Noncontact safety interlock switches are machine safety products that provide a compact, non-contact way to interlock doors, guards, gates and covers. They are made of plastic or chemically-resistant steel and are designed to restrict or prevent access to a variety of hazards. Non-contact safety interlock switches use several types of contacts. Positive-break contacts are normally closed (NC) electrical contacts which, upon actuation, are forced open by a non-resilient, mechanical drive mechanism. In the event of a mechanical failure such as the breakage of a spring or weld, the contact point remains in an activated position. True spring-actuated safety interlock switches are not considered to be positive-break devices. Noncontact safety interlock switches with normally open (NO) contact pairs and pairs of changeover contacts are also available. Typically, changeover contacts are used in single-pole double-throw (SPDT) devices. 

Noncontact safety interlock switches consist of an actuator, one or more read heads, and a control unit that is connected to multiple power contactors. Each read head transmits an electrical signal or electromagnetic field to the actuator, which processes the data for transmission to the control unit. If the control unit recognizes the transmitted information, the safety outputs are closed. Each actuator includes a uniquely coded element that cannot be reprogrammed. Most control units provide external device monitoring (EDM) and can be operated in manual or automatic-start mode. Typically, a control unit manages two or four solid-state outputs and provides individual monitoring of each read head. For applications that require the guarding or more than four doors, control units are connected in series according to safety standards such as EN954-1. 

Noncontact safety interlock switches vary in terms of electrical specifications, sensing range, break range, and special features. Electrical specifications include maximum current, maximum AC voltage, and maximum DC voltage. Sensing range is the distance between the actuator and the sensing head to activate the relay outputs. Conversely, break range is the distance between the actuator and the sensing head to deactivate the relay outputs. Features include emergency overrides and visual indicators. Devices with integral control units can be used to monitor multiple switches. Products with manual time delays continue to protect equipment even after machine controls are turned off. Some noncontact safety interlock switches are hermetically-sealed or explosion-proof. Others are tamper-resistant or include a rotating head for multiple entry points.