iTechGuides is reader-supported. When you buy through links on our site, we may earn an affiliate commission. As an Amazon Associate I earn from qualifying purchases. Learn more
A safety instrumented system (SIS) is an instrumented system that implements one or more safety instrumented functions (SIFs) to achieve or maintain a safe state for a specified hazardous event. In process industries, its protection path typically runs from sensors through a logic solver to final elements that act on the process.
What a safety instrumented system includes
An SIS is the complete instrumented safety system, not just a controller, shutdown panel, or software component. Its boundary generally extends from the sensor that detects a relevant process condition through the logic solver and out to the final element that acts on the process. ISA describes this sensor-through-final-element scope in its ISA84 committee information.
- Sensors provide measurements or status inputs about process conditions.
- Logic solver evaluates those inputs against the safety requirements and determines whether action is needed.
- Final elements carry out the action, such as stopping flow or shutting down equipment.
A programmable SIS may use input modules, a logic solver, and output modules, but no single hardware arrangement is mandatory in every installation. The trip conditions and intended safe state depend on the specific hazard and safety requirements; there is no universal SIS trip setting.
Recommended Free Tools
How SIS, SIF, and SIL differ
| Term | Meaning | What it applies to |
|---|---|---|
| SIS | The instrumented system used to implement safety functions. | The system and its components. |
| SIF | A specific safety function intended to achieve or maintain a safe state for a defined hazardous event. | The function addressing a particular hazard. |
| SIL | Safety integrity level, describing the required or assigned integrity of a safety function in relation to risk reduction. | The safety function, not a general badge for a product or plant. |
One SIS can implement multiple SIFs. The required SIL for a SIF is considered alongside other protection layers addressing the same risk. Calling a controller or an entire facility “SIL-rated” without identifying the relevant safety function obscures what is being assessed. ISA’s explanation of SIF and SIL appears in its ISA84 material.
#1 Best Overall
Where an SIS fits in process safety
An SIS is one layer in a broader risk-reduction strategy. Other layers may include safer process design, mechanical protection, and administrative measures. An alarm or basic process control system is not automatically an SIS merely because it helps an operator or process run safely; the SIS is the instrumented system assigned to carry out specified safety functions.
When evaluating two SIS implementations, the meaningful comparison is tied to the hazard and function, not just the controller brand or panel. Consider the required SIL, sensor/logic-solver/final-element architecture, separation from the basic process control system where applicable, fault tolerance, and evidence that lifecycle activities have been addressed. ISA identifies issues such as field-device and logic-solver selection, wiring, power, common-mode impacts, and fault tolerance in its lifecycle guidance. These considerations support an engineering evaluation; they do not replace project-specific safety requirements.
Rank #2
- Used Book in Good Condition
Which standards cover process-industry SIS?
IEC 61511 is the principal functional-safety standard series ISA identifies for SIS in the process-industry sector. ISA describes Part 1 as covering the framework, definitions, system, hardware, and application-programming requirements; Part 2 provides application guidance; and Part 3 provides guidance for determining required SILs. ISA’s standards listing identifies the U.S. adoption of Part 1 as ANSI/ISA-61511-1-2018, based on IEC 61511-1:2016 with Amendment 1:2017. See the ISA84 committee scope and ISA standards catalogue.
Do these 3 things before closing this tab:
1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsThe standard series concerns a safety lifecycle, not only hardware selection. Lifecycle work includes specification, design, installation, operation, maintenance, and decommissioning. The ISA84 scope excludes nuclear power safety systems, so IEC 61511 should not be treated as a universal standard for every sector. Edition and jurisdiction matter: check the currently applicable standard before making a compliance or design decision.
Quick Recap
Rank #4
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
