1. Circuit Breakers

2. Definition
Device 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 types
Air
Oil
Vacuum
Gas (SF6 or SF6/CF4)

5. Two methods circuit breakers sense circuit current
Thermal
Magnetic

6. 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

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

8. Thermal Circuit Breaker
Schematic symbol
Schematic

9. 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

10. 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

11. Magnetic Circuit Breaker
Schematic symbol
Schematic

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

13. Four Major Types
Air
Oil
Vacuum
Gas (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)