I think it has to do with reflected current flows in the E/N wires. All circuits connected to the council point on the E/N circuits are connected in parallel to all the different loads, however when there is a break on the council side supply line, as is the case described by ACEsterhuizen, and because of high current flows and inductance in these circuits, a pulse is injected into the neutral line towards the load side. THE RCD , by nature of its design, is to trip when there is an imbalance in the currents in the L and N circuits. When this pulse comes down the line, there is a mismatch between the L and N, and the RCD can not differentiate where the imbalance is coming from, source or load, and the mechanism then trips.
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