An “overcurrent relay” is a type of protective relay which operates when the load current exceeds a preset value. The ANSI device number is 50 for an instantaneous overcurrent (IOC), 51 for a time over current (TOC).
In a typical application the overcurrent relay is connected to a current transformer and calibrated to operate at or above a specific current level. When the relay operates, one or more contacts will operate and energize to trip (open) a circuit breaker.