In the event of a Neutral conductor failure, domestic load current from your property and and (possibly neighbour's) consisting of several 10's or 100's of amps will still return to the sub-station transformer via the Electricity Co.'s bonding Neutral-Earth at the distribution point. The earth connection obviously has some impedance, so the current flowing through it raises point A by V=I*R volts. So now our mains earth is floating at "V" volts above the general mass of the earth.

In our shack this means our nice shiny radio chassis is at V volts above real earth. But outside we have an aerial with a ground connection, feeding into the shack, by, say, co-ax. The braid of this co-ax is at real earth potential which is currently below our mains earth. If I now undo this co-ax lead from the rig, I am holding in one hand "real" earth (co-ax), and in the other mains earth, which is higher, therefore current flows through me.

It doesn't take much to see that this electrocution current has flowed through my RCD in both directions, so it is still balanced and has no reason to trip.

Good-night!