Why we use Circuit Breakers?
Circuit breakers are mechanical device designed to close or open contact members.
Thus closing or opening an electrical circuit under normal or abnormal conditions.
A circuit breaker has fixed and moving contacts.These contacts remain in contact whenever the circuit breaker is in normal operation.
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| Why we use Circuit breakers? |
Whenever fault occurs the contacts separate out resulting in arc between the electrodes.
The production of arc not only delays the current interruption process but it also generates enormous heat which may cause damage to the syatem or the breaker itself.
Thus the main problem is to extinguish the arc whenever it occurs is circuit breaker.
The temperature of arc may be as high as 6000C°.
Methods of arc creation
Commonly used methods of arc creation are mentioned below:
(1) High resistance interruption
By increasing the effective resistance with time current can be reduced to a value insufficient to maintain it.
The arc resietance can be increased by
✓ Arc lengthening
✓ Arc cooling
✓ Arc splitting
✓ Arc constraining
(2) Low resistance interruption
Here, the arc resistance is kept low, in onder to keep the arc energy to a minimum.
This can be achieved by
✓ Cooling
✓ Gap lengthening
✓ Blast effect
The normal frequency r.m.s. voltage that appears across the breaker contacts after final
arc extinction has occured is called the recovery voltage.
The transient voltage that appears across the contacts at the instant of arc extinction is
called the restrildng voltage.
The rate of rise of restriking voltage depends on:
✓ Active recovery voltage which is the instantaneous value of the recovery voltage at the instant of arc extinction.
✓ Natural frequency of oscillations
Types of circuit breakers
1) Low voltage air circuit breakers
2) Oil circuit breakers
3) Water type circuit breakers
4) Air blast circuit breakers
Circuit breakers have the following ratings:
✓ Breaking capacity
✓ Making capacity
✓ Short time capacity
Clearing and closing time of a circuit breaker
Figure 1 includes the clearing and closing time of a circuit breaker.
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| Fig.1 clearing and closing time of circuit breakers |
Reactor
The purpose of reactors incorporated in circuit breakers is to limit the short-circuit current
flowing to a safe value thus providing protection of instrument.
They consists of large coils of high self-inductance and very low resistance.
The reactors are of two types namely:
1.Open type
2.Oil immersed type
Oil immersed type reactor used for voltages above 33 kV.
Reactors in a power system can be located as follow:
✓ In series with generators
✓ In series with feeders
✓ In bus-bars
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