instantaneous overcurrent relays for motor protection
An AC motor may draw 5x to 8x the rated max and reduces as speed builds up so after this period the trip level is reset rapidly so if the motor is overloaded and continues to draw &gt;2x rated current, it will break  and at 3x rated current even faster. This is carefully timed for the expected rating and inertia of the system and power rating to prevent heat rise damage.