The Siemens 3RB1025-1SB0 is an electronic overload relay in the SIRIUS 3RB10 series, rated for 3...12 A motor protection with Class 10 trip characteristic. It mounts directly on a contactor in size S0, using screw terminals, and is configured for manual reset only.
The 3...12 A current range covers small to mid-size induction motors — think 2.2 kW at 400 V three-phase, or a 5 HP motor at 480 V. Class 10 trip means it will ride through normal starting inrush on most pump, fan, and compressor loads (typical start time under 10 seconds), but will trip before the motor overheats on a stalled rotor. The manual reset only configuration means the relay latches after a fault and requires a physical button press to re-close the circuit — no auto-restart, which is the safer choice for unattended or hazardous machinery where unexpected re-start could injure personnel.
Size S0 is the standard Siemens contactor footprint for this current class. The relay snaps onto the contactor without additional hardware, and the screw terminals accept 0.5...4 mm² wire. Wiring is straightforward: the three-phase motor leads pass through the relay's current sensing apertures, and the N/C trip contact (95-96) breaks the contactor coil circuit on overload. Verify the contactor's coil voltage matches the control circuit before wiring the trip contact.