The Siemens 3RH1122-1KG40-0LA4 is a SIRIUS auxiliary contactor in size S00 — a compact 45 mm wide block that fits a standard DIN rail or screws directly to a panel backplate. It carries a 125 V DC coil with an integrated varistor surge suppressor, so you don't need to add a separate suppression diode or RC snubber across the coil terminals. That saves a wiring step and a part on the BOM.
Contact ratings — what they mean for your load
The contact set is 2 NO + 2 instantaneous contacts (the 2 in the order code). Rated operational currents are given per voltage: 10 A at 24 V, 1 A at 110 V, 0.27 A at 220 V, 6 A at 230 V, 3 A at 400 V, 2 A at 500 V, and 1 A at 690 V. These are AC ratings for general-purpose switching — think control transformers, relay coils, signal lamps, PLC inputs. The 10 A at 24 V handles a bank of contactor coils; the 1 A at 690 V tells you it can switch a small motor contactor in a 690 V line. Contact reliability is specified at one incorrect switching operation per 100 million cycles at 17 V, 1 mA — that covers dry-contact PLC-level signals without stuttering.
Mounting and environment
Fastening is screw and snap-on mounting — clips onto a 35 mm DIN rail or screws flat. The mounting position is flexible: +/-180° rotation on a vertical surface, plus +/-22.5° tilt forward/backward. That matters when you're shoehorning it into a tight corner of a panel. IP20 on the front means it's protected against solid objects larger than 12.5 mm (fingers) but not against water — keep it inside an enclosure. Operating temperature range is -40 to +70 °C, storage and transport -55 to +80 °C, so it survives cold warehouses and hot enclosures alike. Pollution degree 3 means it's rated for industrial environments with conductive dust or occasional condensation.
Coil power and wiring
Closing power of the magnet coil at DC is 11 W; holding power drops to 4 W. That's typical for a DC-operated contactor — the inrush is higher to pull in, then the holding circuit reduces power. The varistor across the coil absorbs the back-EMF spike when the coil de-energizes, which is the main reason DC coils need external suppression. Here it's built in. Terminal capacity accepts 2x (0.5...1.5 mm²), 2x (0.75...2.5 mm²), or 2x 4 mm² solid/stranded — covers common control wiring sizes without needing ferrules for the smaller strands.