In article ,
Roger Mills wrote:

In an earlier contribution to this discussion,
Andy Wade wrote:
Sensitive "earth leakage" clamp meters like this
http://www.martindale-electric.co.uk/clamp.htm#cm67

Fascinating! Can you hire them - 'cos you could replace quite a
lot of potentially faulty kit for the £300 cost of buying one?

I made a cruder version of that a couple of years
ago, when trying to trace an RCD trip. It had
very little measuring accuracy, just an indication
whether the unbalance current was larger or smaller
than 15mArms.

Started off with one of those RCDs built into a
13A plugtop. Stripped all comps off the internal
pcb except the Current Transformer and the two
mains wires looping through it. Then terminated
the secondary of the CT with a pair of diodes and
a burden resistor (10k to start with afair). Then
brought out a couple of wires for connection to
a digital multimeter, ( on the 200mVrms range).

I wired a 15k resistor across L-E in a 13A plug
and plugged that into the modified RCD as a
15mArms calibration current and noted the reading
on the DMM.

It worked ok. I was surprised at the very low
unbalance current for most items.

BTW: It did not help in locating the RCD trip
because the fault was actually an N-E short that
only produced a tripping unbalance during the
transient startup surge of a heavy load, (say a
vacuum cleaner, or computer, etc).

--
Tony Williams.