I have an ongoing problems with nuisance RCD trips.

The supply is a TT system with a "whole house" 100mA RCD on the
incoming feeder. Downstream of the RCD are two consumer units:

1. House circuits (Shower, cooker, socket ring main downstairs, socket
ring main upstairs, lighting ring main down stairs, lighting ring main
upstairs)

2. Electric Heating and Garage (electric heating ring downstairs,
electric heating ring upstairs, garage supply, immersion heater)

The garage supply is 2 x 2.5mm FTE and it also has a 30mA RCD at the
garage end (approximately 50 yards away)

Over the last month we have been continuously experince random nuisance
trips. (it quietened down over the last couple of weeks, but picked up
again in the wet weather - significant?) We have tried narrowing it
down, but with little success. Differentiation using RCBOs has been
suggested, but that will require a new consumer unit. (The current one
uses Wylex plug in mcbs.) It is interesting that sometimes just the
"whole house" 100mA RCD goes, at other times the garage RCD (some 50
yards away) will also trip out.

Last night I turned off Consumer Unit 2 at the isolator and we suffered
a trip at about 02:30. Therefore there is nothing on those circuits
that can contribute to the trip. (Apart from the tails from the "whole
house" RCD to the consumer unit). Tonight I plan to turn off consumer
unit 1. I am hoping for a trip as that will rule out my house wiring
as a cause.

If that is the case, what's the next step? I believe Western Power
Distribution are the local distribution company in the South West, I
assume that I aske them to check there supply side.

If it doesn't trip it means that it may be my problem. In which case,
anybody in Wiltshire got a Megger I can borrow.....

Has anybody else had this sort of problem? Have people experienced
trips due to "outside" influences? How have they been resolved?

Cheers

Peter