Locating distance of earth fault in buried swa

[plantpot]

Hi
I have a earth fault on a customers drive with 30 drive over fittings.
The swa runs past each fitting and has a resin branch joint,with one cable going into fitting.
Have narrowed fault down to cable by dismantling fittings etc.
Is there any tester or method of measuring from the end of cable to fault.
We know the route of cable.
I relize I cold do the old dig cable

 

[Hellmooth]

Step up Marconi.......

 

[davesparks]

Is it a dead short or a partial fault?
If it's a dead short you can measure the resistance of the fault from each end of the cable and work out a rough distance along the cable to the fault.

There are more advanced ways of doing it too

 

[plantpot]

0.00 at 250v and 500v l-e n-e
Are you saying set mft to ohms and test between l-e at both ends and note reading then I can go from there.
Sorry never done this before.
Out tonight so won't be ignoring replys
Thanks for quick replys at least I know there is a way now

 

[Hellmooth]

Marconi will give you some in depth details, this is right up his street.

 

[Pete999]

Wheatstone Bridge method, sorry I can't help anymore,It's been a long long time since I used this method, I used to be able to work it out, but age and red wine has dulled my thinking capacity, perhaps one of the more youthful members can help. Or wack some DC down the cable and use a cable meter that causes the ground to thump near the fault, don't know how it works but I have seen the contractor that maintained my U/G system use it and it worked very well

 

[davesparks]

0.00 at 250v and 500v l-e n-e
Are you saying set mft to ohms and test between l-e at both ends and note reading then I can go from there.
Sorry never done this before.
Out tonight so won't be ignoring replys
Thanks for quick replys at least I know there is a way now

0.00 what? Elephants?

I mean measure the resistance of the fault, you can use whatever test instrument you like. But personally I don't use an MFT for fault finding.

 

[Octopus]

0.00 at 250v and 500v l-e n-e
Are you saying set mft to ohms and test between l-e at both ends and note reading then I can go from there.
Sorry never done this before.
Out tonight so won't be ignoring replys
Thanks for quick replys at least I know there is a way now

are those readings taken with the lamps in it out?

 

[HandySparks]

Hi
I have a earth fault on a customers drive with 30 drive over fittings.
The swa runs past each fitting and has a resin branch joint,with one cable going into fitting.
Have narrowed fault down to cable by dismantling fittings etc.
Is there any tester or method of measuring from the end of cable to fault.
We know the route of cable.
I relize I cold do the old dig cable

If the fault is fairly low resistance, measuring resistance L to E at each fitting should get you pretty close. The fitting with the lowest resistance reading will be nearest the fault.

(Disconnect/isolate both L and N at the supply end.)

 

[Deleted member 26818]

Is this a short between two conductors, or between a conductor and the armour?

 

[marconi]

Hi
I have a earth fault on a customers drive with 30 drive over fittings.
The swa runs past each fitting and has a resin branch joint,with one cable going into fitting.
Have narrowed fault down to cable by dismantling fittings etc.
Is there any tester or method of measuring from the end of cable to fault.
We know the route of cable.
I relize I cold do the old dig cable

A suggestion on how to proceed building on Davesparks #3, Handysparks #9 and Spinlondons #10:

Call one end of cable 'Supply' and the other 'End'. Disconnect all lamp fittings first and both ends of cable.
Use an accurate dc ohmmeter - an instrument more precise than an MFT to take some resistance measurements as follows.

1. First confirm continuity of each of the conductors and the armouring too.
2. Measure L-N, L-E and N-E at Supply and End. Record measurements.
3. Connect L-N at each end and measure L-N at each of the 30 fittings. Record results.
4. Repeat 3 for L-E and then N-E.
5. Take some graph paper and mark off the x(horizontal axis) with a scale up to the length of the swa.
Mark of the y(vertical axis) with a scale up to the maximum resistance reading found in steps 3 and 4.
6. If you know the distances between each fitting then mark these along the x axis. (If you do not then I will provide a way to deal with that in another post.)
7. Plot the fitting readings from 3 and 4, using a different colour for each.
8. Join the points to create the curve of resistance for each of L-N, L-E and N-E.
9. You should see a smooth symmetrical curve with points lying on it. if you see a change in slope away from the expected smooth curve, this is where there will be near the discontinuity in the cable's characteristics, or in other words near the fault between conductors.

You might be able to narrow down to a stretch of cable between numbered fittings.

[Only a half an hour's thought gone in to this. But the principal is based on comparing a perfect cable's resistance properties to the faulty cable's. A prefect cable is symmetrical about its mid-point whereas a faulty cable is not. A big assumption is that for each conductor the joint box connections are all the same. ]

 

[andyb]

What's wrong with just measuring the earth fault from both ends and going from there? should be no need to remove lamps.

So, if you get for example 0.7 ohms from the supply end and 1.3 ohms from the other end then the fault is 35% of the length from the supply end.

Or use the same readings coupled with the resistance/metre to work out how far from each end.

 

[marconi]

What's wrong with just measuring the earth fault from both ends and going from there? should be no need to remove lamps.

So, if you get for example 0.7 ohms from the supply end and 1.3 ohms from the other end then the fault is 35% of the length from the supply end.

Or use the same readings coupled with the resistance/metre to work out how far from each end.

Nothing wrong, albeit I would remove the lamps to measure only the properties of the cabling.

I missed out from my post using the 3 readings at step 2 to divide the length of the cable as you and Davesparks suggest, and drawing on the graph 3 vertical lines (again in the colour for each pair).

I think my scheme provides a little more information to study because measurements are taken at each fitting.

I realise there needs to be some economy of effort: I would want to be as sure as I might be beforehand, using measurements that are possible and relatively easily taken, before digging up a cable.

It is a judgement call. As Lord Sainsbury once said, an ounce of experience is often worth more than a ton of theory! I don't have the experience of faulty swa cables to know whether what you suggest is all that is required - so I am happy to defer to yours and others experience if what you say is successful.

(These are just puzzles for me to engage my mind on - but with the intention of being helpful perhaps if I can. I look forward to learn what the OP actually did and found.)

 

[Midwest]

Shame the original electrician didn't just loop in & out of each fitting.

 

[Octopus]

We need more info but 30 drive over fittings, no indication of all the lamps have been taken out, nor if he has inspected all the lamps for moisture ingress either.

My money is on water in one of the fittings.

 

[HandySparks]

We need more info but 30 drive over fittings, no indication of all the lamps have been taken out, nor if he has inspected all the lamps for moisture ingress either. ...

...
Have narrowed fault down to cable by dismantling fittings etc.
...

......

 

[anderlect]

Hi plantpot
this is a rough method
We are assuming that the cable is isolated LNE at the dist-board,
and is disconnected from all the lamps or gear connections in the fittings.
That would leave the just the cables and the joints, do an insulationresistance test on the open ends, if that is clear the fault is on one of thelights, (as Murdoch says its probably water or damp in the fitting/fittings)

If it is down you will need a low reading digital multimeter to measure the resistanceof the ends L-E, N-E, L-N
You will also need the table for the K value of cable in milli ohms/meter (notethis reading is listed as a whole number so you will have to divide this by1000 to get it into milli ohms)
Zero the leads.
now measure down the line, through the fault, back down the neutral thisreading needs dividing by two as you have measured both conductors.
Now divide this measurement by the value of K for that cable size.
it may give you a number less than the length of the cable.
That figure is how far down the cable the fault is.
mark the floor.
Now repeat from the far end. if the two marks are close to each other get yourshovel out.

 

[snowhead]

Now repeat from the far end. if the two marks are close to each other get your shovel out.

If there's a big gap, get yer Minidigger out.

AS you've got access to 30 branches off the main run, test from each branch.
If you start 1 end, the fault resistance should get less the closer you get, then start to increase as you go past the fault.

 

[plantpot]

Thanks for all your help guys.sorry if its a basic question but is the multimeter suggested a run of the mill meter or special that needs buying or hiring

 

[marconi]

Thanks for all your help guys.sorry if its a basic question but is the multimeter suggested a run of the mill meter or special that needs buying or hiring

If using the MFT you have you follow my method, then if you send me the figures I will do the graph and study it to see if I can conclude anything from these readings. Note well the requirement to connect together the pairs of conductors, L-N, L-E and N-E at each end, one at a time.

I do have other methods up my sleeve you might be able to use but let us keep it simple to begin with.

Did you get any further in answering the query of spinlondon on the nature of the fault? Is it a complete short circuit between L, N and E/Armouring?

Please let me know:

a. CSA of SWA.
b. Number of cores.
c. Total length.
d. Is it a linear layout ie: a-b-c-d-e-f.... or are their branches?
e. Any measurements you have so far.
f. Any history to the fault.

 

[UKMeterman]

The multimeter function you could also find on the low ohms function of your MFT and or standalone megger. What is the resistance of the fault?

 

[Lucien Nunes]

What is the resistance of the fault?

Good point. All we know so far is that we're looking for 0.00. If that's megohms (probably is) it could still be 10k which we're not going to find with any amount of end-to-fault cable resistance measurement. All is speculation without more precise numbers.

 

[marconi]

Posts #21 and #22 got me thinking again on how a practical electrician might use what he already has or can easily obtain/buy to locate the fault, be it a high resistance or low resistance inter-conductor(s) fault, in the situation as described in post #1 when one has tap points along the swa.

What do you reckon to this? I am assuming for my explanation an L-E fault, high or low resistance.

1. Buy a box of five 12V 55W halogen lamps. Connect them in parallel - soldered connections or careful use of chocolate block connectors - whatever.
2. Using 2.5mm cable obtain a 12V supply from your van's battery. Run it through a switch for control.
3. Wire the halogen lamps in series with the positive lead from the switch and then connect to 'supply' and 'end' of swa L conductor. Thus, one can now switch on and pass circa 20A through the L conductor. This will create a volt drop along it.
4. Take a voltmeter, switched to measure 12V dc and attach a wandering lead between meter's negative lead and to E conductor at 'Supply' end. You can now move along the tap points holding the meter and with the positive test lead in hand.
5. On a piece of paper draw a line and mark 30 dots along it representing the lamps tap points.
6. At each tap point in turn starting at Supply touch the L conductor and note down the polarity of the voltage reading. It is only the polarity the scheme needs.
7. Through the fault, the E conductor is connected to the L conductor somewhere along its length. Thus the negative input to the voltmeter is at potential relative to the End of the swa cable - call it Vfault.
8. Whenever, the positive test lead is connected to a tap point nearer the Supply than is the point of the fault, the polarity will be POSITIVE (Vmeter > Vfault). When the tap point it is below the fault the polarity reverses (Vmeter<Vfault).
9. Studying the the polarity recordings on the paper will identify the section between taps along the swa cable which is at fault
10. One may need to reduce voltage scale as one moves nearer to the point of the crossover of polarity.
11. This might work and is less laborious than my earlier scheme.

 

[Lucien Nunes]

It's handy having all the tap off points, you can't normally get measurements along the cable this way. Without them, you would have to rely on the ratio of end-end and end-fault voltages being the ratio of the lengths. I've done something similar before on the spur of the moment, without a bunch of halogen lamps to hand. I opened the car fuse box, took out the fuse for the rear demister and ran two wires from the fuse terminals. This gives a handy resistive ballast delivering about 15A although neither lead is at chassis potential, so wet tyres might influence the reading if the fault is very high resistance!

 

[marconi]

There is one more step I have just thought of to add to the scheme in post #24;

After Step 10 do step 11 if required:

Step 11: In order to resolve whether the fault is at the tap rather than in the cable up stream or downstream of it, with the voltmeter positive test lead connected to the tap:
a. Disconnect the 12V supply at Supply end and connect it to the tap - observe the voltmeter. If the meter shows a positive polarity non-zero reading (albeit perhaps a small voltage) then the fault is downstream. Reconnect 12V supply at supply end.
b. Disconnect the 12 V supply at the End and connect it to the tap - observe the meter. If the meter shows a negative polarity non-zero reading the fault (albeit perhaps small voltage) then the fault is upstream.
So, if a and b show the fault is neither upstream or downstream of the tap it is at the tap or very close to it.

 

[plantpot]

Update
Had a good go at it today in more of a structured manner has other visits were just quick afternoon visits.
Took all lamps ( led) out and all connectors off cabling leaving just cables to test.
40 Mohm with 500v insulation tester L-E N-E.so cable OK.
Went to each fitting and added connectors and lampholders testing each time.as I added lampholder resistance go lower.
Reading with no lamps in 11Mohm.
Connected back up to supply and RCD held.
Has I started putting led lamps in I noticed about 17 of the lamps had about half the individual LEDs lit.
Could a combination of all these have been the problem.
Left with 15 drive lights and 3 wall fittings working fine.new lamps been ordered.
What have I learnt?
Work in a structured manner when fault finding rather than dipping in and out not achieving much other than confusion.
Thanks for everyone's comments on this issue a great help

 

[Octopus]

Update
Had a good go at it today in more of a structured manner has other visits were just quick afternoon visits.
Took all lamps ( led) out and all connectors off cabling leaving just cables to test.
40 Mohm with 500v insulation tester L-E N-E.so cable OK.
Went to each fitting and added connectors and lampholders testing each time.as I added lampholder resistance go lower.
Reading with no lamps in 11Mohm.
Connected back up to supply and RCD held.
Has I started putting led lamps in I noticed about 17 of the lamps had about half the individual LEDs lit.
Could a combination of all these have been the problem.
Left with 15 drive lights and 3 wall fittings working fine.new lamps been ordered.
What have I learnt?
Work in a structured manner when fault finding rather than dipping in and out not achieving much other than confusion.
Thanks for everyone's comments on this issue a great help

Why didn't you test L-N?

 

[plantpot]

Did but never posted

 

[UKMeterman]

You might want to consider sealing the drive over units with an acetic acid free sealant, such as 1705055 - LOCTITE - SEALANT, TEMPFLEX, 5145, 40ML | Farnell element14 or a good quality silcone grease.

 

[HandySparks]

Has the ground dried out a bit since your last visit? Maybe the fault only occurs after heavy rain?