During the operation of power system, it is often necessary to switch on or off the various circuits (e.g., transmission lines, distributors, generating plants etc.).
The switching on or off could be under both normal and abnormal conditions. In earlier days, switches and fuses were placed in series to control (ON or OFF) the various circuits under both normal and abnormal (faulty) conditions.
However, such a means of control presents two disadvantages.
• It takes sometime to replace fuse and restore supply to consumers when they blows out.
• Fuses do not successfully interrupt heavy fault currents that result from faults on modern high-voltage and large capacity circuits.
Switches and fuses are therefore limited to low-voltage and small capacity circuits where frequent operations are not expected such as 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 to employ 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;
• no-load• full-load
• short-circuit conditions.
This characteristic of the circuit breaker has made it a very useful equipment for switching and protection of various parts of the power system.