Cable fault location meters , anyone used them before ?

[JD6400]

We had a break down call out on Thursday to a stable yard we have just completed , it turned out that the 500m 95mm three phase sub main that was installed as a single phase for the time being until they can afford a three phase meter upgrade has been compromised on the currently used brown phase and also on the black phase which is dormant at the moment !
The grey phase and neutral bell out fine .
So we have got the yard up and running at the moment , but we need to go back and locate the problem now !
Have any other forum members had dealings with cable fault location gear and can you recommend which is best ?
We have been quoted around £550 + to hire some , but i always prefer to look at buying reconditioned kit at a all be it higher cost rather than pay a dead loss hire fee .
It would not be so bad but the run was moled under two private tarmac roads and strait across the main access yard and lorry park park for the whole complex , without being ducted ( clients request to keep cost down along with using their own labour to lay it ) , BUGGER !!!!!!!!!!!!

 

[lamb]

Good private job for a BT engineer, they all carry TDR's.

 

[JD6400]

Hi Lamb , thanks for the fast reply !
Would this work on a 95mm , have started looking on flee bay for stuff and have come across these but it only seems to mention for telecoms ?

 

[lamb]

Yes, a TDR will work on any cable that has two cores, eg L1 + N, L3 + SWA etc.
For each cable size/conductor type you need to find the signal transfer rate and set this on the machine.
The machine then tells you the distance (roughly) from the test point to each anomaly in the conductor (cable joint, termination, cable break etc), you then have to interpret the signal to work out where your fault may be.
So if it bounces back 70m, 250m and 500m, and you originally put a joint half way along the run, disregard the 250m (joint) and 500m (termination) and investigate the 70m reading first.
Using a cat and genny find the 70m from test point (will probly be +/- 2m or 3m), dig up and have a look.
Other methods include wheaton bridge but you need a degree to interpret the results (too complicated for me) or old school cut into cable half way, test each half, cut bad section in half again, etc, until you get down to last 20m eg. then just overlay new cable on that section.

 

[lamb]

Forgot to say, you can also use a cable thumper, again it's specialist to set up, pumps mega volts through the cable at regular intervals, you then walk along the run and listen for the bang as the cable fail point flashes over.

 

[La Poste]

Forgot to say, you can also use a cable thumper, again it's specialist to set up, pumps mega volts through the cable at regular intervals, you then walk along the run and listen for the bang as the cable fail point flashes over.

Great stuff, I'd like to have a go or at least see it in action.

 

[JD6400]

Forgot to say, you can also use a cable thumper, again it's specialist to set up, pumps mega volts through the cable at regular intervals, you then walk along the run and listen for the bang as the cable fail point flashes over.

This is the one that we were looking to hire if we can not find anything else , but at £550+ it is some what pricey !

 

[lamb]

TBH by far the best method is common sense - are there any joints in the run, has the ground been disturbed eg other trenches/ demolition works, can be a lot cheaper to ask these questions than hire in specialist equipment and specialists to interpret the results.

 

[Knobhead]

How good is the fault? Dead short or high resistance?
A dead short can be found if you have access to a ductor by measuring the resistance between the faulty cores from each end. This will give you a ratio that you transpose on to the cable length.
R2+RF+R4/R1+RF+R3

View attachment 11974

 

[JD6400]

Have walked the run and nothing obvious , it only went in a couple of months ago !
My gut feeling is it is at the furthest end of the run through the yard , as the we were not on site when this was back filled and it was some what a hostile environment in that part of the trench !
The black phase has been so badly damaged and because the run is up a fair slope , there is actually water running out of it !

 

[JD6400]

Sorry my thanks button has now disappeared , No Tony only ever herd of them and never seen one in real life , and it is a dead short .

 

[Knobhead]

Try it with any low resistance meter. It’s cheaper than hiring equipment. I will admit it’s donkeys years since I’ve used it.

We always had access CAT detectors or if neaded the group engineering pulse reflection gear and the dreaded thumper.
Be warned with a thumper it’s not something to be used unless you’re trained. Last time I had anything to do with one we ended up with two faults, the thumper blew an 11KV joint.

 

[JD6400]

This is what worries me the most Tony , i do not really no what to expect with the thumper and as the owner of the yard is up at the top of his game , there are a lot of very , very expensive four legged friends about that have a bit of a aversion to getting a lick !

 

[UKMeterman]

If there are four legged friends about forget the thumper and just hire in a TDR.

 

[Mark_Burgess]

Would be a TDR for me, just input the cables velocity of propagation and test, just as lamb suggested.

 

[i=p/u]

sounds intersting , im fed up installing 10A light switches and doing domestic, i kn ow already

 

[Heisenberg]

Just out of curiosity, who's gonna be paying for all this? As it was only installed a couple of months ago, is the client blaming you and expecting you to cover the cost?
Sounds like the sort of job that could turn a profitable year into a not very profitable one if that's the case :dead:

 

[lamb]

If there is water running out of the cable, there could be a lot more to chop out than just the damaged section.
If any joints or terminations are lower than the cable fail point, they will keep getting damp.

 

[Knobhead]

The outer sheath is compromised so the armours are at risk. I know it’s late now but this is where testing the armouring to earth pays off, proves the outer sheath isn’t damaged.
DP, what tests were done on the cable before putting in service?

Sorry but the more I think about it this cable is going to be a write off.

 

[lamb]

Agreed, a large section will need to be cut out at least.
Steel has a co-efficient of expansion of 6:1 when oxidised, so when the armour rusts it will further ruin the sheath and pierce/compromise the bedding and insulation.
I would still try to salvage any good part of the cable if it is dry and sheath not leaking to earth.