Safety Instrumented System

Also known as: SIS

A Safety Instrumented System (SIS) is a control system designed to prevent or mitigate hazardous events in a process plant by taking the process to a safe state when predetermined conditions are violated, governed primarily by IEC 61511 in the process industries.

Last updated: April 2026

Key Facts

Term: Safety Instrumented System
Abbreviation: SIS
Governed by: IEC 61511 (process industries); IEC 61508 (general)
US equivalent standard: ANSI/ISA 84
Components: Sensors, logic solver, final elements (valves, actuators)
Risk classification: Safety Integrity Level (SIL) per safety function within the SIS
Independence requirement: Must be independent from the Basic Process Control System (BPCS)
Industries: Chemical, petrochemical, oil and gas, power generation, water treatment
Typical course duration: 2 to 5 days

What is a Safety Instrumented System?

An SIS combines sensors, a logic solver, and final elements (such as valves and actuators) to perform one or more Safety Instrumented Functions (SIFs). Each SIF carries a SIL rating that defines its required reliability.

SISs are deployed in process plants to handle hazards that the basic process control system cannot manage, including overpressure, runaway temperature, level deviation, and emergency shutdown. The design, installation, operation, and modification of SIS hardware and software are governed by IEC 61511 and, in the United States, ANSI/ISA 84. Engineers responsible for SIS work need competency in hazard analysis, SIL determination, and verification methods.

SIS Components

Component	Function	Examples
Sensor	Detects the hazardous condition	Pressure transmitter, level sensor, temperature sensor
Logic solver	Processes sensor input and decides action	Safety PLC, dedicated safety controller
Final element	Takes the action that brings process to safe state	Shutdown valve, emergency depressurisation valve, contactor

How SIS Differs from BPCS

BPCS (Basic Process Control System): Handles normal automated control of the process under non-hazardous conditions.
SIS: Independent system that only acts when the BPCS fails to keep the process safe.
Required separation: Industry guidance requires the two to be physically and functionally separate to avoid common cause failures.

How Process Safety Training Providers Deliver SIS Courses

Process safety training providers deliver dedicated SIS courses covering hardware architecture, logic solver selection, sensor and final element verification, common cause failure analysis, and proof testing.

A typical course runs two to five days and combines theory with hands-on case studies. Delegates include process engineers, instrumentation engineers, control system designers, safety integrators, and assessors. Providers commonly bundle SIS training with IEC 61511 foundation and SIL determination workshops. Delivery formats include open-enrolment public cohorts, private corporate cohorts at process plant locations, and blended online programmes for distributed engineering teams. The training operation needs identity verification at exam, verifiable certificates, audit-ready records, and branded portals for client cohorts.

Common Questions

What is the difference between SIS and BPCS?

A Basic Process Control System (BPCS) handles the normal automated control of the process. A Safety Instrumented System (SIS) is independent and only acts when the BPCS fails to keep the process safe. Industry guidance requires the two to be physically and functionally separate to avoid common cause failures.

Who needs SIS training?

Process engineers, instrumentation engineers, control system designers, functional safety engineers, safety integrators, and assessors working in process plants. Operators and maintenance staff also benefit from awareness-level SIS training.

How does SIS relate to SIL?

A Safety Instrumented System contains one or more Safety Instrumented Functions, and each SIF has a SIL rating. The SIS as a whole inherits the SIL constraints of its constituent SIFs.

What is proof testing in an SIS context?

Proof testing is the periodic verification that each component of an SIS works correctly. Proof test intervals are set during design based on the target SIL and contribute to the overall probability of failure on demand calculation.