The Siemens Industrial Controls 3RH2921-1DA11 Auxiliary Switch Block is a compact electrical accessory that provides one normally open (N/O) and one normally closed (N/C) contact for use with contactors and control devices in HVAC systems. It operates by changing contact states in response to the primary device’s actuation, enabling additional control or feedback functions within the ... https://www.partshnc.com/aux-switch-lateral-1no1nc