The Siemens 3RV1902-1AQ0 is a SIRIUS undervoltage release designed for motor circuit breakers in frame sizes S00, S0, S2, and S3. It operates on a 380 V AC control supply at 50 Hz, tripping the breaker when the control voltage drops below its dropout threshold — a standard safety interlock for motor starters that prevents automatic restart after a supply dip. This is a phased-out product. The manufacturer's stated successor is the SIRIUS 3RV2 series, which consolidates the undervoltage release function into a newer platform with updated ratings and a different physical interface.
The 18.5 mm width and 68 mm depth mean it occupies a single modular slot, leaving adjacent spaces free for auxiliary contact blocks or other add-ons. The 90 mm height is standard for the S00–S3 frame accessory footprint.