Circuit Breakers

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Circuit Breakers
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Definition

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

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

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Four major types

• Air
• Oil
• Vacuum
• Gas (SF6 or SF6/CF4)

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Two methods circuit breakers sense circuit current

• Thermal
• Magnetic

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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

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Thermal circuit breaker

• There is a time delay before the circuit opens
• The time delay depends on the amount of
  overcurrent

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Thermal Circuit Breaker

• Schematic symbol

• Schematic

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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

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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

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Magnetic Circuit Breaker

• Schematic symbol

• Schematic

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Thermomagnetic circuit breaker

• Most common type
• Schematic symbol
• Employs both thermal and magnetic current sensors

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Four Major Types

• Air
• Oil
• Vacuum
• Gas (SF6 or SF6/CF4)

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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)