If the RCD still operates fine on the bench and it isn't a mechanical problem, could it be that on the bench you're running it from a cleaner supply?
If so, then maybe it is all the hash on your power grid that's causing the problem.
Just another possibility: I know it shouldn't make any difference to the RCD, but on the bench are you applying the phases in a different order to when it is installed?
I'm just clutching at straws here and thinking that it might have an effect if there's excessive differential noise between phases or on one particular phase to neutral (i.e. the phase that it used inside the RCD to power the circuitry).
Re grounding/earthing, it would be interesting to get some more comments from this side of the Big Pond with our variety of systems. I have a particular dislike for the TT system!
If you read through some of the other posts, you'll see that grounding in the States also regularly poses certain questions and discussions.
[This message has been edited by pauluk (edited 08-06-2002).]