I agree with the previous diagnosis that the neutral is bad somewhere along the line. It might be a bad connection in the main breaker, or possibly intermittent contacts on it.
At the times when there's a relatively high resistance through the neutral side due to the fault, you'll measure 230V phase-to-neutral and 0V neutral-to-earth with no load because the meter is pulling a tiny current.
As soon as you connect a substantial load, the resistance of the fault becomes significant and on the load side of the fault you'll read 230V neutral-to-earth due to the circuit completed through the loads.
There's no reason for the RCDs to trip, as the small current on their two poles will still be equal.
If you could temporarily disconnect the feeder from the load side of the incoming main breaker and wire up a suitable load at that point, you could determine if the fault is on or before the main breaker or somewhere further down the line.
[This message has been edited by pauluk (edited 05-24-2005).]
