Donkey's moons ago there used to be things called TDRs, Time Domain Reflectometers, which would show the location of discontinuities in a transmission line by the squiggles on a built-in oscilloscope screen.
Hard to interpret and I imagine it would be even more difficult if all you were looking for was a fairly high leakage resistance rather than a short, and even more difficult if the circuit branched.
But treating live and earth as a transmission line and using a TDR to examine its characteristics might just possibly reveal something.
OTOH one might have more luck with a couple of bent hazel sticks !
That's a double-fault scenario (live-to-case short *and* case not earthed). One doesn't normally attempt to protect against such unlikely occurrences. In any case an entirely disconnected earth isn't very plausible when the stainless steel lamp column physically enters the ground!
Seems entirely reasonable to me. If the solution to a problem will inevitably be difficult, expensive and time-consuming, putting it off as long as possible (without compromising safety) makes perfect sense!
In N. America they use 115v but they also use RCDs (they call them GFIs) with a sensitivity of only 5 mA. not 30 mA. So they're still worried even though the voltage is lower.
If you measure the resistance between two of those columns (which will be about twice the resistance of one column to earth) you may be surprised - it'll likely be of the order of K ohms.
There're still around used on the other week to find a fault on a buried co-ax cable it showed it up very accurately. Two discontinues one major one not so. There used to be a design around for a brew you own but needed a scope to go with it not that practical. Theres a BT test unit comes up on ebay from time to time thats very good...
Well that might tell you the line is there but not much else. Seen it done on buried water pipes, seems to work very well with some people but not all..
Yup. You've got a fair chance of getting a lot of the circuit operational quite quickly. But it is also possible the OP has more than one leaky cable or jbox, or that the leak is in the one and only feed line for the lot.
There's really no other way to fix it than divide & conquer. The only other options are workarounds that keep going with wet cable or jboxes, which have a high risk of later total failure.
With luck you may be able to trace much of it with a basic metal detector.
Or not... With the 'fault' being a fairly high resistance leak to earth the reflection coefficient probably won't be high enough to show up.
Sort of. My suggestion is as follows:
Disconnect the feed-point from the RCD. Strap the L & N outgoing conductors together and energise them at mains voltage via a precautionary low value fuse (1 or 2 A, say). The point(s) of leakage will now be injecting some current into the soil. If you're worried about safety then reduce the applied voltage using a suitable transformer or auto-transformer. 50 V might be enough.
Now use a highish impedance AC voltmeter (an ordinary DMM will do) and probe for ground potential gradients - i.e. earth one side of the meter to a reference, the mains earth should do, via a long flying lead. On the other side, rig up a probe a few inches long to stick in the ground
- this ought to a have an insulated handle for obvious reasons. Probe around, following the cable routes, if known. Look for the highest voltage and dig there.
If water has got into a joint box it will only be a matter of time before the situation develops into a low impedance L-N fault. I fear that feeding via an isolating transformer would only give a short-term fix.
Might be worth adding a step of doing an additional probing before energising the circuit, just to establish that you are not going to be lead up the garden path (so to speak) by stray ground currents from other sources not connected with the actual circuit in question.
and could ultimately end up costing in electricity keeping the ground warm if you are unlucky.
Good idea - and in retrospect my suggestion to use mains earth as the voltmeter's reference point wasn't too clever, especially it's PME. A 'clean' reference spike somewhere in the garden, but known to be well away from the lighting cables, should be a much better bet.
:) If you pull the fuse or switch the mcb off, and clip a flashing light bu lb across it, you then get a mains feed that goes on and off frequently. Yo u can thus tell with your ground probe how much difference the mains feed i s making, it just means you can do one sweep not 2. Its a classic old fashi oned way to trace circuits.
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