Why ELCB can't work if N input of ELCB do not connect to ground ?

  ELCB is used to detect earth leakage fault. Once the phase and neutral are connected in an ELCB, the current will flow through a phase and that much current will have to return neutral so the resultant current is zero. Once there is a ground fault in the load side, current from phase will directly pass through earth and it will not return through neutral through ELCB. That means once side current is going and not returning and hence because of this difference in current ELCB will trip and it will safeguard the other circuits from faulty loads. If the neutral is not grounded, fault current will definitely high and that full fault current will come back through ELCB, and there will be no difference in current.