There are two types of ELCB : voltage ECB abd current ECB.
In voltage ELCB, one of the terminals of the sensing device is connected to the main body of the equipment to be protected. The other terminal is connected to the earth so that in the case of any accumulation of charges on the non - current carrying conducting part of the equipment, so that the excess charges may flow to the ground/earth, due to the voltage difference that has been created. However there is a sensing device that is connected in between, as well. So, if the voltage difference caused, crosses a fixed/set value, then the current through the relay coil becomes sufficient top cause the tripping of what is incoming from the equipment.
A current ELCB is also called as RCCB ( Residual Current Circuit Breaker ) or RCD ( Residual Current Device It is more sensitive than the voltage ELCB. There is acone that encircles both - the Phase and the Neutral windings. Both these windings have the same magnitude but opposite direction currents flowing through them. So, the resultant effect of both the current flows is zero. However, if some fault occurs in the phase, then current will flow to the earth and then the magnitudes will differ and thus, there will be a residual flux in the cone. Thus, the RCCB operates.
Thus, the main principle of RCCB/RCD is that with unbalance in the line currents due to earth fault, there is a residual flux in the cone that encircles all the windings, and the relay, then, thus senses it, with the help of a winding wound ON the cone.
This principle applies for both - single phase, as well as 3 - phase RCCB.