Earthing Switch is connected between the line conductor and the earth. Normally, it is open when isolation work of opening is over the earthing switch is closed. Due to this closing, whatever the voltage is trapped on the line is discharged to the earth and working becomes safe. (Though the line is disconnected by isolators there is some voltage on the line to which capacitance between line and earth is charged). Normally. the earthing switch is mounted on the frame of isolator. The symbol of earthing switch is shown in Fig. 1.
Fig. 1: Earthing Switch
Installation of Earthing Switch
It is installed along with the isolator to discharge the voltage on circuit capacitance to earth and provides safety (o the operator doing maintenance/ repair/ inspection work.
Function of Earthing Switch
Earthing switch provides phase to ground discharge path to capacitive charges trapped in the circuit (particularly a high voltage circuit has inherent capacitance).
Even after opening the circuit by C.B. and isolating by isolator, the voltage remains trapped on the capacitors and there remains a chance of getting shock to the operator. Hence earthing switch is necessary.
Working of Earthing Switch
(1) During opening of circuit breaker
Fig. 2 shows the sequence of operations as
- Opening C.B
- Opening Isolator
- Closing earth switch.
(2) During closing circuit breaker
Fig. 3 shows the sequence of operations as
- Opening earth switch.
- Closing Isolator
- Closing C.B.
