EEE 4508 Switchgear and Protection
Lecture Notes 5
Protection Circuit Breakers:
Circuit breakers are switching devices and equipment that are designed to break/open and
make/ close a circuit ON LOAD. Circuit breakers have very high speed of operation and are
fitted with mechanisms of quenching the arc that is created when contacts are either making or
breaking. Circuit breakers are therefore classified or categorised by their method of quenching
the arc or under which medium the contacts break and make.
5.1 Operation of Circuit breakers
Circuit breakers are mechanical devices designed to make and break the electrical circuits
under normal and abnormal conditions. Circuit breakers are generally used for the protection
of electrical circuits, are equipped with a trip coil connected to a relay designed to open the
circuit breaker automatically under abnormal conditions such as a short circuit or any fault
whose threshold current exceeds the current settings for the circuit breaker.
Figure 5.1 Circuit breaker principle of operation
A circuit breaker consists of fixed and moving contacts which touch each other and carry the
current when the circuit breaker is closed. The circuit breaker can be opened and closed
manually for switching and during maintenance. Therefore, whenever a fault occurs on any
part of the power system, the trip coil of the circuit breaker get energized and moving contacts
are separated by some mechanism. Figure 5.1 shows the parts that comprise a circuit breaker
and how operation is effected under normal and abnormal conditions.
1
, The separation of the current carrying contacts under load conditions creates an arc between
them. Once an arc is formed between contacts, the molecules of the medium surrounded by arc
become extremely hot and get ionized, i.e. the insulating property of it is destroyed, and it
breaks down becoming conductor of electricity.
Therefore, the arc is maintained even if the contacts are further drawn. This arc not only delays
the current interruption process but it also produces heat in a huge quantity which may cause
damage to the system or to the circuit breaker itself.
Therefore, the main issue in a circuit breaker is to extinguish the arc in the shortest possible
time so that heat generated by it may not reach up to a hazardous value.
The basic construction of a circuit breaker requires the separation of contacts in an insulating
medium. This insulating medium extinguishes the arc between the contacts when the circuit
breaker opens. It also provides insulation between contacts and from each contact to earth. The
insulating mediums commonly used for this purpose are commonly used in the classification
of circuit breakers as follows:
5.1.2 Methods of Arc Extinction in Circuit breakers
There are two methods of arc extinction that are used in circuit breakers; the high resistance
methods and low resistance or current zero interruption methods.
5.1.2.1 High resistance methods: In this method, the effective resistance of the arc increases
with increase in time. So the arc current reduced up to a very small value, and it is unable to
maintain the arc. Thus current is interrupted, and the arc is extinguished. The resistance of the
arc can be increased by
cooling the arc,
increasing the length of the arc,
reducing the cross-section of the arc,
splitting the arc into a small number the arcs in series.
High resistance method of arc extinction is generally used in D.C. circuit breakers, and air
break type A.C. circuit breakers of low braking capacities.
5.1.2.2 Low resistance or current zero interruption: This method is applicable only in A.C.
circuit breakers because in an A.C. system current drops to zero after every half cycle. This
property of A.C. circuit is exploited for the current interruption. In this method, the arc
resistance is kept low until the current is zero where the arc is extinguished naturally and is
prevented from re-striking after current zero instant.
2
Lecture Notes 5
Protection Circuit Breakers:
Circuit breakers are switching devices and equipment that are designed to break/open and
make/ close a circuit ON LOAD. Circuit breakers have very high speed of operation and are
fitted with mechanisms of quenching the arc that is created when contacts are either making or
breaking. Circuit breakers are therefore classified or categorised by their method of quenching
the arc or under which medium the contacts break and make.
5.1 Operation of Circuit breakers
Circuit breakers are mechanical devices designed to make and break the electrical circuits
under normal and abnormal conditions. Circuit breakers are generally used for the protection
of electrical circuits, are equipped with a trip coil connected to a relay designed to open the
circuit breaker automatically under abnormal conditions such as a short circuit or any fault
whose threshold current exceeds the current settings for the circuit breaker.
Figure 5.1 Circuit breaker principle of operation
A circuit breaker consists of fixed and moving contacts which touch each other and carry the
current when the circuit breaker is closed. The circuit breaker can be opened and closed
manually for switching and during maintenance. Therefore, whenever a fault occurs on any
part of the power system, the trip coil of the circuit breaker get energized and moving contacts
are separated by some mechanism. Figure 5.1 shows the parts that comprise a circuit breaker
and how operation is effected under normal and abnormal conditions.
1
, The separation of the current carrying contacts under load conditions creates an arc between
them. Once an arc is formed between contacts, the molecules of the medium surrounded by arc
become extremely hot and get ionized, i.e. the insulating property of it is destroyed, and it
breaks down becoming conductor of electricity.
Therefore, the arc is maintained even if the contacts are further drawn. This arc not only delays
the current interruption process but it also produces heat in a huge quantity which may cause
damage to the system or to the circuit breaker itself.
Therefore, the main issue in a circuit breaker is to extinguish the arc in the shortest possible
time so that heat generated by it may not reach up to a hazardous value.
The basic construction of a circuit breaker requires the separation of contacts in an insulating
medium. This insulating medium extinguishes the arc between the contacts when the circuit
breaker opens. It also provides insulation between contacts and from each contact to earth. The
insulating mediums commonly used for this purpose are commonly used in the classification
of circuit breakers as follows:
5.1.2 Methods of Arc Extinction in Circuit breakers
There are two methods of arc extinction that are used in circuit breakers; the high resistance
methods and low resistance or current zero interruption methods.
5.1.2.1 High resistance methods: In this method, the effective resistance of the arc increases
with increase in time. So the arc current reduced up to a very small value, and it is unable to
maintain the arc. Thus current is interrupted, and the arc is extinguished. The resistance of the
arc can be increased by
cooling the arc,
increasing the length of the arc,
reducing the cross-section of the arc,
splitting the arc into a small number the arcs in series.
High resistance method of arc extinction is generally used in D.C. circuit breakers, and air
break type A.C. circuit breakers of low braking capacities.
5.1.2.2 Low resistance or current zero interruption: This method is applicable only in A.C.
circuit breakers because in an A.C. system current drops to zero after every half cycle. This
property of A.C. circuit is exploited for the current interruption. In this method, the arc
resistance is kept low until the current is zero where the arc is extinguished naturally and is
prevented from re-striking after current zero instant.
2
