Content Types of circuit breakers Operating principles of circuit breakers Inverse time/current characteristic of circuit breakers Circuit breakers and fault loop impedance Circuit breaker selection

Parts of A Circuit Breaker Thermal/Magnetic

Parts of A Circuit Breaker Bimetal strip Used to detect overloads Bends when heat from current passes through it. It then mechanically trips the breaker

Parts of A Circuit Breaker Bimetal strip

Parts of A Circuit Breaker Solenoid Used to detect Short Circuits At a pre set current the coil attracts a metallic part of the trip, tripping the breaker

Parts of A Circuit Breaker Solenoid

Thermal/Magnetic Three basic time/current curves  Type B  Type C  Type D Used to protect Generators Long submains General loads Power & Light Motors

Moulded Case

Hydraulic Magnetic magnetic force produced by a load current flowing through a series connected coil Hermetically sealed tube containing an iron core, a spring and dampening fluid.

Overload Magnetic flux in the solenoid pulls on the core. Movement toward the pole piece is regulated by the hydraulic fluid. Controlled time delay which is inversely proportional to the magnitude of the current.

Overload If overload persists, core reaches the pole Reluctance of the magnetic circuit drops The armature is attracted to pole face Breaker trips.

Short Circuit Magnetic flux produced is large enough to attract armature to the pole face and trip the breaker even though the core has not moved. Called the instantaneous trip region of the circuit breaker.

Hydraulic Magnetic Advantages Because magnetic operation, Trip curve not affected by ambient temperature Can be re-closed immediately as there is no heating element Possible to obtain any variation of time/current characteristic