3. Application to control system design

Today's control systems are made up of hardware and software. It might be tempting to study their safety by considering the two entities software and hardware in series from a reliability diagram point of view, as is often practiced for computer systems. However, we can't approach the safety of a control system without taking into account the environment that interacts with it, i.e. the controlled system. The failure modes of the latter must be known to the control system, as well as its operational modes, so that it can contribute to overall operating safety. Similarly, the control system's failures must not induce dangerous behavior in the controlled process. Of course, we must ensure the safety of the control hardware and software, but this is not enough.

The definition of the level of safety required by a control system is the result of a preliminary risk analysis....