Circuit breakers are critical links in a fault-clearing situation. When a fault occurs on the electrical system, the associated current must be interrupted quickly and reliably to prevent catastrophic damage to equipment and nearby personnel.
Basic Components
- Frame – Outer enclosure for other components.
- Operating Mechanism – Responsible for the mechanical charge, trip, and close cycle.
- Primary Contacts – All circuit breakers have contacts, and when the contacts separate during a fault clearing operation, they draw an arc. The electric arc forms between the contacts and is drawn out in length as the contacts open.
- Arcing Contacts – Arcing contacts are designed to make-before and break-after the main contacts. This helps keep high temperatures away from the main contacts which could result in damage.
- Primary Disconnects – Physically connects the circuit breaker to the switchgear bus bar.
- Secondary Disconnects – Drawout circuit breakers have an additional disconnect point to power accessories and communications.
- Protective Device – The Electronic Trip Unit is a microprocessor controlled, multifunction overcurrent protective device for application with low voltage power circuit breakers. The adjustment flexibility provided by electronic trip units allow users to easily accommodate load changes and other protective requirements while still assuring optimum coordination. Typical functions, long delay (overload), short delay (ignore inrush), instantaneous (no intentional delay), and ground fault.
- Accessories – Responsible for status indication, remote operation. Aux switch, Bell alarm, motor, shunt trip, UV trip, electric lockout, electronic close, operations counter, position switch, etc.
Circuit Breaker Indicators
- Racking Indicator – shows the position of a circuit breaker within its cell. May be connected, test, disconnected, or withdrawn. No equipped in bolt-in breakers.
- Spring Indicator – shows whether the circuit breaker operating springs are charged and ready to close.
- Contact Indicator – shows whether the mechanism is closed, open, or tripped.
Methods of Arc Extinction
- Air Gaps – Convection cooling air assists in keeping the operating components within reasonable temperature limits.
- Vacuum – Similar to air circuit breakers with the main difference being that the contact assembly is housed in a vacuum bottle.
- Oil – All contacts and mechanism assemblies are housed within an oil-filled tank, which provides both an insulation and arc-quenching means.
- Gas – SF6 circuit breakers are similar to vacuum circuit breakers with the main difference being that the contact assembly is housed in a pressurized bottle. One distinct advantage gas breakers have over vacuum breakers is if the gas bottle loses pressure, the residual SF6 gas can be enough to allow the breaker to open safely under a normal load.
Circuit Breaker Maintenance
- Visual and Mechanical Inspections
- Bolted connections and contact resistance
- Insulation resistance
- Trip Functions via Primary or Secondary Injection
- Auxiliary Devices
- Control Wiring
