cable fault finding - homemade tdr?

[Woody.v8]

I had a similar thing a bit back, inevitability it involved getting the spade out
http://www.mig-welding.co.uk/forum/threads/swa-earth-leakage.32909/#post-359541

 

[MrFluffy]

Yeah I decided to stop farting around, dug a access hole at the rear of the house as it comes from under the slab and cut it in a way that gave a chance of doing a resin encased joint to join a new segment on. Then put a short across both stubs and same readings inside the house end, but fine on the run under the patio and yard/drive etc.
So the wire is fubar under the house, then got some grips and tried the wire to see if a new wire could be pulled into the conduit or if it had collapsed with the weight of stuff during slab construction and its free. In fact two of the conductors were more free than the others and came away in my hand and had a burnt end about 4ft from my inspection hole. So no tdr or meter needed now, fault positively identified and theres enough cable good left tho act as a fish for a new section.
Now have to order a single length of 4 core 16mm SWA direct burial and pull it in all the way again. Its a expensive pain but at least laying it in conduit has meant we stood a chance to renew it. If I can pull it all the way to the workshop panel I wont joint it either, just one run again.
Still no clue how it has gone, the house is on a slate bedrock with no foundations and has been here hundreds of years, and on top of that its got a poured concrete slab, so I think we can rule out rodent and moles and as for contraction with the cold weather, the slab is heated so although theres some isolation between the heated section and the subfloor, its still warmer than the ground around it, and it was free to slide. Maybe just a lurking defect.

 

[Woody.v8]

It's bitter sweet isn't it? A bummer but least you know!

 

[dobbslc]

That's odd!
Could be the cable had a slight whack when it was installed and gradually burnt away at the insulation then that took out the next phase core.
Was it wet when it came out?

 

[Woody.v8]

That's odd!
Could be the cable had a slight whack when it was installed and gradually burnt away at the insulation then that took out the next phase core.
Was it wet when it came out?

I really need to sort my head out, I see innuendo everywhere and giggle like a kid, tehe

 

[dobbslc]

Careful Finbar! Fnar fnar

 

[Woody.v8]

Careful Finbar! Fnar fnar

Haha , Finbarr Saunders, it wasn't just me who read VIZ then!

 

[dobbslc]

I think it was Sid the sexist that got us all drinking Newcastle Brown. Bottle of Dog please....

 

[awemawson]

What size was the faulty cable, and what were you running off it? Big motor starting currents etc come to mind. Possibly started getting warm at the most insulated bit and melted the pvc?

 

[knighty]

What size was the faulty cable, and what were you running off it? Big motor starting currents etc come to mind. Possibly started getting warm at the most insulated bit and melted the pvc?

says 16mm above, so good for 60ish constant amps ?

 

[awemawson]

It 16 mm csa he's proposing to pull back in - I don't see where he says that's what's in at the moment which iirc is in singles

 

[MrFluffy]

its 4x16mm individual cables in there already, we're putting no more than 30amp/phase in to it because thats what the breaker set at the incoming supply to the house is so 7Kw per phase, at the time it went, it was a nighttime and the 3 phase was disconnected (I have a big MEM switch control on the wall that isolates the 380v 3 phase when I'm not in the workshop or running a machine) so it was tiny residual loads, 0.1amp for the in ground dog fence, a few amps on p2 powering the ups that feeds the rack as we dont have chillers on this time of year, p3 was running a trickle charger doing battery maintenance on the motorhome in the barn etc.
Yes the singles were wet on the outside when they came out the conduit, I found a small cut which may have been the point of ingress too and its near where I fixed a leaking drain a few months back, so the insulation must have broke down over time and then one went, and took out its neighbor is the most likely cause. I have no confidence thats going to be the only breech though. My theory is the gfci didnt operate because the water sat inside the conduit already there was isolated from the outside so wasn't at gnd potential but had allowed the cable to short out between phases damaging it.
Moneys tight here, and 4g16 double insulated armoured is going to run 300+ euros easily. I can in theory get away with 6mm over the 30 odd metre length, but 10mm would be more comfortable for me & I can more easily pull 10mm 4g10 swa in place of 4 singles. A double insulated armored direct burial cable in the existing conduit would seem to be the better option as there's no way we'll ever have more than 30amp/phase feed because the industrial tarif is insanely expensive (and 21kw... its enough for a shed now I dont run my old BOC adr 375 tig set).

 

[awemawson]

Let's hope that your luck picks up - what with this AND the 101 issue you certainly don't want any more excitements

 

[the snooper]

Water in the cable boil it out with a thumper

 

[dobbslc]

I assumed it was SWA cable!
Singles could have got cut pulling them in, it won't take much. Then once water is in the cable it's had it!
Is the gfci an RCD / earth leakage sensor?
I suppose if the water inside the tube was somehow isolated from the outside it won't have sensed it and tripped. I bet that cable was pulling some current and getting warm down there!
Would it be possible to pull a cloth through that tube to try to clean it?

 

[awemawson]

At least blow it through with a blast of compressed air - probably over several days to see if water is still penetrating

 

[Robotstar5]

...Is the gfci an RCD / earth leakage sensor?..

Yes, description here

 

[MrFluffy]

SWA wasn't in the normes when we put that in and we couldn't even buy it (still think you can't get it easy here) so had to run singles in a conduit of a defined size because we were being inspected under Consuel for conformity before EDF would move us from a temporary chantier(works) supply to a "domestic" one.
I'll blow it out while the cable is being sent (cant get anything but singles local), but if its direct burial cable surely it should be able to cope with a damp environment? I don't want to risk pulling a rag through to clean up the nasty black wastewater sludge that got in the tube because if it gets dislodged half way, that's game over for pulling the replacement through. Me fixing the drain leaking so that whole area isn't saturated all the time will do more to prolong its life too.

 

[awemawson]

SWA is absolutely fine in the damp until the first nick in the outer sheath allows water to get at the outer wire armour. Even that is ok for a while until the thin electro-deposited zinc on the wire armour leaches out and the wires start to rust.

I'd be concerned to see what damage has been done to your conduit to allow water in. If a sharp spiky thing has worked its way through the wall of the conduit it's going to slice the outer of any other cable that you pull through.

Can you not use one of the existing cables to pull though a decent bit of strong polypropylene rope twice the length of your run, then put a 'bobbin' between two knots that is about 10% less in diameter than the conduit and pull it through like the 'pig' they use to clean out subsea oil pipelines. That way if it jams to can pull it back from the other end.

... of course you may have a root growing into the conduit

 

[MrFluffy]

The main culprit I think was a large stone in the sand backfill under the pallet that sits under the dog kennel all of which was supposed to make it easier to excavate in time, the action of dogs jumping in and out of kennel for 7 years probably drove it down over time and nicked through one of the ribs.

Theres no chance of a root I think, we're a good 100m from the nearest tree and the fosse tank is between that and the house and we hit no roots excavating for that. I think it just got nicked by the stone, got wet from a leaking drain outlet (I sort of knew it was leaking as it was soggy round there when the washing machine was on but I've been a bit occupied over the past 6 months to want to point that out to the wife and have her fret or dig that corner up and see what the cause was) then blew out when expansion and contraction with the weather let the sheath break down a tiny bit.
Hopefully if I take care with the swa not to nick it, cover the conduit with sand again and then a flat slab that wont drive through over time before backfilling, it'll be ok. Its 240 quid for a suitable length of swa with shipping so I hope its a one time event. Next time I'm in the UK with a big van I'm going to buy a 100m bobbin from a wholesalers to do the barn knowing the weight (its a kg aprox per metre) and how much shipping costs.

I saw your comment earlier about luck, someone said to me if I didnt have bad luck this year I'd have no luck at all as some other more major stuff has happened too, but the way I look at is I'm still here and able to make my own luck, what more than that can we ask for?