The operating time of a conventional open-air LS is 2–3s, while that of an LS set in GIS is 0.5s The LS has to trip leakage current which flows through a small leakage capacitance C.
In Fig left shown a substation with a double bus structure , and feeder line 1 is connected to bus A (i.e. LS11 closed, LS12 open). Now the connection of feeder line 1 is change from bus A to bus B.
- t1: breaker BR1 is tripped
- t2: LS11 is opened. The line switch LS11 has to break the leakage current which is flowing across the leakage capacitance C of the small energized section connecting Br1/LS11/LS12.
- t3: LS12 is closed, so the bus connection changing processes of feeder line 1 from bus A to bus B.
Concerning the behavior of LS11 at time t2 when LS11 is break the leakage current i=j*w*C*E, the waveform of the voltage switching surge caused by LS11 opening is shown in Fig right.
The voltage at point b(one terminal of LS11) is obviously AC system voltage Vb=E* cos wt with a sinusoidal waveform, while the waveform for the voltage Va at point a (another terminal of LS11) is of repeated stepping form with quite high oscillatory transient terms.