what is residual current?

Simple version...

I an AC circuit, electric current must pass from one source to another, be it another circuit out of phase from the first or to a Neutral point or to Earth. If you measure the current in both paths (or all 3 n the case of 3 phase) and sum the vectors, the total should be zero because when the current is flowing one direction in one path, it is flowing the opposite direction in the other(s). If you divide the total vectors by the number of paths and there is any remainder, that is called "residual current" and it means there is current flowing somewherethat is not getting back to one of the other "normal" paths. Since current MUST flow to somewhere, the only path left must be earth (ground) and therefore, the Residual Current represents a condition in which one or more of the circuits must be flowing to earth, or what we in the US call a Ground Fault. If the circuit uses a Neutral path, even if it is eventually connected to Earth at some point, you still measure the Neutral current and anything flowing to Earth but NOT through the Neutral path frst, also represents a Ground Fault.