Buried SWA cable fault finding

[timhoward]

Out of curiosity though, how would an insulation resistance test work and would it help me locate the break? Or just confirm the damage exists?

It's exactly the same concept as a resistance test with a multi-meter, it just uses 500v dc not (probably) 9v. It can be thought of as a pressure test to confirm the condition of the cable and effectiveness of the insulation between the cores / armour. Any sparks worth their salt would do this test before considering re-energising the circuit. (you obviously aren't anywhere near that stage yet)
As Tel said if you were indeed measuring the actual fault with a multi-meter then it's a very low resistance fault indeed. An IR+continuity test device (most Megger BM devices do both) can measure resistance very accurately (to roughly hundredth of an ohm), much more accurately than most low-mid range multimeters where the good ones do +- 0.2 ohms.
So it would help confirm the cable's integrity and (best case) gain an idea where the issue is.
I do expect you to need a sparks at some point. It would be nice if you can find where the issue is though.

 

[marconi]

Could I confirm/ask some things please?

1. 3 core swa in one length?

2a. As far as you can tell with your multimeter the only continuity/contact is between one core and the steel wire armouring?

2b. There is continuity end-to-end on each core and for the SWA?

3. Please send me a photo of your multimeter.

4. Please draw me a plan of the garden, garage, CUs, cable route and tell me whether you can put your car in the garage - I'd need access to the car's 12V battery.

What is the first half of your post code?

 

[Lucien Nunes]

Testing with a multimeter and/or an insulation tester cannot easily locate the damage. The plan of measuring resistance to the fault from each end doesn't always work because the end-end resistance of the cable is usually so much lower than that of the fault. It could be like trying to locate a pinhole in the channel tunnel by filling the tunnel full of water first from the English end and then from the French end, and comparing how fast the water flows in.

There are two standard methods to locate a fault. One does use the resistance of the conductor from each end to the fault, but in a balanced bridge configuration that is very sensitive and largely independent of the resistance at the fault. This type of Murray or Varley loop test can be done with a dedicated test instrument, or one can rig up a method using a reasonably high current source, some resistors and a multimeter. I think this is what Marconi is hinting at.

The other method is Time Domain Reflectometry, which injects a pulse into the cable and measures the time taken for it to be reflected by any features along its length (including the fault) that alter its impedance. Again it is normally done with a dedicated test instrument.

 

[timhoward]

I’m learning a lot tonight. So it seems the principle of both Murray and Varley loop tests are that when the middle of a Wheatstone bridge measures zero volts you know the ratios of the two pairs of resistors are equal, so if three of the values are known the fourth can be calculated.

It would seem that whichever of the two tests are used one would need a voltage source, two known resistance values and a variable resistor to balance the bridge. I’m particularly fascinated how @marconi was going to improvise the variable resistor

 

[pc1966]

I think this is the method that @marconi and @Lucien Nunes are discussing:

Here the 3 conductors are joined at one end. One of the "good" conductors is used to feed a high-ish test current (say 5A from car battery & 60W lamp), the 2nd is used as a Kelvin connection to sense the voltage at the far end of the faulty 3rd conductor, and then the two voltages are measured:

• V1 = drop from both R1 and R2 at I test
• V2 = voltage across R1 due to the test seen by the fault to armour

You need enough current to get a measurable voltage, and you want it to be above the mV sort of point to avoid thermoelectric offsets.

Then the distance ratio = resistance ratio allows you to guess the R1 length as a fraction of total length (R1 + R2) from V2/V1

 

[pc1966]

Probably should have though this through and made the resistor numbers and voltage numbers a little more consistent!

 

[timhoward]

Well that’s certainly a bit easier than figuring this out…!

I’d got as far as A and B being equal,
y adjusted so G is zero, meaning….
A/B=R/(X+y)
A/B is 1 so R=X+y or X=R-y
y is known.
That leaves half of a pair of simultaneous equations and I gave up and opened the 1664 at that point. I’ll return to this one day…

 

[Markyd]

Brilliant vid

 

[danielz000]

Hi guys,

Thanks again for all the responses! I'll watch that you tube video and read through them diagrams shortly.

Marconi, thanks for your offer. In short, last night my brother randomly decided he knew where the damage was and so pulled up a load of blocks and exposed the cable (it wasn't where he thought obviously).

Today we agreed to just chop the cable in half where it was exposed and test the two halves (removing half the block paving is a lot easier than all of it right...)

Turns out the problem is at the house end.

I'm totally curious as to how someone with the know how and tools would solve the riddle so have answered your questions below. Out of curiosity, can you see my location? Are you local to Romford/Essex?

1. Yes, it WAS 3 core and 1 complete length (about 25 meters)

2a. Yes, there was only continuity between live and the armour.

2b. Yes, I believe so for 2 reasons, 1, the circuit was working for 2 weeks and no digging occurred in that time and, 2, we connected different pairs up at one end and checked continuity along them at the other (the leads don't reach across the garden so that was the only was to do it).

3. LAP AC/DC Digital Multimeter 600V - https://www.NoLinkingToThis/p/lap-ac-dc-digital-multimeter-600v/161fg that's the multimeter I got but when I tried to measure resistance across the live and armour it just read "ERR". Nothing in the manual about how or why that can happen...

4. The car battery could get directly on top of the wire at the house end and within about 6 foot of the CU at the garage end. I've attached an image of my garden before all the work was carried out. It's basically 2 meters of exposed swa cable at both ends with the garage end running under 5m of concrete slab and then under about 15m of block paving before before rising above ground on the outside wall of where the CU is inside the house.

Thanks again all!

Dan

 

[pc1966]

Well that’s certainly a bit easier than figuring this out…!

I would prefer to to have any significant current going via the fault as we don't know its exact resistance, other than "low".

However, that is something else to check to make sure the resistance of good and faulty conductors is more or less the same, in case there is a chunk of copper blown off it where the fault originally tripped. In my drawing you could also measure a 3rd voltage from conductors 1 & 2 at the sending end - that should be more or less identical to the voltage V1 between 2 & 3 as they should be the same R for #1 (good) and #3 (faulty) under normal circumstances.

 

[timhoward]

That leaves half of a pair of simultaneous equations and I gave up and opened the 1664 at that point. I’ll return to this one day…

To answer my own question in case anyone searches the forum for varley loop test in future…
The 2nd equation is derived from twice the resistance of a length of one conductor being equal R+X+y. Substituting that in leaves a ratio of resistance X to resistance of the total loop. I think.
I might try this and @pc1966 method out when I have time just for fun.

 

[Jim_e_Jib]

Deleted - misread post!

 

[marconi]

danielz000 - I am no more than 20 minutes drive from you. Do you want me to call in? If the method my colleagues have explained does not work on the damaged 25m length I will improvise a time domain reflectometer and use it during a second visit.

If he was my brother he would have been given an Essex smack for not waiting for some testing to be done ;-)

We will pass circa 50 Amps through the cable in order to produce a volt drop of the order 50 x rho = 50 x 0.00183 Ohm/A/m = 50 x 0.002 = 0.1 V per m or 25 x 0.1 = 2.5V over the length of the 25m cable. May even use an even larger current briefly.

Shall I buy the necessary items or do you want me to guide you through what you need and how to do it yourself? For safety's sake is the house end of the cable removed from the CU so that you would not be working near anything live?

 

[littlespark]

And was the fault actually under the blocks or concrete? Did you do a load of digging for nothing?

 

[DPG]

danielz000 - I am no more than 20 minutes drive from you. Do you want me to call in? If the method my colleagues have explained does not work on the damaged 25m length I will improvise a time domain reflectometer and use it during a second visit.

If he was my brother he would have been given an Essex smack for not waiting for some testing to be done ;-)

We will pass circa 50 Amps through the cable in order to produce a volt drop of the order 50 x rho = 50 x 0.00183 Ohm/A/m = 50 x 0.002 = 0.1 V per m or 25 x 0.1 = 2.5V over the length of the 25m cable. May even use an even larger current briefly.

Shall I buy the necessary items or do you want me to guide you through what you need and how to do it yourself? For safety's sake is the house end of the cable removed from the CU so that you would not be working near anything live?

The usual knowledge and helpfulness from Marconi, mixed in with a slight hint of violence ?

Be interested to see how this works out.

 

[littlespark]

“Hi, brother…. This is the guy I met on the internet that says you’re a prat”

 

[pc1966]

To answer my own question in case anyone searches the forum for varley loop test in future…
The 2nd equation is derived from twice the resistance of a length of one conductor being equal R+X+y. Substituting that in leaves a ratio of resistance X to resistance of the total loop. I think.
I might try this and @pc1966 method out when I have time just for fun.

My concerns with the Varley Loop / Murry loop sort of tests here is the need for high current / low resistance adjustments. With ~25m of 16mm copper the total resistance of each conductor is of the order of 0.03ohms and you will struggle to find rheostats of that sort of value and ~10A rating just sitting around.

My approach is really a "Wheatstone Bridge" arrangement but instead of using the other arms to null (zero) a galvanometer, today we just use a decent DMM that will be > 1M impedance so can be ignored and if 1mV resolution for the meter we can get a meaningful reading / distance resolution with only a 100mV or so total volt drop. The balancing arm ratio then becomes maths on the two measured voltages.

So then test currents of the order of 5A are feasible and sustainable without worrying too much about how to get that much current (mid-size laboratory PSU, car battery with some current-limiting load like a 60W bulb, etc).

 

[pc1966]

3. LAP AC/DC Digital Multimeter 600V - https://www.NoLinkingToThis/p/lap-ac-dc-digital-multimeter-600v/161fg that's the multimeter I got but when I tried to measure resistance across the live and armour it just read "ERR". Nothing in the manual about how or why that can happen...

The Screwfix site lacks detailed specifications, such as the voltage ranges/resolutions supported. At least it is CAT-II rated, but that is not quite enough if measuring at the CU (instead of at the appliance after 13A fuse, etc) See:

Measurement category - Wikipedia

en.wikipedia.org

The usual reason for getting an error when attempting to measure resistance is it is seeing some residual voltage there. It could be the fault is intermittent so only manifesting at higher test voltages so it is picking up capacitively coupled voltages, or it could be there is a small AC voltage of the order of 0.1V or so induced from some other energised circuit or current flowing back via earthed metalwork in the garage, etc.

As general advice, always check for voltage before you measure for resistance! Multimeters are not considered safe for "proving dead" as there are so many ways you can get it wrong, possibly resulting in a fatal shock if you failed to detect high voltages, or an inadequately rated meter exploding if subject to a very high energy fault.

That is what the likes of GS38 health & safety advice call for the use of a dedicated voltage tester, along with some means of proving it is working before and after you checked for the presence of high voltages. This is the sort of thing you would be looking at for a low budget:

https://www.NoLinkingToThis/p/lap-ms8922b-ac-dc-2-pole-voltage-tester-with-rcd-400v/669hy

The proving units are significantly more expensive (in the £70++ range) but for occasional use you can check it works on a circuit you did not switch off before/after you test one you plan on working on.

4299 Proving Unit

Intelligent voltage ramp testing for voltage indicators and multimeters

www.cef.co.uk

 

[HappyHippyDad]

@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.

 

[pc1966]

If he was my brother he would have been given an Essex smack for not waiting for some testing to be done ;-)

THIS !!!

Though quite why you would give him a twin-foresail fishing boat is a bit of a mystery:

Betty CK145 History of the Essex-smack

Essex oyster smack Betty CK145 homepage

www.betty-ck145.com