The Siemens 3RB1253-0FG00-0FM0 is a solid-state overload relay that combines motor overload protection with integrated thermistor monitoring and ground-fault detection. It is rated for a current range of 50 to 205 A and accepts a control supply of 110-120 VAC at 50/60 Hz. Ground-fault monitoring is achieved through the connection of an external summation current transformer, which lets you detect leakage currents that a standard overload relay would miss.
The 50-205 A setting range covers a wide band of motor sizes — from a 30 kW motor on the low end up to roughly 110 kW at 400 V. The 110-120 VAC control supply is common in North American panel designs; verify your control transformer taps before wiring. The solid-state design means no bimetallic strip to drift with ambient temperature, so the trip curve stays repeatable across the enclosure's thermal gradient. The relay provides two sets of isolated contacts: 1 NO/1 NC for overcurrent/thermistor alarm, and a separate 1 NO/1 NC for ground-fault indication. Both are monostable, so they return to the de-energized state when the fault clears — no manual reset required unless the circuit logic demands it.