Safety circuit to prevent electrocution - has parallel resistance and few-turns coil in mains earth lead controlling mains switch via close-coupled sensing coil of control circuit

摘要

The connector is for supplying current to an electrical appts. and has live and neutral terminals connected through an electromagnetically operated switch to the main circuit and an earth terminal connected to the mains earth through a parallel circuit of a 0.05 OMEGA to 5 OMEGA resistor and a coil having few turns. The coil is the primary of a current transformer having a primary-secondary turns ratio of 200:1 whose secondary coil is an input to a symmetrical amplifier. The switch is held closed by its energised operating coil until an operator of the apparatus accidentally touches the line supply. The resulting earth current sensed by the current transformer is amplified, rectified and triggers a thyristor which short circuits the switch coil to open the switch and disconnect the supply before the operator is electrocuted. An electrostatic screen coil screens the transformer and the power supply for the amplifier and switch coil is derived from the mains theough a rectifier and smoothing circuit.