Continuity / Ohms testing help / Short ?

[king185]

Can i give you some advice?? You will never find the fault...

1, as said, you are wasting your time with a multimeter, what voltage does it use to test?? about 2 volts or something. An insulation tester, mine goes up to 1000v. entirely different. With one of these, you "might" find the fault is there all the time.

2, Next problem, you are planning on cutting sections to test. Where this will go wrong, is that the fault is intermittant, so you will be cutting out sections and probably finding nothing..

3, By the time you do all this, you will most likely find that it was cheaper just to have replaced the cable at a cost of about £360...

john..

Cable is going to cost between £500 and £600 not including installation as its 60m length.
Better to cut but it will be checked with a insulation resistance tester by a professional.

 

[king185]

I take it it was all working fine before the fence went in?

yes

 

[kerching]

In that case I would go with someone who has a GOOD ohm meter AND can do a bit of maths
Res per core
Res good core to fault cable from end A
Repeat end B
This MAY be able to give you resistance to,fault

Won't work too well if there is more than one fault

Someone with a TDR would be better

Mine has picked up faults to 1m over 200+ metres and my JDSU will trace to about 2m in 3000

 

[binky]

yes

Then taking out the fence section is a viable option

 

[Murdoch]

yes

Who put the fence in?

 

[Andy™]

Who put the fence in?

this bloke

 

[mort2376]

The other question is do you have enough slack to joint the cable in situ or are you replacing a full section therfore adding 2 joints ?

 

[SPECIAL LOCATION]

Cable is going to cost between £500 and £600 not including installation as its 60m length.
Better to cut but it will be checked with a insulation resistance tester by a professional.

You mentioned earlier that "There are signs of damage which have been fixed"..
Also mentioned that "MCB was already replaced and it did not fix the issue. Not a faulty MCB."

If you have had repeated tripping of a 32A MCB, this indicates a serious Live to Neutral or Live to Earth fault.. (as we are not talking about small earth leakage detected by RCD's).

And the MCB was working correctly because it was tripping, when a fault current was detected. So no point replacing any MCB's until the fault has been identified and fixed..

Fitting a replacement MCB is about as useful as using a different tap if trying to fix a leaking garden hose. The hose needs to be fixed not the tap replaced.

And repeated short circuit faults can cause greater damage to the cable itself,
Dead short Live to Earth or Neutral between a pair of 16mm conductors can give faults currents of several hundred amps, causing significant arcing, sparking, welding etc. between conductors, plus associated heat effects on the cable insulation.
They can also over stress and damage the inner workings of the MCB itself.

What was the damage you previously identified and how was it fixed.?
I am assuming the outer sheath has been damaged...
allowing possible damp ingress, which may have crept along the cores of the steel armour introducing corrosion..

Also If the cable has had any significant impact damage,
the inner cores may have physical damage to their insulation.

I think you could end up running yourself around in circles trying to do a cut & paste fix a bit here, fix a bit there. But still end up with later tripping problems

It also could be wise to shop around on your cable suppliers with those quoted supply only costs.. And.. do you really need a 3core? why not 2core & TT the shed?

A quick google for 60m 2core 16mm SWA, has returned three suppliers at... £360, £380 or £420 inc VAT.

 

[Andy™]

And.. do you really need a 3core? why not 2core & TT the shed?

no idea why people always insist on 3 core for single phase using a conductor as an earth, the armour is almost always suitable and is pretty much the standard way its done everywhere other than a house

 

[mort2376]

I prefer to put a 3core in as it's a belt and braces approach. Your garenteed a good earth.

 

[Kot]

OP says it trips randomly

My money is on an old damaged cable - damage by spade or tree trunk

A shed???? 16mm2 cable???? do you really need that? about 75A buried for a shed?

 

[Andy™]

A shed???? 16mm2 cable???? do you really need that? about 75A buried for a shed?

and did you consider volt drop etc? or what the load is in the shed?

 

[SPECIAL LOCATION]

A shed???? 16mm2 cable???? do you really need that? about 75A buried for a shed?

I suspect your design calculations are a bit off the mark if you think you could run 75A along 60m of 16mm... you may need to brush up on regulation group starting 525 ?

I know of some customers who are into ceramics and a have mini kiln in their garden workshop/shed requiring a lot more than just a light and a plug for the electric lawn mower..

 

[Murdoch]

There could be a dwelling on the end of this cable

 

[king185]

The other question is do you have enough slack to joint the cable in situ or are you replacing a full section therfore adding 2 joints ?

2 joints

 

[king185]

Who put the fence in?

neighbour

 

[king185]

A shed???? 16mm2 cable???? do you really need that? about 75A buried for a shed?

You mentioned earlier that "There are signs of damage which have been fixed"..
Also mentioned that "MCB was already replaced and it did not fix the issue. Not a faulty MCB."

If you have had repeated tripping of a 32A MCB, this indicates a serious Live to Neutral or Live to Earth fault.. (as we are not talking about small earth leakage detected by RCD's).

And the MCB was working correctly because it was tripping, when a fault current was detected. So no point replacing any MCB's until the fault has been identified and fixed..

Fitting a replacement MCB is about as useful as using a different tap if trying to fix a leaking garden hose. The hose needs to be fixed not the tap replaced.

And repeated short circuit faults can cause greater damage to the cable itself,
Dead short Live to Earth or Neutral between a pair of 16mm conductors can give faults currents of several hundred amps, causing significant arcing, sparking, welding etc. between conductors, plus associated heat effects on the cable insulation.
They can also over stress and damage the inner workings of the MCB itself.

What was the damage you previously identified and how was it fixed.?
I am assuming the outer sheath has been damaged...
allowing possible damp ingress, which may have crept along the cores of the steel armour introducing corrosion..

Also If the cable has had any significant impact damage,
the inner cores may have physical damage to their insulation.

I think you could end up running yourself around in circles trying to do a cut & paste fix a bit here, fix a bit there. But still end up with later tripping problems

It also could be wise to shop around on your cable suppliers with those quoted supply only costs.. And.. do you really need a 3core? why not 2core & TT the shed?

A quick google for 60m 2core 16mm SWA, has returned three suppliers at... £360, £380 or £420 inc VAT.

Your right i probably dont need 100amps 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.

Yes it 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 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

 

[Murdoch]

Tested before it’s joined up.

You need to test with proper test kit to confirm you’ve located the fault

And once complete, are you going to bill your neighbour?

 

[king185]

Tested before it’s joined up.

You need to test with proper test kit to confirm you’ve located the fault

And once complete, are you going to bill your neighbour?

No, however i did mention the fault to him.

 

[Murdoch]

And don’t forget that both lengths will need to be thoroughly tested for continuity and IR and then again when it’s been connected …
best you find an accommodating spark

where are you ?