High-integrity systems definition

Hence high-integrity systems are defined as those with reliability requirements more onerous than 10 on demand (for low demand systems) or 10 per year (for high demand systems). These are normally referred to as SIL 3 and SIL 4 systans. The exact definitions of these terms may vary from industry to industry, and from country to country. For example, the aviation industry tends to refer to failure rates per hour, whereas the process plant industries tend to refer to failure rates per year. Since there are (24 x 365) = 8760 h in a year - or roughly 10,000 - this means that 10 failures per year (fpy) equates to approximately 10 failures per hour, etc. Also, the European nuclear industry has imposed a more restrictive reliability limit on the SIL band definitions, as shown in the right hand column of Table 2.1. 

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