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How does a thermal magnetic trip unit work?

FA90910

01 February 2021

Issue:
How does a thermal-magnetic trip unit work in a circuit breaker?

Product Line:
Circuit Breakers

Environment:
Miniature and Molded Case Thermal-Magnetic Circuit Breakers

Resolution:
Thermal magnetic trip units trip under short circuit conditions instantaneously, with no intentional delay. Below the instantaneous trip current, they have a delay established to protect conductors while allowing momentary current surges such as for motor starting and transformer inrush. In some cases, they have adjustable instantaneous trip current settings.