Control-Reliable – refers to Category 3 and 4 devices.  A stand-alone control-reliable device, which fully meets the standard must:

• Be redundant in function
• Be self-monitoring and locking (lock and prohibit further operation if a fault occurs), without any dependency on external machine controls or other safety circuitry.

          NOTE: The device must also monitor for diminished performance for applications involving safe stopping time.

• “Fail” to the safest position (fail-to-safe)
• Have a dedicated, specific reset function, and prohibit reset by removal or reapplication of air or electric power
• Not be able to be reset if the solenoids are energized in the run mode
• Be designed specifically for safety.  This means that a control-reliable valve is basically the fluid power equivalent to a safety relay in electrical applications
• NOTE: Other valves can meet control-reliability standards with external monitoring