A trip unit is the part of a circuit breaker that opens the circuit in the event of a thermal overload, short circuit or ground fault. An open circuit will not conduct electricity because either air, or some other insulator has stopped or broken the flow of current in the loop.
There are two types of trip units, electromechanical (also called thermal magnetic) and electronic. An electromechanical trip unit has moving parts; it combines a current-sensitive electromechanical device with a thermal-sensitive device and the two devices work together to determine when to mechanically open the circuit. An electronic trip unit is programmable device which measures and times current flowing through the circuit breaker and initiates a trip signal when appropriate.