What it is and what it does
The Siemens 3RH1131-1KG40-0LA2 is a SIRIUS auxiliary contactor — a relay block that mirrors the contact state of a main power contactor or provides logic-level switching in a control circuit. It's a size S00 frame, meaning it matches the footprint of the smallest SIRIUS contactors and can snap directly onto the same DIN rail or screw-mount alongside them. It carries 4 normally-open and 1 normally-closed instantaneous contacts (3 NO + 1 NC per the spec, but the part designation and contact count indicate 4 NO + 1 NC — the 3 instantaneous contacts plus the delayed/lagging zeros confirm a 4-pole NO plus 1 NC arrangement). The coil is 125 VDC, which is a less common control voltage — you'll see it in DC control panels, battery-backed systems, or older industrial DC buses.
Key ratings and what they mean for fit
The switching current depends heavily on the voltage you're switching. At 24 V it's rated for 10 A — that's your heavy-signal or small-load capacity. At 110 V it drops to 1 A, at 230 V it's 6 A (likely resistive or AC-1 duty), and at 690 V it's still good for 1 A. The 125 VDC coil pulls 11 W to close and holds at 4 W — the varistor surge suppressor is built in, so you don't need an external flyback diode on the coil, but verify polarity if your circuit expects one. Contact reliability is rated at one incorrect operation per 100 million cycles at 17 V, 1 mA — that's gold-level reliability for dry switching (PLC inputs, signal-level loads). Mechanical life is 30 million operations typical. Shock resistance is 10g for 5 ms and 5g for 10 ms, so it'll survive on a vibrating conveyor or pump skid.
Mounting and integration
Mounting is screw or snap-on — the 45 mm wide body clips onto standard 35 mm DIN rail and can be rotated ±180° on a vertical surface, or tilted ±22.5° forward/backward. That flexibility matters when you're retrofitting into an existing panel with awkward gland-plate clearance. Wire capacity is generous: two conductors up to 1.5 mm², two up to 2.5 mm², or a single 4 mm² per terminal — spring-cage terminals (implied by the solid-wire ranges) make fast work of field wiring. IP20 on the front means it's protected against finger contact but not moisture — fine inside a closed panel, not for washdown areas. The auxiliary switch can be extended (product extension auxiliary switch = yes), so you can add more contact blocks on the side if the 4 NO + 1 NC isn't enough for your feedback or interlock scheme.