Safety requirement specification failure contribution

Broadly, what is required is a pragmatic approach to the application of HF methods and techniques for human safety requirements specification at the Logical Level and the demonstration of satisfaction of human safety requirements at the Physical Level. Figure 8 shows different HF analyses that can be undertaken for the specification of human safety requirements (function, performance and integrity) and the realisation of those requirements and their contribution (both success and failure) to safety assurance typically provided by a system safety case. 

From the percentage failure contribution diagram in Fig. X/1.0-1 it is clear that the development activities for specification need special attention. First, we shall look at how a system specification is developed from system requirement definitions, then we shall look at the requirements for a safety system, that is, SRS. 

As we have seen specification errors contribute a large proportion of safety system failures. Recognizing and imderstanding the safety problem to be solved is the first essential step in avoiding this problem. This in turn requires that we imderstand the nature of hazards and the contributing factors. Hence the emergence of systematic hazard study methods. 

The potential for systematic failure in the process specification, programming, and checkout of the SIS is not considered in the safety system PFDavg calculation. That failure rate may have a significant contribution to the potential for loss of the ability to bring the process to a safe state. When there is a failure, whether you have redundancy or not, you need to provide a way to bring the process to a safe state. Clause 11.3 presents requirements for system behavior when a fault is detected. The manual shutdown does not have to be an emergency stop button, which activates the final elements of the SIS, Independent of the logic solver, but some alternate shutdown method should be provided. 

The contribution of each structure, system or component to the facihty hazard can be measured through the probabilistic concept of the performance goal. The performance goal for a structure, system or component in relation to a specific external event is defined as the probability of failure (Pp) of the structure, system or component to perform its required safety function in the case of that external event. The performance goal for an external event may be lower than the performance goal for internal accidents. 

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