RCD trips when all Circuit Breakers are on. only sometimes

[tlarge]

I have two fuse boxes, one never trips, on the other the main RCD trips occasionally when all MCB's are on, if I switch two MCB's off on the lighting circuits, or one of the power MCB's the RCD does not trip. It does not seem to care which MCB I switch off on the power circuit, but the RCD does not trip. I originally thought that because I have 9 strip lights in the garage that may cause the problem? But as I can switch any one power circuit off and then the RCD does not trip. Both are Wylex fuse boxes.
Any idea's please.

 

[plugwash]

sounds like a combination of different leakages adding up to take you over the limit.

the first thing to do would be to test the wiring properly (which sadly requires tools diyers generally don't have) then if the wiring is known to be sound move on to trying to eliminate appliances.

 

[andy]

could possible be a N-E fault. if it is, it wont matter which MCB is off