The Siemens 3RV2902-4AV0 is a SIRIUS undervoltage release — a control-voltage accessory that drops the motor circuit breaker's trip mechanism when its coil voltage falls below the dropout threshold. It's designed specifically for the 3RV2 family of motor circuit breakers in frame sizes S00, S0, S2, and S3. The ring cable lug connection keeps wiring clean inside the panel.
Snap-on mounting onto a DIN rail. Width is 18.5 mm, depth 68 mm, height 90 mm. Front is finger-safe against vertical contact. Protection class is IP00, for enclosed panel installation.