Continuity / Ohms testing help / Short ?

[pc1966]

If the MCB is not tripping immediately then it is not a hard short, so you would need a high voltage insulation test to see the problem (though a multi-meter showing less than 10M at low voltage is suspicious).

Some of the fault-locating techniques such as Varley / Murray loop need a hard fault to work, others like time-domain reflection can work on a fault seen when flashing over at typical operating voltages.

How long is the cable? Was it buried directly in rocky ground rather than having sand around it?

I am asking as 16mm 2C SWA is about £7.50/m inc VAT and hiring fancy reflectrometry test gear, plus the resin joint (or multiple joints if more than one fault) may well cost more than 30+ m of cable, so you might be as well to simply have the cable IR tested and if faulty replace the lot.

With sand around it. Or in twin-wall duct if you want to put a network cable (or better still fibre) or there at the same time. If buying sand then look at bulk delivery from a quarry or merchant, works out way cheaper than bags of it.

 

[king185]

To set expectations, an IR tester will confirm there is a fault on the cable and show the nature of the fault. It unfortunately won't help with the location of it.

Other test gear or approaches will be needed to estimate the fault location. These range from modestly priced Wheatstone bridge based testers which enable old school methods (Varley test), to very expensive modern time delay reflectometers. Being honest, there isn't a high chance that an average domestic sparks would have either (though the latter can be hired).
You're probably going to need to find an above average sparks to help pin-point the fault.
Which part of London are you? There are some geniuses who lurk on here that love challenges....

Near heathrow buddy.

 

[king185]

If the MCB is not tripping immediately then it is not a hard short, so you would need a high voltage insulation test to see the problem (though a multi-meter showing less than 10M at low voltage is suspicious).

Some of the fault-locating techniques such as Varley / Murray loop need a hard fault to work, others like time-domain reflection can work on a fault seen when flashing over at typical operating voltages.

How long is the cable? Was it buried directly in rocky ground rather than having sand around it?

I am asking as 16mm 2C SWA is about £7.50/m inc VAT and hiring fancy reflectrometry test gear, plus the resin joint (or multiple joints if more than one fault) may well cost more than 30+ m of cable, so you might be as well to simply have the cable IR tested and if faulty replace the lot.

With sand around it. Or in twin-wall duct if you want to put a network cable (or better still fibre) or there at the same time. If buying sand then look at bulk delivery from a quarry or merchant, works out way cheaper than bags of it.

Cable from house to shed is approx 60m. Very expensive to replace. So i think it makes sense to do some test with an insulation resistance tester and cut and test and keep repeating till we have two good pieces of cable then i can purchase the length of the gap and it can be joined.
cable is buried in the garden just under the grass and soil, some parts go under the conrete base of my neighbours fence.

Thanks

 

[Other_Tucker]

Check the cable first, disconnect, really disconnect both ends so they're pointing out into free space, both ends. Check isolation between conductor 1+2, 1+3, 2+3. Ideally the multimeter should read >200MOhms. Run out a long lead and test 1-1, 2-2, 3-3 they should be as expected and similar.

If it fails either open or shorts tests then run your fingers literally along it, all the way that's above ground, you'll see the slice or kink or crush mark. If you have a dead short you're in luck, 1.42 Ohm from one end and 4.26 Ohm from the other is exactly 0.25 * the way along, normally where someone's been digging or a fence post. We only used Time Domain Reflexometers if say a buried camera cable was open circuit hundreds of metres up a racetrack.

Your multimeter should be fine if you didn't buy a stupid auto-ranging one, I was an electronics designer and used every function of my cheap one, a https://www.distrelec.de/en/digital-multimeter-250v-200mohm-uni-ut131d/p/30158662 while other broadcast engineers spent £££s on Fluke's they never understood how to use.

 

[king185]

Wow, thanks for the advice. Alot of people seem to find a multimeter hilarious for resistance checks to find an issue in my situation.
I will try and get this checked. Although mine is auto ranging, it can also be set into manual mode. Thanks again

 

[king185]

Some more information.

I probably dont need 100 amps with a 1000v cable at the moment, but i want to replace like for like, and future proof it, also if i ever get the shed extended or build a back room in the garden its good to have the correct cabling.


I dont want to replace the whole cable as alot of the cable goes out and around the house from the front then comes out at the rear in the corner of the house then makes its way down into the ground and covers a distance of about 20-35m meters to the shed.
The plan is to cut it before it goes into the ground and replace approx between 25-35m worth.
All the cable under the ground will be replaced and that should do the trick, and of course it will be tested before its joined up.

The cable was previously damaged only on the outer sheath and a little bit of the earth steel armour was exposed.
This then got patched up with insulation tape and duck tape on top.
(this damaged part will be replaced)

Possible damp ingress and corrosion cannot be ruled out.

The last thing we want is to running around in circles, that is why i am willing to have the underground bit (25-35m) chopped off and replaced!
Surely that will fix the problem.

Also the cable i currently have is 2 core with the armoured steel earth. I will use the same type of cable again, so i will not be spending extra money to buy a 3 core as that is unnecessary.

I can source the cable and joint connectors myself.

Any ideas of how much this should cost and electrician to do? Also how long would it take?

Thanks