician]

I was kind of assuming the damaged had been done when the trench was open?

I heard of a load of subbies damaging SWA and PYRO on a rewire when they didn't get paid. They 2mm HSS drill in a battery drill, drilled into cable and then snapped the drill bit off.

 

[sosa]

dig up the cable in the centre of the run and then test which side of the cable the fault was on

How did you do that?

There was a mini digger and driver on site.

Must have been a very long screw, or the neighbor is very good at gardening / ground works to hide the spot where they had dug it up to put the screw in!

I say it was a screw as i looked like there had been one there it had been put in the removed and when we went along the length of cable we found the hole because of the bubbling water coming out of the cable.

 

[EFLImpudence]

Could you not have used a continuity tester to give you a reasonable idea where the fault was?

 

[sosa]

Could you not have used a continuity tester to give you a reasonable idea where the fault was?

How would that have been able to pin point the exact area of the fault though, i forgot to mention in the original post that we obviously found the middle of the cable CUT it and then tested which side it was on.

 

[EFLImpudence]

How would that have been able to pin point the exact area of the fault t

If there was a direct contact between Brown & SWA and you measured at each end, the readings would have been proportional to the distance.