The following are some important components common to most of the circuit breakers:
Bushings.
Bushing serves as the essential insulation when a high-voltage conductor passes through a metal sheet or frame that is at earth potential. Bushing's main purpose is to stop electrical breakdown between the enclosed conductor and the metal of the surrounding earthed metal.
The high voltage conductor passes through the bushing made of some insulating materials(porcelain). Although there are many different varieties of bushing, including condenser and oil-filled types, they all serve the same purpose by protecting the conductor from the earth tank.
Circuit breaker contacts.
It is necessary for the circuit breaker contacts to carry both normal and short-circuit current. When carrying normal current, it is preferable that neither the temperature nor the point of contact's low voltage drop exceed the predetermined ranges.
The primary consequences to be dealt with when carrying breaking and making short-circuit currents are melting and vaporization caused by arc heat as well as those caused by electromagnetic forces. Therefore, the design of contacts is of considerable importance for satisfactory operation of the circuit breaker.
Instrument Transformers.
The circuit of a modern power system runs at extremely high voltages and can carry currents of thousands of amps. If put directly on the power system, the measurement tools and safety equipment will not function properly.
These difficulty is overcome by installing instrument transformers on the power lines. The function of these instrument transformer to transform the voltage or current to such a value which are convenient for the operation of measuring and protection instruments.
Related post: What are the different ratings of a circuit breaker?
There are two types of instrument transformers:
1. Current transformer (C.T.)
2. Potential transformer (P.T.)
The power line is connected to the current transformer's primary. A constant percentage of the current in the line is provided for the instruments and relay by the secondary winding.
Similarly, a potential transformer is connected with its primary in the power line. The secondary provides a voltage which is a known fraction of the line voltage.
The use of instrument transformers permits the following advantages:
1. They isolate the relay and measuring equipment from high-voltage power circuits.
2. Small voltage and currents are carried via the leads in secondary circuits. This makes it possible to utilize wire that is smaller and has less insulation.
Bus-bar and conductors.
The current carrying member in a circuit breaker consists of fixed and moving contacts and the conductors connecting these to the points external to the breaker. These connections are connected directly to the overhead wires if the switchgear is of the outdoor type. The incoming conductors to the circuit breaker are connected to the bus in the case of indoor switchgear.
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