Current interruption in a high-voltage circuit-breaker is obtained by separating two contacts in a medium, such as SF6, having excellent dielectric and arc quenching properties. After contact separation, current is carried through an arc and is interrupted
when this arc is cooled by a gas blast of sufficient intensity. Gas blast applied on the arc must be able to cool it rapidly so that gas temperature between the contacts is reduced from 20,000 K to less than 2000 K in a few hundred microseconds, so that it is able to withstand the transient recovery voltage that is applied across the contacts after current interruption. SF6 is generally used in present high-voltage circuit-breakers.
Interrupting Phenomena
SF6 gas in the cylinder is compressed by the downward movement
Gas is forced into the nozzle where the arc is drawn.
In early stages pressure inside the cylinder is raised.
Nozzle concentrates the gas flow to the area of the arc.
Parameter Definition
Rated Voltage
The maximum voltage at which a circuit breaker can operate for extended periods without undue degradation or safety hazard.
Rated Short Circuit breaking Current
The highest electric current which can exist in a particular electrical system under short circuit conditions.
Rated Operating Sequence
This denotes the sequence of opening and closing operations which the circuit breaker can perform under specified conditions. The circuit breaker should be able to perform the
operating sequence as follows:
O-t-CO-T-CO
Where, O = opening operation
t = time required to be ready to receive closing order from auto- reclosure relay
CO = close operation followed by open operations
T = time required to insulating media for regeneration and operating mechanism
Rated Lightning Impulse Withstand Voltage
Lightning impulse voltage rating which the gas circuit breaker can withstand.
Rated Short Duration Power-Frequency Withstand Voltage
R.M.S. value of sinusoidal power frequency voltage that the breaker can withstand during tests made under specified conditions and for a specified time.