Safety Requirements
Safety requirements and their role in determining the acceptability of a safety-critical system.
Many safety standards use the concept of a safety requirement to ensure that the system carries out the functions needed to make it acceptably safe. For the safety requirements to achieve this (in terms of the risk reduction being both sufficient and necessary), an adequate risk assessment must have been carried out.
A safety requirement in AS 61508 "Functional safety of electrical/electronic/programmable electronic safety-related systems" has two components: a safety functions requirement (which details how the system should avoid the occurrence of a hazard), and a safety integrity requirement (which details how often the function can fail and allow the hazard to occur). Between the two components, the safety requirements for a system state are used to derive how often all the hazards of the system can occur, and from this if the system is acceptably safe.
Other safety standards are less restrictive about what a safety requirement is, making general statements about safety requirements being those requirements needed to reduce the risk to an acceptable level.
Safety requirements may relate to the frequency of various failure modes of components of a system, procedural mitigations, or processes involved in the development of a system.
Why?
Without adequate identification and derivation of safety requirements, the system may not be preventing the right or any hazards, and you may build a high integrity system which is reliably unsafe.
Similar?
Safety requirements also often lead to the determination of Safety Integrity Levels (SIL) as one way of expressing the relative importance of meeting each safety requirement.