Buried SWA cable fault finding

[danielz000]

Hi all,

My first post here, thought I'd see if it was possible to get some advice or tips...

Basically, I had a large amount of block paving and large concrete base laid at the far end (25m) of my garden a few months ago. Under this I had 3 core 16mm SWA laid.

(I most definitely can't vouch for the quality of the work done by any of these guys)

More recently me and my brother built a large garage on the concrete base and had an electrician add a new CU in the garage and connect the other end of the SWA to an MCB in the CU in the house.

Today, after about 2 weeks of not having any problems, the RCD in the house tripped out and wouldn't switch back on unless I turned off the MCB to the garage. It's worth noting, it rained fairly heavily last night.

After trying a few different things, I've eventually disconnected the SWA from the CUs at both ends.

I've tested for continuity across all cable/ armour combinations and noticed there's continuity (beeping) when I touch the live and armour.

Is my assumption that someone has damaged the cable, breaking the live wire insulation and water has entered this and created a circuit between the live and the armour a reasonable one? Are there any other ways this continuity could occur given both ends are now fully disconnected?

Assuming this assumption is correct, I'm curious if there's a simple way for me to locate this fault without digging up 30m of block paving?

For example, I was wondering if, by using a multimeter and testing the resistance across the live and armour at both ends I might be able to work out how far down the wire the break is?

So maybe if I got a resistance 10 times higher at one end than the other I can assume the damage to the 25m long cable is 10% (2.5m) from the end of the cable with lower reading?

I guess I don't need to be super accurate and that there are industrial tools that could accurately locate the fault, all I really want to know is roughly were to start digging in the hope of keeping the damage to the paving to a minimum.

Thanks in advance for any help or suggestions.

Dan

 

[marconi]

Is this a "continuation" of your suggestion in post #25 in that you require this pulsating system.?

Yes

 

[pirate]

I truly don't understand this at all, but I'd love @marconi to wire my Christmas lights!
Seriously, I love to try and follow the logic, but the theory is way beyond me. However I also love to see the ideas out there! Thanks,!

 

[marconi]

#62 reminds me of our Glaswegian lecturer in electrical engineering science, who if you interrupted him during a lecture and asked him a question had the stock answer - 'd'ya knaw listna tu ma lecktures?' - and then he carried on with the lecture.

(With apologies to all Glaswegians for any failure to represent in italics how he said it).

 

[Lucien Nunes]

I'd probably cheat and use AC, having checked what error the cable inductance is likely to cause (top of head it's very small.) Wrap a couple of turns of 16mm through any hollow-centre toroidal transformer to make a secondary and wind it up on a variac, or ballast the primary with a suitable lamp. Any decent pot to make a bridge, use multimeter to find the null then read off the pot resistance ratio out of circuit.

Of course it would be better to fetch the bridge test set but that's far away in deep storage.

 

[timhoward]

@Lucien Nunes in my vast pile of junk I'm sure I have a post-office test box. I acquired it as a curious item that looked nice and never twigged it's purpose, but this thread suddenly resulted in a light-bulb moment and I realised that all along I had a bridge in a box without realising it.
In theory, assuming I find it, would a DC multimeter work instead of a proper galvanometer?
It's a bit like this one (from memory)

 

[marconi]

Work in progress.

 

[marconi]

Power on trial without electronics. Electronic control finished and to be connected to board next.

 

[telectrix]

in my cave i have a bridge megger. could this help . it's finer functions are beyond my old brain these days.

 

[littlespark]

I’ve got a couple of bits of bent fence wire… does the trick…

Sorry, that’s for water

 

[timhoward]

in my cave i have a bridge megger. could this help . it's finer functions are beyond my old brain these days.

Noting @pc1966 's comments earlier in this thread about the Varley test's shortcomings, yes, I recently discovered the Megger Bridge series were designed to conduct Varley and sometimes Murray tests. Is it either of these:

 

[marconi]

freddo: Do you have a preferred first test on the cable? If you do I will jot down a test schedule including how to connect 'what to what'. Otherwise I will decide.

Do you think 5ish seconds for one voltage measurement immediately followed by 5ish seconds for another will give you enough time to read your dvm and then record the values?

Looking at my little video clip I am minded to allow 5ish seconds for the current to settle before the voltage measurements are taken.

The test routine would be: Press a button - wait 5 seconds - in the next 5 seconds interval take a voltage reading - and then in the final 5 seconds interval take a second voltage measurement.

In practice - press a button - watch the dvm until it is settled - record the value - and then when you hear the click of a relay operate wait until the dvm is again settled - record the value.

 

[telectrix]

Noting @pc1966 's comments earlier in this thread about the Varley test's shortcomings, yes, I recently discovered the Megger Bridge series were designed to conduct Varley and sometimes Murray tests. Is it either of these:
View attachment 90047
View attachment 90048

xame as the 2nd pic.

 

[timhoward]

xame as the 2nd pic.

Yeah, that one does the Varley test. Don't know if yours still has this plate on it (from an ebay picture):

From what I gather, when in Bridge mode it measures resistance between EARTH and LINE, you crank the handle and adjust the ratio and the 4 digit values - I think the needle at infinity position shows you have it right. (I'm not sure how it indicates higher or lower, other models use a galvanometer and the needle moves both ways to show.) Then repeat with Varley position which measures between Earth and Varley terminals and do the maths shown.

 

[freddo]

freddo: Do you have a preferred first test on the cable? If you do I will jot down a test schedule including how to connect 'what to what'. Otherwise I will decide.

Do you think 5ish seconds for one voltage measurement immediately followed by 5ish seconds for another will give you enough time to read your dvm and then record the values?

Looking at my little video clip I am minded to allow 5ish seconds for the current to settle before the voltage measurements are taken.

The test routine would be: Press a button - wait 5 seconds - in the next 5 seconds interval take a voltage reading - and then in the final 5 seconds interval take a second voltage measurement.

In practice - press a button - watch the dvm until it is settled - record the value - and then when you hear the click of a relay operate wait until the dvm is again settled - record the value.

No preferred test. Yes those times sound practical.

 

[marconi]

Test rig completed. The 3 gold test resistors are so freddo can check the rig works on arrival. The I button pulses the test current and the near/far button switches the voltmeter between near and far ends of cable.