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.

 

[KAS1]

Why on earth wasn't this ducted into the building lol

 

[Guitarist]

Why on earth wasn't this ducted into the building lol

Ducted? Thinking ahead?
Have you actually worked with builders?

 

[SJD]

Thanks for the additional comments.

As to how/why the cable was installed as it was, that was before I got involved, but I gather the customer bought the cable and asked the builder to put it in before it was too late. Possibly the builder knows it was not done properly, or possibly just doesn't understand, as already mentioned.

 

[Engineer54]

Were all knocking the builder here, but what if the homeowner has put his garden fork through this cable, The OP did say that the cable passes through newly finished plant beds?? You could still blame the builder to a certain extent mind, for not burying the cable to a suitable depth!! lol!!

 

[pennywise]

It's always the builder!!!!

 

[guest119]

Give them your opinion of where the fault is, ohms one end, ohms other end, cable length, % etc and let them argue with the builders.

Boydy

 

[SJD]

I've already indicated the fault is approx. 1/8 from the end at the CU, which puts it pretty much at the boundary where it exits the extension and enters under the brick-built raised beds, but there is quite a margin of error in this (a 0.03 Ohm reading, with careful nulling), which is where I began. Inside, the builders are at fault, outside then there are some landscape gardeners who did the flower beds that might be at fault. Yes, cutting at the boundary will give some answers, but there is so little of the cable accessible, and depending on which part is damaged, the owner still has thoughts of reusing the non-damaged part, which is why I didn't cut it to check so far.

 

[Guitarist]

Whoever laid it is at fault. Obviously not at the correct depth or adequately protected. There should have been no digging into it as tape should be buried about 6 inches down to warn it's there.
Sorry, but no excuses here from whoever did it.

 

[benji]

have you tried exposing cable outside of extension and give it a good pull to see if it moves if its only 8mtr you might be able to pull enough to see damage (unless its cast in concrete)

 

[Engineer54]

have you tried exposing cable outside of extension and give it a good pull to see if it moves if its only 8mtr you might be able to pull enough to see damage (unless its cast in concrete)

The 8 metres is set into a concrete slab, the other 50+ metres is buried!! I'd like to see you, or even King Kong, to pull more than an inch, and then only if you're very lucky!! lol!!

 

[richy3333]

Bet the Hulk could pull it out?

 

[ruston]

Hang on I'll be there soon.

 

[ruston]

And I will make the trench a little deeper for you.

 

[SJD]

Update: After some delay, the builder finally brought his electrician along and they replaced the damaged SWA section - the first approx. 8m, that goes under the concrete floor, and probably got damaged when laying the concrete.

There is a nice resin sealed joint to the rest of the cable. However, there is no continuity of the steel armour, only the 3 cores (which all test fine). Yes, I can earth the two halves of the SWA outer from each end separately, since one core will be earth. But should I really be insisting the joint is remade?

 

[Silly Sausage]

No continuity of the armour, that would instantly make me suspicious of the rest of the joint!

 

[SJD]

Well I have tested continuity of each core, testing 2 cores in turn, 0.23 Ohm per any two cores for the approx. 65m length. And IR >1999 MOhm between any two cores. I just think the guy didn't bother to connect the armour, the customer has left a message for him asking why not.

 

[ipf]

Update: After some delay, the builder finally brought his electrician along and they replaced the damaged SWA section - the first approx. 8m, that goes under the concrete floor, and probably got damaged when laying the concrete.

There is a nice resin sealed joint to the rest of the cable. However, there is no continuity of the steel armour, only the 3 cores (which all test fine). Yes, I can earth the two halves of the SWA outer from each end separately, since one core will be earth. But should I really be insisting the joint is remade?

It's OK then, if you don't want more disruption. Earth it and test.

 

[guest119]

Sounds like some Electrical Trainee has done his first resin joint.

Boydy

 

[Octopus]

Update: After some delay, the builder finally brought his electrician along and they replaced the damaged SWA section - the first approx. 8m, that goes under the concrete floor, and probably got damaged when laying the concrete.

There is a nice resin sealed joint to the rest of the cable. However, there is no continuity of the steel armour, only the 3 cores (which all test fine). Yes, I can earth the two halves of the SWA outer from each end separately, since one core will be earth. But should I really be insisting the joint is remade?

Get the builder to bring his sparky back again?

 

[SJD]

Sounds like some Electrical Trainee has done his first resin joint.

I think the guy has been around some time, probably not a Electrical Trainee, just has some odd ideas about regs. For example, he also said to the customer the SWA should be connected directly into a Henley block from the meter tails (all 65m of it), with no protective device.

 

[Guest55]

this tale of electrical woe is chock full of fail.

of course you should connect the swa's together in the joint , thats what the spring clips and braiding are for.

of course you dont connect the swa direct to henley blocks , you use glands and a switch fuse.

these clowns should seriously step away from the work area lol.

 

[7029 dave]

I think the guy has been around some time, probably not a Electrical Trainee, just has some odd ideas about regs. For example, he also said to the customer the SWA should be connected directly into a Henley block from the meter tails (all 65m of it), with no protective device.

No switch fuse? crikey!!

- - - Updated - - -

shot gun biff lol

 

[Guest55]

- - - Updated - - -

shot gun biff lol

bang bang!
both barrels.
;-)

 

[Guitarist]

I think the guy has been around some time, probably not a Electrical Trainee, just has some odd ideas about regs. For example, he also said to the customer the SWA should be connected directly into a Henley block from the meter tails (all 65m of it), with no protective device.

Not actually an electrician then, just someone with a little knowledge "who's been doing it a long time". See it all the time...just because they haven't killed anyone yet, they think it makes them "experienced". pff

 

[Engineer54]

I think the guy has been around some time, probably not a Electrical Trainee, just has some odd ideas about regs. For example, he also said to the customer the SWA should be connected directly into a Henley block from the meter tails (all 65m of it), with no protective device.

One things for sure be he a electrical trainee or not, ....he aint no electrician!!!