The Siemens SIRIUS 3RH1122-1MB40 is a coupling relay for switching auxiliary circuits, built into the S00 contactor footprint. It carries 2 normally-open instantaneous contacts, with a 24 V DC coil that picks up at 0.85 of rated voltage and drops out at 2.15 times rated — that 2.15 factor means the relay holds in through deep sags but releases decisively on undervoltage, a useful trait in control circuits where you want a clean dropout rather than a chattering contact.
The contact rating is 10 A at 24 V, which is the full-load current for a 24 VDC auxiliary load — think solenoid valves, indicator lamps, or PLC input modules. At 230 V AC the contacts are rated 6 A, and at 400 V AC they drop to 3 A, so the relay can handle mixed-voltage control circuits within a panel. The 6 kV surge voltage rating (impulse withstand) covers the transient immunity required in industrial environments with motor starts and contactor switching nearby. Coil power is 1.4 W holding, same as pick-up — a DC economizer circuit is not needed here; the coil simply stays at full power. That 1.4 W is low enough to be driven by a standard 24 VDC PLC output or a small auxiliary contact without overheating the backplane.
Fastening is by screw or snap-on onto a DIN rail. The S00 size — 45 mm wide, 57.5 mm tall, 72 mm deep — fits the standard contactor footprint used across the SIRIUS family, so it occupies the same slot as a 3RT2 contactor of the same frame. Front protection is IP20 — finger-safe but not washdown-rated. The relay is intended for dry indoor panels, not for hose-down or outdoor locations.