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 when a specified hazardous event threatens. 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 does
An SIS monitors process conditions, evaluates safety-related logic, and initiates an action intended to bring or keep the process in its defined safe state. The safe state and the conditions that trigger action depend on the particular hazard and the system’s safety requirements; there is no universal trip setting.
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An SIS is a complete instrumented safety system, not just a controller or shutdown panel. ISA describes the relevant standards scope as extending from sensor through final element. The SIS is one layer in a broader risk-reduction strategy; it is not a synonym for every safety measure in a facility. Process design, mechanical protection, and administrative measures may also contribute to risk reduction. An alarm or basic process control system is not automatically an SIS simply because it supports safe operation.
How the protection path works
A common process-industry architecture follows this path:
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- Sensors measure relevant process conditions and provide inputs.
- Logic solver evaluates those inputs against safety logic and trip settings.
- Final elements act on the process, for example by stopping flow or shutting down equipment.
A programmable SIS may use input modules, a logic solver, and output modules. This is a common configuration, not a requirement that every SIS use identical hardware.
SIS, SIF, and SIL are different terms
- SIS: the instrumented system used to implement safety functions.
- SIF: the specific safety function intended to achieve or maintain a safe state for a defined hazardous event.
- SIL: the safety integrity level associated with a SIF and its required risk-reduction performance.
One SIS can implement multiple SIFs. SIL is associated with a safety function, not a general badge for an entire plant or another name for the SIS. The required SIL for a SIF is considered in relation to the other protection layers addressing the same risk.
Where IEC 61511 fits
IEC 61511 is the functional-safety standard series for SIS in the process-industry sector. ISA lists Part 1 as covering the framework, definitions, system, hardware, and application-programming requirements; Part 2 as application guidance; and Part 3 as guidance for determining required SILs. ISA’s current listing identifies the US adoption of Part 1 as ANSI/ISA-61511-1-2018, based on IEC 61511-1:2016 with Amendment 1:2017. The ISA84 committee scope excludes nuclear power safety systems.
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Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Safety is managed across the lifecycle
Functional safety work extends well beyond selecting a controller. ISA describes lifecycle requirements and guidance that cover specification, design, installation, operation, and maintenance, continuing through decommissioning. Design considerations include separation from the basic process control system where applicable, selection of field devices and logic solvers, wiring, power, common-mode impacts, and fault tolerance.
When evaluating SIS implementations, start with the hazard and safety function rather than a product label. Relevant engineering comparison points include the required SIL, sensor-to-final-element architecture, independence from the basic process control system where applicable, fault tolerance, and lifecycle evidence. These considerations do not replace a project-specific safety requirements specification or the required safety lifecycle work.
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