On 13/02/2014 16:22, Sooner or Later wrote:
The MCB at the head end of my garage supply is tripping. It has been
tripping intermittently for some months but now trips every time it's
switched on.

The head end is fed directly from the meter into a small consumer
unit containing a main switch and a 20A type B MCB. This is the one
that trips. This connects to around 100m of 6mm SWA, partially
buried, partially running above ground through some brambles (will
get around to burying that one day!). I have completely disconnected
the SWA in the garage.

So its a cable fault then, or a faulty MCB. Try feeding it from any
other MCB as a test to eliminate the MCB.

There was a resin-filled join in the SWA around a third of the way,
so I cut that out as that was what I suspected of being the problem.
If I connect just the first third of the SWA to the head end it
doesn't trip. If I connect the rest it does trip, so the problem
obviously exists in the final two thirds of the cable.

sounds like it

I've used a standard multimeter to test the resistance between L&N,
L&E and N&E but all read open circuit (the meter shows a reading for
an instant, then reads open circuit again). I don't have access to a
Megger unfortunately.

I would have to be a fairly catastrophic insulation failure of some
description to sink the 100A+ necessary to open the MCB immediately.

I've visually inspected the parts of the cable I can get to, so now
I'm stuck for ideas.

Any suggestions for how to proceed would be gratefully received!

If the MCB checks out, replace the second section of cable.

--
Cheers,

John.

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