Main purpose of using pre insertion resistor is to
Answers
PIR is pre-insertion-resistor. This is a resistor of about 200-400 ohms which gets temporarily closed before closing the circuit breaker. The sequence is (close order)->closing of PIR->10-12milliseconds->closing of main break.
But while opening, PIR is first disconnected by the Breaker operating mechanism and then i.e. after 10 ms, main contacts of Breaker are opened. The main purpose of PIR is to limit the initial charging current of line. As the charging current of long line will be much more, it is advisable to use Breaker having PIR in long transmission line.
When electrical circuit having capacitance or transformer which can have inrush. Also transformers may have large inrush current. Transmission line (due to its capacitance) is another such circuit.
For closing in such circuit first the PIR comes in circuit to limit the inrush current. When the main contacts close, the current through PIR becomes zero as the shunt path is bridged by main contact.