An SIL can be considered a statistical representation of the availability of an SIS at the time of a process demand. A process demand is defined as the occurrence of a process deviation that causes an SIS to transition a process to a safe state.
An SIL is the litmus test of acceptable SIS design and includes the following factors:
• Device integrity
• Systematic and common cause failures
In modern applications, a programmable electronic system (PES) is used as the core of an SIS.