© Handermann 2013 FunctionalSafety.net The term safety-related is used to describe systems that are required to perform a specific function to ensure risks are kept at an accepted level. Such functions are, by definition, safety functions (SF). Two types of requirements are necessary to achieve functional safety: -Safety function requirements (what the function does) and -Safety integrity requirements (likelihood of a safety function being performed satisfactorily). The layers of protection differing in prevention layers like plant design, basic process control system (BPCS), operator intervention or safety instrumented system (SIS) or mitigation layers like rupture disk, dike, plant or emergency response. SIL, safety integrity level is a qualitative measure of safety and a quantitative realibility metric, not only a probability calculation ! There are 4 SIL levels with different safety avalability (SA) and risk reduction (RR) SIL 1: SA = 0.9 - 0.99; RR = 10 - 100; SIL 2: SA = 0.99 - 0.999; RR = 100 - 1000; SIL 3: SA = 0.999 - 0.9999; RR = 1000 - 10000; SIL 4: SA = 0.9999 - 0.99999; RR = 10000 - 100000;