It is important to realize and understand the fundamental difference between process control and safety control.
Process control systems are active, or dynamic. They have analog inputs and analog outputs, perform math and number crunching, and have feedback loops.
Process controls act positively to maintain or change process conditions. They are there to help obtain best performance from the process and often are used to push the performance to the limits that can safely be achieved.
Hence, most failures in these systems are inherently self-revealing. PCS must be flexible enough to allow frequent changes. Process parameters (e.g. set points, PID settings, MAN/AUTO, etc) require changing. Portions of the system may also be placed in bypass, and the process may be controlled manually.
They are not built with safety in mind and are not dedicated to the task. Because they are operating at all times they are not expected to have diagnostic routines searching for faults.
Safety systems, however, are just the opposite of process control systems. They are dormant, or passive. They sit there doing nothing and hopefully will never be called into action. An example would be a pressure relief valve.
Normally the valve is closed. It only opens when the pressure reaches the set value. If the pressure never exceeds that value, the valve never operates. Many failures in these systems may not be self-revealing. If the relief valve is plugged, there is no immediate indication.
A PLC could be hung up in an endless loop. Without a watchdog timer, the system would not be able to recognize the problem. There is a need for extensive diagnostics in dormant, passive safety-related systems.
Safety systems should be incorruptible – need to be kept to a fixed set of rules and access for changes carefully restricted. And they must be highly reliable and be able to respond instantly when a hazardous situation develops.