On 21/06/2013 23:57, Andy Wade wrote:
On 19/06/2013 23:36, Windmill wrote:

But treating live and earth as a transmission line and using a TDR to
examine its characteristics might just possibly reveal something.

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.

OTOH one might have more luck with a couple of bent hazel sticks !

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.

Nice plan...

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.

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.

and could ultimately end up costing in electricity keeping the ground
warm if you are unlucky.


--
Cheers,

John.

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