Target safety assessment risk tolerance

A quantitative target for measuring the level of performance needed for safety function to achieve a tolerable risk for a process hazard. It is a measure of safety system performance, in terms of the probability of failure on demand. There are four discreet integrity levels, SIL 1-4. The higher the SIL level, the higher the associated safety level and the lower the probability that a system will fail to perform properly. Defining a target SIL level for a process should be based on the assessment of the likelihood that an incident will occur and the consequences of the incident. Table S.2 describes SIL for different modes of operation. 

Nevertheless, these laws and requirements neither specify how one should go about establishing a target tolerable risk for an activity, nor address the methods of assessment of proposed designs, nor provide requirements for specific safety-related features within design. 

Having established a SIL target it is insufficient merely to assess that the design will meet the Maximum Tolerable Risk target. It is necessary to establish whether further improvements are justified and thus the principle of ALARP (as low as reasonably practicable) is called for as good practice. In the UK this is also arguably necessary in order to meet safety legislation ( all that is reasonably practicable is called for in the Health Safety at Work Act 1974). 

Apportionment of Safety targets and quantification of requirements. To demonstrate that the risks resulting from the changes to the railway are tolerable, ALARP and no greater than the level of risk in the current Railway System, quantitative risk assessments are carried out, and a set of requirements related to the new hardware system constituents have been quantified. 

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