Glossary · Functional safety
Safety instrumented system (SIS)
Also known as: SIS, emergency shutdown system
German: Sicherheitsgerichtetes Instrumentierungssystem (SIS)
In process safety under IEC 61511, a safety instrumented system (SIS) is an instrumented system used to implement one or more safety instrumented functions. It is composed of any combination of sensors, logic solvers and final elements.
- Functional safety
- Integrated systems
In one sentence
A safety instrumented system (SIS) implements one or more safety instrumented functions with sensors, logic solvers and final elements.
Example
The SIS of a gas processing plant uses a certified safety controller, pressure and gas detectors, and shutdown valves to carry out 40 SIFs, including emergency depressurization.
How it applies
- Engineering: The SIS is designed from the Safety requirements specification (SRS). Its architecture, component choice and diagnostics must achieve the SIL of every Safety instrumented function (SIF) it carries.
- Independence: IEC 61511 expects the SIS to be separate from the basic process control system unless shared elements are justified in the analysis.
- Operation and maintenance: Proof tests, bypass management, access control and management of change keep the SIS in its designed condition over decades of operation.
- Documentation: SIS documentation includes the SRS, design and verification records, the safety manual conditions of all devices, test procedures and operating instructions. The documentation team should keep one master list of SIFs and make sure every document refers to the same identifiers and revisions.
SIS vs. safety-related control system
SIS is the IEC 61511 term for the process industry. In machinery, IEC 62061 speaks of a Safety-related control system (SCS) and ISO 13849-1 of safety-related parts of control systems. The concepts are similar, but the requirements and calculation methods differ.