CIRCUIT BREAKER

Definition  Device designed to open the circuit automatically under abnormal conditions without damage to itself when properly applied within its rating.

 Circuit breakers are categorized by the method employed to interrupt the circuit current (extinguishing the arc) when the current contacts open.

Four major types  Air  Oil  Vacuum  Gas (SF6 or SF6/CF4)

Two methods circuit breakers sense circuit current  Thermal  Magnetic

Thermal circuit breaker  Sense circuit current through the production of heat  Generally uses some type of heating element in series with the load  The heater is located close to a bimetallic strip  The bimetallic strip is mechanically connected to the moveable contacts

Thermal circuit breaker  There is a time delay before the circuit opens  The time delay depends on the amount of overcurrent

Thermal Circuit Breaker  Schematic symbol  Schematic

Magnetic circuit breaker  Also known as Instantaneous trip circuit breaker  Senses circuit current by connecting a coil in series with the load  When current flows thought the circuit, a magnetic field is established around the coil  The magnetic field attracts the metal arm of a solenoid

Magnetic circuit breaker  When the magnetic field becomes intense enough the metal arm mechanically opens the contacts  There is very little time delay in opening the contacts when an abnormal condition occurs

Magnetic Circuit Breaker  Schematic symbol  Schematic

Thermomagnetic circuit breaker  Most common type  Schematic symbol  Employs both thermal and magnetic current sensors

Four Major Types  Air  Oil  Vacuum  Gas (SF6 or SF6/CF4)

Air circuit breaker  Divided into four basic types  1 Moulded Case CB (120V – 750V)  2 Low-Voltage Power CB (120V – 750V)  3 Medium-Voltage CB (1.5kV – 69kV)  4 High-Voltage CB (115kV – 230kV)