Principle
Definition :
The transformer neutral is earthed; (
) 
The frames of the electrical loads are also connected to an earth connection. (
) 
The insulation fault current is limited by the impedance of the earth connections and the faulty part is disconnected by a Residual Current Device (RCD).
Note :
The RCD should open the faulty circuit if the voltage between the ground and the metallic frame
is above the limit voltage
according to the premises where the equipment is.
The limit voltage
The limit voltage
is 50 V for dry premises and 25 V for wet premises. The maximum time (in Sec) for maintenance of contact voltage as in standard IEC 60364 is given by the table below:
The device (RCD/ELCB) should open the circuit before the maximum time to avoid any damage.
