What it is and where it fits
The Siemens 3RH1122-1BG40 is a SIRIUS auxiliary contactor in size S00 — a compact switching block that extends the control-circuit contacts on a larger contactor or starter assembly, or stands alone as a signal repeater. Rated 125 V with two normally-open instantaneous contacts, it carries 10 A at 24 V and derates to 1 A at 690 V. The 45 mm wide body snaps onto DIN rail or screws to a panel backplate, and the mounting position allows ±180° rotation on a vertical surface plus ±22.5° tilt forward/backward — useful when you're squeezing it into a crowded gland-plate layout.
Ratings that decide fit
The 2 NO instantaneous contacts switch without delay — no leading or lagging action, no make-before-break overlap. That means the contacts close and open at the same instant the coil energizes or drops, which matters when you're using them for a safety feedback loop or a fast interlock where timing skew between contacts could cause a race condition. The contact reliability spec — one incorrect switching per 100 million operations at 17 V, 1 mA — tells you this part handles dry circuits (PLC inputs, low-power signal loads) without needing gold-plating. Mechanical life is 30 million cycles typical, so in a machine that cycles once per minute you get about 57 years of mechanical life before the springs fatigue.
Wire termination and environment
Terminals accept solid conductors: 2x 0.5–1.5 mm², 2x 0.75–2.5 mm², or 2x 4 mm² — that covers the common control-wire sizes from 20 AWG through 12 AWG. Pollution degree 3 means it's rated for conductive dust and occasional condensation, typical in an industrial panel without climate control. Operating temperature is -25 to +60 °C; storage and transport handle -55 to +80 °C. IP20 on the front means finger-safe from the front but not sealed against dust ingress — mount inside a cabinet, not in the washdown zone.