TABLE OF CONTENTS As well as there being a focus of interest on major accident hazards there is a growing awareness that many failures relate to the control and safety systems used for plant operation and protection. Examples of this type of equipment are Fire Detection Systems, Emergency Shutdown Systems, Distributed Control Systems, Rail Signalling, Automotive Controls, Medical Electronics, Nuclear Control Systems and Aircraft Flight Controls.
Terms such as safety-related and safety-critical have become part of the engineering vocabulary. The distinction between them has become blurred and they have tended to be used synonymously.
Safety-critical has tended to be used where the hazard leads to fatality whereas Safety-related has been used in a broader context. There are many definitions all of which differ slightly as, for example:
some distinguish between multiple and single deaths
some include injury, illness and incapacity without death
some include effects on the environment
some include system damage
However, the current consensus distinguishes them as follows:
Safety-related systems are those which, singly or together with other safety-related systems, achieve or maintain a safe state for equipment under their control.
Safety-critical systems are those which, on their own, achieve or maintain a safe state for equipment under their control.
The difference involves the number of levels of protection. The term Safety-Related Application implies a control or safety function where failure or failures could lead to death, injury or environmental damage.
The term Safety-Related applies to any hardwired or programmable system where a failure, singly...
