Buried SWA cable fault finding

[nicebutdim]

@danielz000 if marconi is offering to come and help, I would jump at the chance. His Knowledge is vast. You may not understand a 100% of what he is explaining to you though ?.

Ps.. I love @Lucien Nunes explanation of why a standard resistance test from each end may well not work! (i.e pin hole and filling channel tunnel). Really helps you see the problem, especially when you cannot understand all of the physics involved.

I might understand somewhere in the region of 5% on a good day (plenty of sleep and a decent meal the night before).

Hopefully the OP jumps at this chance and then reports back here on the experience.

 

[johntheo]

I might understand somewhere in the region of 5% on a good day (plenty of sleep and a decent meal the night before).

Hopefully the OP jumps at this chance and then reports back here on the experience.

If I'm interpreting it correctly the voltage drop (across R1+R2) is measured between the two ends of the faulty cable and then between one end of the faulty cable and the armour (just R1), one of the healthy cables is just being utilized as a 25 meter M.meter lead for the first reading. Not quite sure about the right calcs but assuming that the fault is say only 5 meters from the joined ends then the volt drop across RI might be V1x20/25. Would love to see actual test readings from this method.

 

[marconi]

If I'm interpreting it correctly the voltage drop (across R1+R2) is measured between the two ends of the faulty cable and then between one end of the faulty cable and the armour (just R1), one of the healthy cables is just being utilized as a 25 meter M.meter lead for the first reading. Not quite sure about the right calcs but assuming that the fault is say only 5 meters from the joined ends then the volt drop across RI might be V1x20/25. Would love to see actual test readings from this method.

I will do you a demo. All a bit academic now cos two joins underground not great future for this run of swa cable- one maybe tolerable but two …,…

 

[UNG]

All a bit academic now but was there any tests done before the cable was put into service / buried to check it's integrity

 

[freddo]

I must have a go at that Varley test. I have a fault on a 4 core swa , 2 cores low resistance to each other and to earth. Would be cool to find the location of the fault, though most of the cable is now under a building extension. I do have a 20A variable PSU and some big resistors and power variable resistors

 

[pc1966]

If I'm interpreting it correctly the voltage drop (across R1+R2) is measured between the two ends of the faulty cable and then between one end of the faulty cable and the armour (just R1), one of the healthy cables is just being utilized as a 25 meter M.meter lead for the first reading. Not quite sure about the right calcs but assuming that the fault is say only 5 meters from the joined ends then the volt drop across RI might be V1x20/25. Would love to see actual test readings from this method.

Most of the methods are doing this - establishing the resistance either side of the fault to armour. The different methods really come down to available test equipment, etc, and some minor variations in accuracy for a given assumption of test equipment, simil;arity of available 2nd conductors, etc.

One method mentioned that is not doing that is time-domain reflectometry (TDR) where a pulse is sent down the cable and where it encounters a change of characteristic some of it is reflected back. By measuring the time taken for this first reflection (you get another large one when the pulse reaches the far end) you can estimate the distance to the fault either directly (using the known velocity factor for the insulation) or by ration (from known length and far-end delay).

This is commonly used in RF systems but often indirectly (not a pulse, but maths performed on a swept-frequency reflection measurement), and in fibre optics using a laser pulse. Whereas in power systems they originally used the closing and then fault-triggered reopening of the supply breaker to generate the surge, though probably these days it is a safer low energy plust that is used!

 

[pc1966]

I must have a go at that Varley test. I have a fault on a 4 core swa , 2 cores low resistance to each other and to earth. Would be cool to find the location of the fault, though most of the cable is now under a building extension. I do have a 20A variable PSU and some big resistors and power variable resistors

Let us know how you get on with the different methods, would be good to have some hands-on experience of how easy each/any of them were.

 

[pc1966]

Overall though the underlying problem was most likely poor workmanship.

While you can bury SWA directly in the ground it should have sand or similar around it for several cm to make sure that sharp stones are not crushing the armour - there is always a limit to what it can take! Though it is just possible that very bad handling of the cable might have caused internal damage.

As others have also said, putting in duct is better for many reasons, though it also should have sand or very fine gravel surrounding it for the same physical protection reasons. The big advantages are: less likely cable damage, the ability (if sane length and route) to pull through a fault cable and bring its replacement in, and also to allow other cables if needed (say network for internet, etc).

Finally it should have been properly tested after initial burial and before final work over that area. And that means a proper high-voltage insulation check as well as the typical conductor / armour resistance chances you would do to verify any new circuit.

 

[marconi]

I must have a go at that Varley test. I have a fault on a 4 core swa , 2 cores low resistance to each other and to earth. Would be cool to find the location of the fault, though most of the cable is now under a building extension. I do have a 20A variable PSU and some big resistors and power variable resistors

I do not have a long length of swa with which to do a realistic demo using a high current. How about me sending you my test rig - the parts for which are on order and due today - and then you use it on your defective length of swa cable and see how it performs in practice? You can keep it afterwards.

What is the csa and length of your cable?

 

[freddo]

Yes I would be interested in playing with your bits! The CSA is 25mm2 length ~40M. Someone with some modern test gear was going to take a look last week but couldnt make it. I ended up retesting the 2 undamaged cores at 2500V today to prove they were still ok, got greater than 2500Mohm in all directions once I had cleaned the exposed insulation in the cabinet outside, as needed to get a reduced supply back on feeding half of the lighting. The fault is between yellow and red phases and armour today it measures around 100ohms on a continuity test. The 2 damaged cores are now extra cpcs. As most of the cable route is now built on it is likely a new route will have to be dug soon. A ground working company are looking in next week.