nick smith wrote:

I have a standard 4 bed house with an outbuilding which has power fed to it.
The electrics are in "generally good shape" (i.e. no rubber wire etc) and I
have had a new split load consumer unit fitted with each side on an RCCB main
switch, with the lights not on a RCCB and the fridge / freezer on its own RCCB,
so that another circuit tripping does not take them out with it.

Occasionally, like two or three times a year, one of the circuit trips out - it
might do this two or three times (re-setting in between) over a period of a
couple of hours then be fine again for another few months.

The problem is that it this occurs so infrequently that I can't build up a
sample of tests to narrow down where the problem is occurring. If I had a
selection of specialist test equipment I guess I could measure each circuit and
item to test for earth leakage, but only have "basics" like a multimeter, scope
(and a working but basic old uncalibrated PAT tester).

Can anyone suggest a course of actions and tests to help identify the problem
with what I have got, or can I hire the necessary piece of kit ?


You may actually have no problem at all.
RCD's seem to trip on quite small imbalances, and I spent ages trying to
ascertain whgat was causing mine to go - In teh end I put i a 100mA one
and its been fine since, except

- tyhe washing machine debveloped an earth short in the motor
- I left and extension plugged into an outside socket, and it rained...

Ther is alwys some earh leakage: Its damned hard to identify what it is.

There are things you can do, like check for earth neutral shorts. One of
these somewhere in the house will cause random tripping on a seemingly
arbitrary basis.

The way to check this is to unhook the erath wire (on a dead CU of
course) from every circuit and check it shows no neutral connection -
use a sensitive analog ohmmeter - e.g. cheap dial based meter.

If those check out OK, I duno what else to suggest. What one really
wants is a very sensitive clamp on ammeter that you can pass both live
and neutral through and measure current impbalance, and then switch
everything on and off to try and work out what is causing it. If only
RCD's hade meters on them...

A little damp in e.g. a cooker can be the culprit, or voltage spikes
coupled with RFI filters on e.g. TV's etc.


Thanks

Nick