Why does an RCD trip when a Neutral to Earth fault occurs on a circuit which has its live ( line ) disconnected.
When there is no fault all the current through any loads on any energised circuit protected by the RCD returns to the neutral bar and from there through the RCD to the neutral incomer.
When there is neutral to earth short anywhere on circuits connected to that neutral bar there is a second path for current to reach the incoming neutral and this second path by-passes the RCD.
In this diagram the CPC is connected to the neutral in the cut out as for a PME or similar supply method. Where the CPC is not connected to the neutral in the cut out the second route is then via the mass of the earth ( earth rod ) to the neutral which is earthed at the substation or transformer.