Prepared by -

Circuit Breker
Prepared by
Kazi Md. Shahiduzzaman
Outline of this Chapter
 Definition of Circuit Breaker
 Technical Particulars of Circuit Breaker
 Working principle of Circuit Breaker
 Classification of Circuit Breaker
 Arc phenomenon and it’s principle of Extinction
 Methods of arc extinction
 Some important terms
 Rate of rise of TRV
 Circuit Breaker Rating
Circuit Breaker
 A circuit breaker is a piece of equipment which can
(i) make or break a circuit either manually or by remote control
under normal conditions
(ii) break a circuit automatically under fault conditions
(iii) make a circuit either manually or by remote control under
fault conditions
Technical particulars Circuit Breaker
• Type of medium for arc- extinction.
•Rated voltage
• Structural form
• Rated breaking current
• Type of operating mechanism
• Type of construction
• Total Break time
Working Principle of Circuit Breaker
 A circuit breaker essentially consists of fixed and moving contacts.
 Under normal operating conditions, the contacts remain closed and
the circuit breaker carries the full-load current continuously.
 When a fault occurs, the resulting overcurrent in the C.T. primary
winding increases the secondary e.m.f. This energises the trip coil of
the breaker and moving contacts are pulled down, thus opening the
contacts and hence the circuit.
 When the contacts of a circuit breaker are separated under fault
conditions, an arc is struck between them. The current is thus able
to continue until the discharge ceases.
Classification of Circuit Breaker
The Circuit Breaker can be divided according to the arc extinction
medium into the following categories:
Axial Blast
Air Break C.B
Air Circuit
Circuit Breaker
Oil Circuit
SF6 Circuit
Circuit Breaker
Air Blast C.B
Cross Blast
Bulk oil C.B.
Radial Blast
Low oil Circuit
Arc Phenomenon
 At the instant when the contacts of C.B. begin to separate, the
contact area decreases rapidly and large fault current causes
increased current density and hence rise in temperature. The heat
produced in the medium between contacts (usually the medium is
oil or air) is sufficient to ionise the air or vapourise and ionise the
oil. The ionised air or vapour acts as conductor and an arc is struck
between the contact.
The arc resistance depends upon the following factors :
Degree of ionisation
(ii) Length of the arc
(iii) Cross-section of arc
Principle of Arc Extinction
The factors responsible for the maintenance of arc between the
contacts are:
(i) p.d. between the contacts
(ii) ionised particles between contacts
Methods of Arc Extinction
There are two methods of extinguishing the arc in circuit breakers viz.
1. High resistance method.
2. Low resistance or current zero method
High resistance method: In this method, arc resistance is made to
increase with time so that current is reduced to a value insufficient to
maintain the arc. Consequently, the current is interrupted or the arc is
The resistance of the arc may be increased by :
(i) Lengthening the arc
(ii) Cooling the arc.
(iii) Reducing X-section of the arc
(iv) Splitting the arc
The principal disadvantage of this method is that enormous energy is
dissipated in the arc.
It is employed only in d.c. circuit breakers and low-capacity a.c.
circuit breakers
Low resistance or Current zero method: This method is
employed for arc extinction in a.c. circuits only. In this method, arc
resistance is kept low until current is zero where the arc extinguishes
naturally and is prevented from restriking inspite of the rising voltage
across the contacts.
All modern high power a.c. circuit breakers employ this method for arc
A.C. arc interruption is to rapidly deionise the medium between
contacts as soon as the current becomes zero so that the rising contact
voltage or restriking voltage cannot breakdown the space between
The de-ionisation of the medium can be achieved by:
Lengthening of the gap
High pressure
Blast effect
Important terms
 ArcVoltage: It is the voltage that appears across the contacts of the
circuit breaker during the arcing period.
 Restriking voltage: It is the transient voltage that appears across the
contacts at or near current zero during arcing period.
• Recovery voltage: It is the
normal frequency (50 Hz) r.m.s.
voltage that appears across the
contacts of the circuit breaker after
final arc extinction. It is
approximately equal to the system
Thank you

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