The Siemens 3RV6921-1M is a SIRIUS short-circuit signalling switch designed to snap onto 3RV6 motor circuit breakers in sizes S00 and S0. The switch breaks the circuit in a maximum of 20 ms when a short-circuit trip occurs, giving you a discrete signal that the breaker has operated on a fault rather than an overload.
This signalling switch snaps directly onto the side of a 3RV6 circuit breaker — no DIN rail footprint of its own, no extra panel space beyond the breaker's own width. It's sized for the S00 and S0 frame sizes, which covers the 3RV6 range from the smallest motor-protective breaker up through the 25 A class. The 19 mm width means it sits flush against the breaker body; you don't lose a DIN slot.