An RCD monitors the current flow in a circuit by measuring the current flowing through the phase and comparing it with the current flowing back through the neutral conductor. Normally, both currents are equal.
However, if a fault occurs, such as an earth fault (when current flows through the body or earth), the currents differ. The RCD recognises this deviation and interrupts the circuit.
This means that somewhere you have a connection from phase to earth or protective earth or neutral to earth or protective earth. If there is a connection from phase to neutral, the fuse would blow.
