What an SIS does
A safety instrumented system can contain sensors, a logic solver and final elements that perform defined safety instrumented functions when specified hazardous conditions occur.
IEC 61511
The IEC 61511 series addresses functional safety and safety instrumented systems for the process industry sector. Its scope includes lifecycle requirements for specification, design, installation, operation and maintenance of safety instrumented functions and systems.
Protection layers and independence
Process safety uses multiple protection layers. A safety function's required independence, architecture and integrity depend on formal hazard and risk assessment — not on simply adding another transmitter to the basic control system.
Proof testing and lifecycle activities
Safety functions require defined lifecycle activities such as validation, proof testing, management of change and functional-safety assessment. Exact intervals, methods and acceptance criteria belong to the engineered safety lifecycle.