Risk of Failure

Failure can come about due to imperfections of design, physical flaws from the manufacturing process, weakness from environmental stress, or damage from human error. Distributed processors for control replaced centralized analog control installations, and the dependability of these latter systems is rated differently than the dependability of digital computer installations. The dependability of the distributed processors in control is somewhere between individual loop failure and failure of the entire process represented by mainframes and centralized analog control installation. System integrity does what it should, when it should. That means all the components function the way that readers expect them to function. Reliability that contributes to improved mean time between failure involves fault prevention, fault limiting, and fault tolerance. Software backup is a variation of hardware backup in that the control functions, which are located in the software, are replaced by software in other devices.