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Operating With Confidence
As noted so succinctly in Wikipedia, the online encyclopedia, fail-safe or fail-secure describes a device or feature which, in the event of a failure, fails in a way that will cause no harm (or at least a minimum of harm) to other devices or create a dangerous hazard to personnel.1 What does failsafe mean for you?
Consider this: You may be designing, commissioning, maintaining or operating a process. And you may wonder if you could be ambushed by an unpredictable fault or failure that threatens the process, or a person. No system is infallible, so you would do well to look for potential hazards. Your bunch of control components and their interactions call for a hard look at each component and its function in the process. Study and deal with any that could fail to danger.
The Controller
The temperature controller has developed into a rich mixture of circuit boards, chips, power supplies, relays and software. Controllers come in packages, from the $100 discrete DIN format to a small piece of circuitry in a SCADA system.
At one time, a controller was an analog circuit that a patient circuit expert could analyze. Then he could predict whether or not the failure of any one or group of components would make the controller fail to danger. Not any more - it is far too complex, even for the person who created it. The controller's main job is to watch and display the process temperature and command some other device (a final control element) to deliver enough heat or cool to get it right. But that is not its only job: The controller can display and act upon deviations from the desired temperature such as a too-fast temperature change. Other tasks it is charged with...