2 DefinitionDevice designed to open the circuit automatically under abnormal conditions without damage to itself when properly applied within its rating.
3 Circuit breakers are categorized by the method employed to interrupt the circuit current (extinguishing the arc) when the current contacts open.
4 Four major typesAirOilVacuumGas (SF6 or SF6/CF4)
5 Two methods circuit breakers sense circuit current ThermalMagnetic
6 Thermal circuit breaker Sense circuit current through the production of heatGenerally uses some type of heating element in series with the loadThe heater is located close to a bimetallic stripThe bimetallic strip is mechanically connected to the moveable contacts
7 Thermal circuit breaker There is a time delay before the circuit opensThe time delay depends on the amount of overcurrent
9 Magnetic circuit breaker Also known as Instantaneous trip circuit breakerSenses circuit current by connecting a coil in series with the loadWhen current flows thought the circuit, a magnetic field is established around the coilThe magnetic field attracts the metal arm of a solenoid
10 Magnetic circuit breaker When the magnetic field becomes intense enough the metal arm mechanically opens the contactsThere is very little time delay in opening the contacts when an abnormal condition occurs
11 Magnetic Circuit Breaker Schematic symbolSchematic
12 Thermomagnetic circuit breaker Most common typeSchematic symbolEmploys both thermal and magnetic current sensors
13 Four Major TypesAirOilVacuumGas (SF6 or SF6/CF4)
14 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)