Circuit BreakersCircuit Breakers

DefinitionDefinition

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

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

Four major typesFour major types

AirAir OilOil VacuumVacuum Gas (SF6 or SF6/CF4)Gas (SF6 or SF6/CF4)

Two methods circuit breakers sense Two methods circuit breakers sense circuit currentcircuit current

ThermalThermal MagneticMagnetic

Thermal circuit breakerThermal circuit breaker

Sense circuit current through the production Sense circuit current through the production of heatof heat

Generally uses some type of heating Generally uses some type of heating element in series with the loadelement in series with the load

The heater is located close to a bimetallic The heater is located close to a bimetallic stripstrip

The bimetallic strip is mechanically The bimetallic strip is mechanically connected to the moveable contactsconnected to the moveable contacts

Thermal circuit breakerThermal circuit breaker

There is a time delay before the circuit There is a time delay before the circuit opensopens

The time delay depends on the amount of The time delay depends on the amount of overcurrentovercurrent

Thermal Circuit BreakerThermal Circuit Breaker

Schematic symbolSchematic symbol SchematicSchematic

Magnetic circuit breakerMagnetic circuit breaker

Also known as Instantaneous trip circuit Also known as Instantaneous trip circuit breakerbreaker

Senses circuit current by connecting a coil Senses circuit current by connecting a coil in series with the loadin series with the load

When current flows thought the circuit, a When current flows thought the circuit, a magnetic field is established around the coilmagnetic field is established around the coil

The magnetic field attracts the metal arm of The magnetic field attracts the metal arm of a solenoida solenoid

Magnetic circuit breakerMagnetic circuit breaker

When the magnetic field becomes intense When the magnetic field becomes intense enough the metal arm mechanically opens enough the metal arm mechanically opens the contactsthe contacts

There is very little time delay in opening the There is very little time delay in opening the contacts when an contacts when an abnormal conditionabnormal condition occursoccurs

Magnetic Circuit BreakerMagnetic Circuit Breaker

Schematic symbolSchematic symbol SchematicSchematic

Thermomagnetic circuit breakerThermomagnetic circuit breaker

Most common typeMost common type Schematic symbolSchematic symbol Employs both thermal and magnetic current Employs both thermal and magnetic current

sensorssensors

Four Major TypesFour Major Types

AirAir OilOil VacuumVacuum Gas (SF6 or SF6/CF4)Gas (SF6 or SF6/CF4)

Air circuit breakerAir circuit breaker

Divided into four basic typesDivided into four basic types

1 Moulded Case CB (120V – 750V)1 Moulded Case CB (120V – 750V)

2 Low-Voltage Power CB (120V – 750V)2 Low-Voltage Power CB (120V – 750V)

3 Medium-Voltage CB (1.5kV – 69kV)3 Medium-Voltage CB (1.5kV – 69kV)

4 High-Voltage CB (115kV – 230kV)4 High-Voltage CB (115kV – 230kV)