What is the role of a safety integrity level (SIL) in functional safety?

The role of a Safety Integrity Level (SIL) in functional safety is to specify the required level of risk reduction provided by a safety function. Functional safety is part of the overall safety that depends on a system or equipment operating correctly in response to its inputs. A SIL is a relative measure of the safety function's performance in terms of its probability of failure on demand (PFD) or its frequency of dangerous failures per hour (PFH). SILs range from 1 to 4, with SIL 4 being the highest level of safety integrity and SIL 1 being the lowest. A safety function is a function that is implemented to reduce the risk of a hazardous event. For example, an emergency shutdown system is a safety function designed to prevent explosions or other hazardous events in a chemical plant. The SIL assigned to a safety function is determined based on a risk assessment, which considers the severity of the potential hazard, the frequency of exposure to the hazard, and the probability of the hazard occurring. The higher the risk, the higher the SIL required. For each SIL, there are specific requirements for the design, implementation, and validation of the safety function. These requirements cover various aspects, including hardware reliability, software development processes, and system testing. SIL ensures adequate safety measures.