The Siemens 3RH1921-2HA22-ZW97 is an auxiliary switch block in the SIRIUS family, designed to snap onto motor contactors for adding extra signaling or control contacts. It provides 2 normally-open and 2 normally-closed contacts in a 4-pole configuration, using spring-type (cage clamp) terminals for tool-free wiring. The 22 mm width matches EN 50012 mounting profiles, so it fits standard contactor add-on slots without adapters.
The 2 NO + 2 NC contact arrangement gives you four independent auxiliary circuits — enough to signal the PLC that the contactor has picked up, interlock a second contactor, and still have a spare NC for a safety feedback loop. Spring-type connection (cage clamp) accepts solid or ferruled stranded wire without screw tightening, which speeds panel wiring and resists loosening under vibration. The EN 50012 profile means this block mounts directly on Siemens 3RT1 and 3RT2 contactors, as well as many third-party contactors that follow the same 22 mm add-on interface.
Deployment Context
Snap-on auxiliary switch blocks like this one are installed in motor control centers and individual starter enclosures. The spring terminals are rated for the contactor's auxiliary circuit voltage and current — typically 24 VDC to 230 VAC control signals. No separate wiring of the main power path is needed; the block simply clips onto the contactor and its contacts are driven by the contactor's armature movement.