Hi, all.

I've been plagued with intermittent nuisance trips on one of the 2 RCDs
in the new 'cheapest way to achieve 17th Edition'CU that our clueless
sparky installed during recent building work.

In due course, I plan to install a new CU which contains all RCBOs.

But in the meantime, I wanted to make some diagnostic measurements to
see what the actual residual leakage was, and where it was coming from.

Clamp meters are quite expensive, so I thought I'd D-I-Y...

I basically needed the current transformer with sense coil.
An old RCD was the donor for this.
I got an old Merlin Gerin 2-pole RCD, and carefully opened it.
I removed all the actual moving parts: toggle, contacts, springs etc.
I soldered the output braids directly to the output terminals.
It's now just a feed-thru.
All that was retained was the housing, current transformer, terminals
and a small PCB where the fragile sense coil wires terminated.
The PCB was held in place inside the housing by the actuation solenoid,
which was no longer required, so the tracks to it were cut.
The sense coil was in fact centre-tapped, and a pair of diodes provided
full-bave rectification to drive the solenoid. I did not use any of
this, and simply ran a pair of wires directly from the outer ends of the
coil, and ran these out to a pair of 4mm sockets.

During calibration, I found that the output voltage drifted about a bit
high when open-circuit, so I measured the original solenoid at 7k-Ohm
DC, and so I added a 10k load resistor across the coil. That settled
things right down.

Calibrarion consisted of using a handy 24v transformer to provide an AC
input, and a pair of Fluke meters. A series of resistors was chosen to
provide nominal calibration points at 0,1,2,3,4,5,6,7,8,9,10 mA, then
15,20,25,30,35,40 mA. The resistors were hooked up in the usual way to
provide an imballance ( 1 leg before the current transformer, one after.

The actual mA imballance was measured with one fluke, and the mV AC
output measured with another. The calibration was very linear, that's
why I went to bigger steps after 10mA. In my example, I got around 45 mV
/ mA.

After calibration at 24v ( keeps the power dissipation in the test
resistors down ), I did a couple of check points at 240v, and it was
spot on. It's easily accurate to within 0.5mA against the fluke, with
a resolution better than 0.1mA.

I'll post a photo.

OK, not the convenience of a clamp-on, but at least I can now make
meaningfull measurements and start fault-finding!

--
Ron