Inverse Time Circuit Breakers
Inverse time circuit breakers have both thermal and instantaneous trip features and are preset to trip at standardized levels. This is the most common type of circuit breaker used in the building trades for residential, commercial, and heavy construction.
- The thermal action of this circuit breaker responds to heat.
- If a motor’s ventilation inlets and outlets are not adequate to dissipate heat from the windings of the motor, the heat will be detected by the thermal action of the circuit breaker.
- If a short should occur, the magnetic action of the circuit breaker will detect the instantaneous values of current and trip the breaker.
- The National Electrical Code requires inverse time circuit breakers to be sized to a maximum of 250% of the motor FLA.
Instantaneous Trip Circuit Breakers
Instantaneous trip circuit breakers respond to immediate (almost instantaneous) values of current from a short circuit, ground fault, or locked rotor current.
- This type will never trip from a slow heat buildup due to motor windings overheating.
- A stuck bearing or a blanket of lint covering the inlets and outlets of the motor’s enclosure will cause the motor to overheat and damage the windings.
- The National Electrical Code allows instantaneous trip circuit breakers to be sized to a maximum of 800% of a motors FLA value.
- They are used where time-delay fuses set at five times their ratings or circuit breakers at three times their rating will not hold the starting current of a motor.
- Some instantaneous trip circuit breakers have adjustable trip settings The instantaneous trip ratings of an instantaneous trip circuit breaker can be adjusted above the locked-rotor current of a motor to allow the motor to start and come up to its running speed.