Electrical systems


Residual current devices

Preventative protection for people from electric current injury is realized by the Residual-current devices (RCD). The low end current limit, considered dangerous to humans, is 30mA. The automatic circuit breakers cannot reliably protect in the cases of current leakage, because they can not guarantee quick reaction in such low current levels, which are actually dangerous to humans. In the design of the RCDs, a current transformer is used, whose primary coils are actually the energized and the neutral conductor wires in the circuit, which has to be protected.  The direction of the winding in this coil is such, that the magnetic fluxes, created by the currents in both conductors mutually cancel each other. When a leakage current is detected, this violates the “status quo” state and in the secondary coil and an electric current is induced, proportional to the leakage current.