Richard Russell writes:

We have an extensive, professionally installed, garden lighting system
which has recently started tripping its dedicated RCD. I have
eliminated all the bulbs, luminaires and accessible cables which,
sadly, means the problem is almost certainly in one of the many
'waterproof' underground connection boxes. How on earth does one
trace a fault like this? Dig up the entire garden?

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 !

--
Windmill, Use t m i l l
J.R.R. Tolkien:- @ O n e t e l . c o m
All that is gold does not glister / Not all who wander are lost