I. Introduction
A residual current device usually comprises coils on a magnetic circuit to carry the phase (three-phase) and neutral current in the opposite directions (Fig. 1). In the balanced conditions no magnetic flux is generated, but if an earth fault occurs in the system, the phase and neutral current imbalance induces an electromagnetic force in the secondary circuit. In the nowadays developed construction of residual current devices, the secondary circuit contains electromechanical relay with a permanent magnet. The permanent magnet keeps the moving armature on the yoke and the spring pulls the moving armature in the opposite directions. In the absence of earth fault current the magnetic circuit of the electromechanical relay is closed. In case of fault the earth fault current (residual current), transformed by the current transformer, in the one half-wave amplifies the magnetic flux of the permanent magnet but in the second half-wave that flux is reduced. If the residual current reaches a predetermined level, the magnetic flux derived from that current is high enough to reduce the magnetic flux of the permanent magnet to the level in which the spring is able to pull out the moving armature of the electromechanical relay and RCD opens the main circuit. Basically the residual current devices do not operate if the residual current is equal to or less than 0.5 , while their operation is required if the residual current equal to or greater than occurs. Simplified diagram of voltage independent residual current device RCD: TR – current transformer, WS – electromechanical relay, N-S – permanent magnet, T – test button, – primary (earth fault) current, – secondary current, – induced secondary voltage, EC – additional electronic components