The Siemens 3RK3221-2AA10 is a SIRIUS modular safety system slice — 22.5 mm wide, 124 mm deep, 105 mm tall — that evaluates up to 4 safety-related inputs (2 single-channel or 2 dual-channel) and drives a 2 A load at 24 V DC or 230 V AC. It's rated Category 4 per EN ISO 13849-1 and SIL 3 per IEC 61508 / IEC 62061, so it handles the highest hazard-level guarding: emergency stops, protective door monitoring, enabling switches, two-hand operator panels, and selector switches. The spring-loaded terminals accept 0.2–2.5 mm² wire; the control supply is 24 V DC.
Mount vertically per spec. The 22.5 mm width leaves room for adjacent slices in a multi-channel safety chain. Spring terminals are finger-safe (touch protection per spec).