The Siemens 3RV6902-1AV0 is a SIRIUS undervoltage release — a bolt-on accessory for the 3RV20 and 3RV10 motor circuit breaker family. That makes it a standard fit for any panel where a loss-of-voltage event needs a manual reset before the line can re-energize. It snaps onto the left side of the breaker via the standard DIN rail clip, no tools needed for the mechanical install. The screw terminals accept the control wiring directly. It covers both S00 and S0 frame sizes, so if your panel has a mix of 3RV10 and 3RV20 breakers, this one release fits both. The 18.5 mm width means it adds less than one standard MCB slot to the rail — tight for retrofit into a packed enclosure.
The release itself is IP20 on the front, finger-safe per IEC 60529 for vertical contact. That's fine inside a locked panel; it's not for exposed or washdown locations. That storage range matters if you're stocking spares in an unheated warehouse. The design uses a fuse for short-circuit protection on the control circuit — that's the 'design of short-circuit protection: fuse' spec. Don't feed it through an MCB without checking coordination; the manufacturer expects a gG fuse upstream on the control transformer secondary.