PMMI Industrial Electricity Practice Test

Protection coordination is about arranging protective devices so they work together to isolate a fault with minimum disruption to the rest of the system. The faults that require this coordinated response are those that create abnormal, high current paths—short-circuit faults, where conductors create a low-impedance path between phases or to ground, and ground faults, where current unintentionally returns to earth through an unintended path. In these situations, protective devices (like breakers and relays) detect the excessive current and trip, with the device closest to the fault operating first to preserve service elsewhere (selectivity). Open circuits aren’t faults that produce abnormal current, so they aren’t the focus of protection coordination in this context. Transient faults may disappear on their own, but protection coordination is designed to handle sustained faults that require interruption to protect equipment and personnel.