The Siemens 3RH2911-2HA12-0KT0 is a front-mounted auxiliary switch block in the SIRIUS family, designed to snap onto 3RT2 power contactors and 3RH2 contactor relays. It carries one normally-open and two normally-closed auxiliary contacts, wired out to terminals 21/22, 31/32, and 43/44 — the fourth position is unpopulated. The spring-loaded terminals accept 2x (0.5 to 2.5 mm²) without ferrule, or 2x (0.5 to 1.5 mm²) with core-end processing, and also take 2x (20 to 14 AWG) stranded.
For inductive loads in AC-15 duty, the rated operational current drops to 1 A at 690 V — that's the figure that governs switching of contactor coils and solenoid valves at higher voltages. Mechanical life is 10 million switching cycles typical, so it outlasts most contactor bodies it rides on.
Snap-on mounting to the front of the contactor or relay — no tools needed, no DIN rail footprint consumed beyond the base device. The block adds 36 mm width and 47.7 mm depth to the assembly, with a height of 41.5 mm. Surge voltage resistance is rated at 6 kV, and insulation voltage at 690 V AC with pollution degree 3 — standard for industrial control panels.