SIL vs ASIL vs PL: Functional Safety Levels Compared

Compare SIL, ASIL, and PL functional safety levels, their standards, risk methods, failure measures, industries, and why direct conversion is unsafe.

SIL vs ASIL vs PL at a Glance

SIL, ASIL, and PL all express the rigor required to control functional safety risk, but they belong to different standards and industries. Safety Integrity Level or SIL is defined by IEC 61508 and sector standards derived from it. Automotive Safety Integrity Level or ASIL belongs to ISO 26262 for road vehicle electrical and electronic systems. Performance Level or PL belongs to ISO 13849 for safety-related parts of machinery control systems.

The practical consequence is simple: a team should select the applicable standard from the product, safety function, operating context, and regulatory or contractual baseline. It should not choose a familiar label first and then force the system into it. The highest labels within the three schemes are SIL 4, ASIL D, and PL e, but those labels are not equivalent. Each scheme uses a different risk model, lifecycle, architecture rules, and body of evidence.

What SIL Means Under IEC 61508

IEC 61508 provides a general framework for electrical, electronic, and programmable electronic safety-related systems. SIL is a property of a specified safety function, not a general quality badge for a controller or component. The hazard and risk assessment identifies the safety function and the risk reduction it must provide. The safety requirements then define its behavior, safe state, response time, interfaces, and target integrity.

The quantitative target depends on demand mode. A low-demand function is evaluated using average probability of dangerous failure on demand, or PFDavg. A high-demand or continuously operating function uses average frequency of a dangerous failure per hour, commonly written PFH. Higher SILs require lower dangerous failure probabilities and stronger controls over systematic faults, verification, independence, competence, configuration, and modification.

A refinery emergency shutdown that acts only when a rare process deviation occurs is a common low-demand example. A railway protection function or continuously active industrial control function may use a high-demand or continuous measure. Sector standards can modify scope and acceptable SIL claims, so IEC 61508 should not be applied without checking the relevant product or industry standard.

What ASIL Means Under ISO 26262

ISO 26262 adapts functional safety principles to electrical and electronic systems in series-production road vehicles. During hazard analysis and risk assessment, the team evaluates hazardous events using severity, exposure, and controllability. The resulting classification is QM or ASIL A, B, C, or D. ASIL D demands the greatest rigor within the ISO 26262 scheme.

ASIL affects the required methods, independence, verification depth, hardware metrics, software development practices, and confirmation measures. It also follows requirements through decomposition and allocation. A safety goal classified ASIL D may lead to technical safety requirements allocated across several elements, with decomposition permitted only under defined independence and coexistence conditions.

ASIL is not a single dangerous-failure frequency band. ISO 26262 uses quantitative hardware targets such as the probabilistic metric for random hardware failures alongside architectural metrics and controls for systematic faults. For a fuller treatment of the lifecycle, see our ISO 26262 guide for embedded product teams.

What PL Means Under ISO 13849

ISO 13849 applies to safety-related parts of control systems that perform machinery safety functions. The required performance level, PLr, is selected from PL a through PL e. When a type C machinery standard does not specify PLr, the risk estimation method considers injury severity, frequency or duration of exposure, and the possibility of avoiding the hazard.

The achieved PL depends on more than a calculated failure rate. The design must account for the control system category, mean time to dangerous failure, diagnostic coverage, resistance to common cause failures, software, and systematic measures. ISO 13849-1:2023 applies to high-demand and continuous modes and does not cover low-demand operation.

A guard interlock that removes torque when an operator opens a machine enclosure is a typical PL application. The complete safety function includes the input device, logic, communication paths, and final switching or actuation elements. Selecting a safety PLC with a PL e data sheet does not make the assembled function PL e.

Why SIL ASIL and PL Are Not Directly Equivalent

SIL and PL include failure-frequency ranges that overlap for high-demand functions. For example, the PL d PFHD range overlaps the IEC 61508 SIL 2 PFH range, and PL e overlaps SIL 3. That numerical overlap is useful when engineers review subsystem data, but it does not create automatic equivalence. ISO 13849 adds machinery-specific categories and design parameters, while IEC 61508 applies its own architectural constraints and lifecycle requirements.

ASIL is even less suitable for conversion because its classification comes from a vehicle-level hazardous event and combines severity, exposure, and controllability. An ASIL D safety goal cannot be translated to SIL 3 or PL e by comparing a single probability target. The vehicle context, driver or road-user controllability, item definition, assumptions, hardware metrics, software methods, and confirmation measures remain part of the claim.

Use cross-standard mapping only as an engineering input. If a supplier provides a SIL-capable component for an automotive ECU, the ISO 26262 project must still justify how that component fits the item, safety concept, assumptions of use, integration evidence, and ASIL requirements. The component certificate may reduce work, but it does not replace the receiving standard's safety argument.

How to Choose the Correct Functional Safety Framework

1. Define the product and safety function. State the system boundary, operating modes, interfaces, safe state, response time, and foreseeable misuse.
2. Identify the governing domain standard. Road vehicle E/E systems point toward ISO 26262. Machinery control safety functions commonly use ISO 13849 or IEC 62061. General or process-sector functions may use IEC 61508 or a sector derivative.
3. Check regulations contracts and type C standards. The applicable product standard may prescribe a method or level and can take precedence over a generic choice.
4. Perform the required hazard analysis. Use the risk parameters and terminology of the selected standard. Preserve the rationale, assumptions, and participants.
5. Allocate requirements before selecting components. Architecture, diagnostics, independence, fault reaction, and verification needs should drive component selection rather than the reverse.
6. Plan evidence across the lifecycle. Trace hazards to requirements, design, code, analyses, tests, production controls, field monitoring, and change management.
7. Validate the integrated safety function. Component certificates and failure-rate data are inputs. The final claim depends on integration and system-level validation.

Frequently Asked Questions

Which is higher: SIL 4, ASIL D, or PL e?

Each is the highest named level in its own scheme, but none is universally higher than the others. The standards define different scopes, risk methods, quantitative measures, and development obligations.

Is ASIL D equivalent to SIL 3?

No. Some engineering targets may look similar in a limited calculation, but ASIL D is an automotive risk classification with ISO 26262 lifecycle and architecture requirements. SIL 3 is an IEC 61508 safety integrity level for a specified safety function.

Can a machine use both SIL and PL?

A machinery project may encounter ISO 13849 and IEC 62061, supplier data expressed in SIL or PL terms, and IEC 61508-certified subsystems. The project must choose and document the applicable design route for each safety function and control cross-standard interfaces carefully.

Does certified hardware set the final safety level?

No. Certified hardware provides capabilities, failure data, and assumptions. The final safety claim also depends on architecture, diagnostics, software, integration, environmental conditions, verification, validation, and lifecycle controls.

What does QM mean in ISO 26262?

QM means the hazardous event does not require an ASIL under the ISO 26262 classification. It does not mean risk-free or exempt from quality, reliability, cybersecurity, regulatory, or contractual requirements.

Conclusion

SIL, ASIL, and PL answer related questions about functional safety, but they do so within different engineering frameworks. SIL expresses the required integrity of a safety function under IEC 61508 and sector standards. ASIL classifies automotive hazardous events and drives the ISO 26262 lifecycle. PL evaluates machinery control safety functions through ISO 13849 risk parameters, architecture, reliability, diagnostics, and systematic measures.

The defensible approach is to select the governing standard first, perform its hazard analysis, and maintain traceability from risk to validated safety function. Conclusive Engineering supports this work through functional safety engineering services and firmware development services covering safety requirements, embedded architecture, firmware, verification, and certification evidence.