Match the Standard to the Application
Select the governing safety-system standard by industry, application, and engineering role. IEC 61508 addresses the complete safety instrumented system (SIS) across the process, transit, and medical industries. IEC 61511 applies IEC 61508 concepts specifically to SIS designers, integrators, and users in the process-control industry.
| Standard | Scope stated in the evidence | Primary selection condition |
|---|---|---|
| IEC 61508, Parts 1–7 | Functional safety of safety-related systems in the process, transit, and medical industries | Use as the general SIS framework |
| ANSI/ISA S84.01-1996 | Safety systems in the United States process industry | Use for the stated US process-industry scope |
| IEC 61511, Parts 1–3 | Safety instrumented systems in the process-industry sector | Use with IEC 61508 for process-control design, integration, and operation |
| NFPA 85 | Operations using single-burner and multiple-burner boilers in the United States | Apply to the stated boiler and combustion-system scope |
| CAN/CSA-C22.2 No. 61010-1-04 | Canadian safety requirements for electrical measurement, control, and laboratory equipment; identified here for burner-management applications | Evaluate for the stated Canadian burner-management scope |
Assign SIL from Required Risk Reduction
IEC 61508 defines four safety integrity levels (SILs), each indexed to a probability of failure on demand (PFD). Assign the SIL from the required risk reduction determined by a process hazard analysis (PHA); do not select it from device capability alone. The evidence does not supply PFD ranges or a calculation method, so verify those values in the applicable standard before completing the assignment.
ANSI/ISA S84.01-1996 uses the IEC 61508 SIL classes and maintains the DIN V 19250 relationships, but it excludes SIL 4. The stated rationale is that process facilities can combine multiple protection layers, whereas the medical and transit cases described for SIL 4 may rely on the safety-instrumented layer as the only protection layer.
Apply the Safety Life Cycle
IEC 61508 treats SIS integrity as a life-cycle result rather than device integrity alone. Control integrity through design, operation, testing, and maintenance. A compliant engineering decision therefore requires evidence that the selected architecture can be operated, tested, and maintained at the integrity assumed by the risk-reduction assessment.
- Use the PHA to establish the required risk reduction.
- Assign the corresponding SIL using the applicable standard and its PFD criteria.
- Select the general, sector-specific, and application-specific standards that match the installation.
- Verify that design, operation, testing, and maintenance preserve the required SIS integrity.
Resolve Overlapping Requirements
For a process-industry SIS, treat IEC 61511 as a companion to IEC 61508 rather than an unrelated alternative. Add the application-specific document when the installation falls within its stated scope: NFPA 85 for US single-burner or multiple-burner boiler operations, or CAN/CSA-C22.2 No. 61010-1-04 for the Canadian burner-management context identified in the evidence. The evidence does not define precedence when requirements conflict, so document the applicable jurisdiction and confirm the governing requirements before approving the design.
FAQ
Which standard applies to a process-industry safety instrumented system?
Use IEC 61511, Parts 1–3 for process-industry SIS design, integration, and use, together with its companion standard IEC 61508, Parts 1–7.
Does ANSI/ISA S84.01-1996 include SIL 4?
No. It uses the IEC 61508 SIL classes but excludes SIL 4 for its stated US process-industry scope.
How is a safety integrity level selected?
Determine the required risk reduction through a PHA, then assign the SIL using the applicable standard's PFD criteria. Include design, operation, testing, and maintenance in the integrity assessment.