I made the suggestion about equipment perhaps being the problem because,I trust Mark has carried out all the usual tests we would normally do,he has happened upon an unusual problem that requires an unusual cause
I made the reference to the no trip test of a loop tester to give comparison to what I was dreaming about
Dreaming,because I have no technical know how whatsoever, regards the intricacies or the workings of the rcd tester
I did however find this bit of information from a study into the effects
DC in Residual Current Devices
The widespread use of semi-conductor
devices in domestic, commercial and industrial equipment may result in the
flow of non-sinusoidal earth fault currents in the event of equipment insulation
failure. This can occur in certain circumstances where the waveform may be
rectified or chopped and these waveforms are said to contain a pulsating d.c.
component which may desensitise a standard Type AC RCD.
The above is somewhere along the lines of what I was thinking,it may be utter bullcarp and the problem will likely be standard, but its certainly an unusual cause
Some of the equipoment listed includes Phone chargers invertors,transformers etc