On 15/12/2012 18:54, harry wrote:
On Dec 15, 2:59 pm, Caecilius wrote:
I've been experiencing occasional nuisance RCD tripping for a few
years now. It's very occasional - sometimes going for up to six months
without tripping. On average I'm getting a trip around once every two
months.

I've done some equipment testing with a megger and basic RCD
sensitivity testing with some home-made leakage plugs (plugs with
resistors between live and earth to give a known leakage current).
Based on this, I think my RCD is over sensitive.

My plan is to replace the RCD, which is fairly cheap and simple. But
I'd like to test the old one and the new one with something better
than leakage plugs to see if there's any change in sensitivity.

Does anyone know if it's possible to hire RCD testers for short
periods at a reasonable rate? When I've looked, most hire shops direct
me to a PAT tester which isn't what I'm looking for. I don't want to
buy one for a single project.

The way to find out if the RCD is faulty is to swop two over.
You can then see if the fault lies with the circuit or the breaker.

You are unlikely to discove ranything on an intermittent fault with
instrumentation.


Depends how D-I-Y you want to get...

I faced a similar problem, and built a D-I-Y leakage meter.

I took an old 2-pole RCD ( Merlin Gerin, as it happened ), and carefully
opened it.

I removed all the mechanical switching parts, leaving only the current
transformer with it's 2 heavy windings passing L and N, and the sense
coil which I wired to a pair of external jacks.

A 10k resistor was placed as a load across the sense coil, to provide a
voltage output rather than a current output. This is important,
otherwise the open-cct output voltage from a current transformer can
posentially get hazardously high.

I then performed a calibration with a 12vAC supply, some test resistors,
and a pair of flukes set up to measure mV out versus mA leakage. I
calibrated in 1mA steps from 1 to 10mA, then in 5mA steps up to 50mA.
The calibration was outstandingly linear, and I printed up a table of mV
to mA. I got 45mV / mA.


Now, by wiring this in-circuit with the troublesome circuit ( both L and
N, obviously ) I was able to make actual measurements and isolated
several faults.

Finally, you may wish to consider replacing the single RCD with multiple
RCBOs, if your CU can take them.