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Discuss Buried SWA cable fault finding in the UK Electrical Forum area at ElectriciansForums.net
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#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).
(With apologies to all Glaswegians for any failure to represent in italics how he said it).
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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.
Of course it would be better to fetch the bridge test set but that's far away in deep storage.
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@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)
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)
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I’ve got a couple of bits of bent fence wire… does the trick…
Sorry, that’s for water
Sorry, that’s for water
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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:
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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.
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.
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xame as the 2nd pic.
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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.
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No preferred test. Yes those times sound practical.
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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.
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Yes - at mate’s rates
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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.
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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.
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.
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