Locating distance of earth fault in buried swa

[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?