SF₆ circuit breakers include 2 gas pressure stages for safe operation.
Stage 1 (Low Pressure)
Normally The closing circuit includes the close contacts for all 3 phases of the gas density relay.
When the gas pressure drops to stage 1 the NC connections become open. This prevents the breaker from closing.
Stage 2 (Very Low Pressure)
The tripping circuit is constructed with normally closed contacts from all three phases of the gas density relay. when the gas pressure falls to stage 2.
NC became open.
This prevents breaker opening.(Closing is already prevented at stage 1).
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