Circuit Breakers
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)