What it is and what it does
The Siemens 3RB2055-1FC2 is a SIRIUS solid-state overload relay — a three-pole device that protects motor circuits against overload and phase imbalance by monitoring current and tripping within the CLASS 10E curve (typically 10 seconds at 7.2× Ie). It can be mounted directly onto a contactor or installed stand-alone in a panel, giving flexibility in cabinet layout. The relay is ATEX certified (PTB 06 ATEX 3001 Ex II (2) GD), so it can be used in applications requiring protection of motors in potentially explosive atmospheres — a detail that often governs the spec for Zone 22 or gas-group installations.
Key ratings and what they mean for fit
Trip class CLASS 10E means the relay will trip within 10 seconds at a locked-rotor current of 7.2 times the full-load setting — standard for most general-purpose motor starting applications where the motor accelerates quickly and doesn't need a longer start-time window. The solid-state design means no heating elements to drift or replace; power dissipation per pole is only 0.02 W (0.05 W total for the main circuit), so it doesn't add significant heat inside the enclosure. Terminals accept solid conductors from 0.5 to 4 mm² (or 2× 0.5 to 2.5 mm²) and finely stranded with ferrules from 0.5 to 2.5 mm² (2× 0.5 to 1.5 mm²) — covers most control wiring gauges used in motor starter circuits. Dimensions are 120 mm wide, 119 mm high, 155 mm deep — fits standard 120 mm-wide DIN-rail or contactor-mounting footprints. Side clearance of 6 mm is required for ventilation; no clearance needed above, below, or behind the unit. The relay is rated for operation at altitudes up to 2 000 m without derating, and can operate in ambient temperatures from -25 to +60 °C with 100 % relative humidity — suitable for most indoor industrial environments.