Circuit breakers. The main purpose of a circuit breaker is to: Switch load currents Make onto a fault Break normal and fault currents Carry fault current without blowing itself open (or up!) The important characteristics from a protection point of view are:
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The main purpose of a circuit breaker is to:
The important characteristics from a protection point of view are:
The first characteristic is referred to as the ‘tripping time’ and is expressed in cycles.
Opening time: The time between instant of application of tripping power to the instant of separation of the main contacts.
Arcing time: The time between the instant of separation of the main circuit breaker contacts to the instant of arc extinction of short-circuit current.
• Total break or clearing time: The sum of the above
Types of circuit breakers
The types of breakers basically refer to the medium in which the breaker opens and closes. The medium could be oil, air, vacuum or SF6
1. Turbulence caused by arc bubble.
2. Magnetic forces tend to force main contacts apart and movement causes oil to be sucked in through ports and squirted past gap.
3. When arc extinguished (at current zero), ionized gases get swept away and prevents restriking of the arc.
• Ability of cool oil to flow into the space after current zero and arc goes out
• Cooling surface presented by oil
• Absorption of energy by decomposition of oil
• Action of oil as an insulator lending to more compact design of switchgear.
• Inflammability (especially if there is any air near hydrogen)
• Maintenance (changing and purifying).
Not suitable for high current interruption at low voltages due carbonization of oil
In the initial stages, the use of high-volume (bulk) oil circuit breakers was more
common. In this type, the whole breaker unit is immersed in the oil. This type had the
disadvantage of production of higher hydrogen quantities during arcing and higher
maintenance requirements. Subsequently these were replaced with low oil (minimum oil)
types, where the arc and the bubble are confined into a smaller chamber, minimizing the
size of the unit.
Double break oil circuit breaker Single break oil circuit breaker
1. Hand operated: Cheap but losing popularity. Speed depends entirely on operator. Very limited use in modern installations that too for low-voltage applications only.
2. Hand operated spring assisted: Hand movement compresses spring over top dead centre. Spring takes over and closes the breaker.
3. Quick make: Spring charged-up by hand, then released to operate
4. Motor wound spring: Motor charges spring, instead of manual. Mainly useful when remote operations are employed, which are common in modern installations because of computer applications.
6. Pneumatic: Used at 66 kV and above. Convenient when drying air is