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