UNIT - I
During the operation of power system, it is often desirable and necessary to switch on or off the various
circuits (e.g., transmission lines, distributors, generating plants etc.) under both normal and abnormal conditions.
In earlier days, this function used to be performed by a switch and a fuse placed in series with the circuit.
However, such a means of control presents two disadvantages.
Firstly, when a fuse blows out, it takes quite sometime to replace it and restore supply to the customers.
Secondly, a fuse cannot successfully interrupt heavy fault currents that result from faults on modern highvoltage and large capacity circuits.
Due to these disadvantages, the use of switches and fuses is limited to low voltage and small capacity circuits
where frequent operations are not expected e.g., for switching and protection of distribution transformers,
lighting circuits, branch circuits of distribution lines etc.
With the advancement of power system, the lines and other equipment operate at very high voltages and carry
large currents. The arrangement of switches along with fuses cannot serve the desired function of switchgear
in such high capacity circuits. This necessitates employing a more dependable means of control such as is
obtained by the use of circuit breakers.
A circuit breaker can make or break a circuit either manually or automatically under all conditions viz., noload, full-load and short-circuit conditions.
This characteristic of the circuit breaker has made it very useful equipment for switching and protection of
various parts of the power system.
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
Thus a circuit breaker incorporates manual (or remote control) as well as automatic control for switching
functions. The latter control employs relays and operates only under fault conditions.