Buried SWA cable fault finding

[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.

 

[7029 dave]

@marconi did you wire Frankenstein's laboratory by chance. ?

 

[marconi]

@marconi did you wire Frankenstein's laboratory by chance. ?

Yes - at mate’s rates️

 

[MoserV8]

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

Heres my penny worth, before you test the cable, make sure there is nothing on the cable both ends, remove all glands from the cable, separate the cores and splay the armour strands out & away from the cores. For testing you need a good insulation resistance tester - if you dont know what one is & how to use it, then my strong advice is to Hire another Electrician to test the cable. Sadly most sparks i know would have undoubtedly layed this cable in a duct, which enables easy replacement, but mainly saves any damage. However you may be lucky.......I genuinely hope you are and its a silly termination fault.

 

[Mark42]

Blimey, I've just seen this - what a brilliant thread ?.
It's great to learn new stuff. And I thought you only wanted that high-current low voltage source to burn off the fault then pretend it was never there, like I once saw an old electrician do with swarf somewhere in a MICC circuit ?. Maybe that was standard practice in the 60's.

 

[marconi]

Test rig posted this morning ?