The Siemens 3RB1253-0FM21 is a solid-state overload relay from the SIRIUS 3RB1 family, rated for motor protection from 50 to 205 A on a 220 to 240 V AC control supply at 50/60 Hz. It integrates thermistor (PTC) motor protection and internal earth-fault detection into a single unit, with separate bistable output contact sets: 1NO/1NC for overload/thermistor status and 1NO/1NC for earth-fault status. That means one device covers three protection functions — overload, overtemperature, and ground leakage — without needing separate modules.
The solid-state design means no heater elements to swap when changing motor FLA — just adjust the 50–205 A range setting. The bistable (latching) outputs hold their state after a trip even if control power is lost, which is useful for remote fault indication on a PLC input card. Earth-fault detection is internal, so no external zero-sequence CT or core-balance transformer needed for ground-fault sensing down to the relay's sensitivity threshold.