GB1473668A - Earth leakage detection and shock prevention circuit - Google Patents

Abstract

1473668 Protective circuits V S SATYANARYANA and B GANDHI 8 April 1974 15525/74 Heading H2K In a protective circuit for disconnecting a load from a power supply, under earth fault conditions or when a human body contacts the live load, comprising an electromagnetic switch S1 operated by a coil K connectable to the power supply by a switch S1 a transistor T1 is connected across the coil K with a resistor R4 connected between its base and the casing of the load L such that when leakage occurs in the casing of the load the transistor T1 is biased to conduct and short out the coil K thus causing switch S2 to open and cut off supply to the load L, and further a current transformer TR1 is connected between switch S2 and the load L to provide shock protection for a human body which contacts the live load the secondary TRs' of this transformer having a capacitor C2 connected across it and the terminals of switch S4, this capacitor also being connected to the base of transistor T1, such that under normal operation with switches, S1, S2 and S4 closed the capacitor C2 carries a charge, but if a human body contacts the live load the voltage in the secondary of TR's increases resulting in an increased charge on C2 which biases transistor T1 to short out the coil K as before to open switch S2. An amplifier or balanced amplifier may be incorporated in the circuit to increase its sensitivity, Figs. 3 and 4 respectively, not shown, two opposed current transformers being used to indicate unbalance due to a human body touching the live load.