Technical Standards
Technical standards are formal documents that set uniform criteria for the design, manufacture, and performance of products, systems, or services. They enable h...
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Safety Standard – Required Level of Safety Performance – Standards
Introduction
A safety standard is a formally documented set of technical and procedural requirements designed to safeguard people, property, and the environment from unacceptable risk. These standards prescribe the minimum criteria for engineering, construction, operation, and maintenance—ensuring that hazards are systematically identified, evaluated, and controlled. Central to safety standards is the required level of safety performance, which defines how reliable and effective safety-related control systems must be to prevent injury, even under foreseeable operating conditions.
This approach is fundamental in industries such as machinery, process, and aviation, where risk reduction is both a legal and moral imperative. International standards like ISO 13849-1 and IEC 62061 provide comprehensive frameworks for analyzing risk, specifying performance targets, and verifying that safety solutions meet actual risk reduction needs. ISO 13849-1 introduces the Performance Level (PL), a graded reliability scale for safety-related components. IEC 62061 uses the Safety Integrity Level (SIL) for electrical and programmable systems. Both emphasize systematic risk assessment, robust documentation, and lifecycle validation.
In high-reliability sectors (e.g., aviation), the required safety performance is managed through frameworks like the ICAO’s Acceptable Level of Safety Performance (ALoSP), monitored via safety performance indicators (SPIs). The integration of these standards into safety management systems ensures not only compliance but continuous improvement.
Key Definitions and Terminology
Safety Standard
A safety standard is an authoritative document created by recognized bodies such as ISO, IEC, ANSI, or ICAO. It prescribes minimum requirements for the design, operation, and maintenance of systems, equipment, or workplaces to ensure controlled risk. Safety standards go beyond technical specs—they incorporate best practices for hazard identification, risk assessment, mitigation, and performance verification.
For example:
- ISO 13849-1: Applied to machinery/automation, specifying how to evaluate and design control systems for reliable safety function performance.
- IEC 62061: Focuses on functional safety of electrical/electronic/programmable systems.
- ICAO Annex 19: Governs safety management in aviation.
Compliance may be legally required or necessary for market access, and standards often integrate with those for risk (ISO 31000), quality (ISO 9001), and environment (ISO 14001).
Required Level of Safety Performance
The required level of safety performance is the explicit minimum effectiveness or reliability a safety function or control system must achieve to reduce risk to a tolerable level. This requirement is derived from risk assessment—considering harm severity, exposure frequency, and possibility of avoidance.
- In machinery: Expressed as Performance Level required (PLr) in ISO 13849-1 or Safety Integrity Level required (SILr) in IEC 62061.
- In aviation: Known as Acceptable Level of Safety Performance (ALoSP), set via performance indicators and targets.
The requirement is dynamic—subject to review as technology or hazards change.
Performance Level (PL)
Performance Level (PL) is a discrete, ordered scale (a–e) classifying a system’s reliability to perform a safety function, even with single faults (ISO 13849-1). PL reflects hardware reliability, system architecture (redundancy, diagnostics), and resistance to common cause failures. PL is determined based on MTTFd (Mean Time To Dangerous Failure), diagnostic coverage (DC), and measures against common cause failure (CCF).
Safety Function
A safety function is a specific action or sequence, performed by safety-related control parts, designed to prevent or mitigate hazardous events. Examples: emergency stops, guard interlocks, two-hand controls, and light curtains. Each function must be specified—including required PL/SIL, response time, and interface details.
Other Related Terms
- Subsystem: A distinct unit within a safety control system, evaluated for compliance with PL/SIL (ISO 13849-1:2023 terminology).
- PFHd (Probability of Dangerous Failure per Hour): Quantifies the likelihood a safety function fails dangerously per hour.
- Risk Assessment: The process of hazard identification, risk estimation, and determination of risk reduction needs.
- SIL (Safety Integrity Level): A 1–4 scale (IEC 62061/61508) for electrical/electronic/programmable systems.
Purpose and Application of the Required Level of Safety Performance
Assigning and achieving the required safety performance level is foundational in risk-based safety management. Its purposes include:
- Quantifying minimum reliability/fault-tolerance needed for each safety function.
- Guiding component/system selection to ensure correct risk reduction.
- Supporting regulatory compliance (e.g., EU Machinery Directive, OSHA, ICAO Annex 19).
- Enabling ongoing verification and validation through calculation, testing, and monitoring.
In practice:
- Manufacturing: PLr/SIL guides safety design of presses, robots, and conveyors.
- Aviation: ALoSP governs safety targets and performance monitoring.
- Process Industry: SIL determines emergency shutdown system architectures.
Risk Assessment Process for Determining Required Performance Level (PLr)
A robust, repeatable risk assessment process is essential for determining PLr/SILr. This ensures hazards are addressed proportionally to their risk.
Key parameters:
- Severity of Injury (S): S1 (slight, reversible) or S2 (serious, irreversible/fatal).
- Frequency/Exposure (F): F1 (seldom/short) or F2 (frequent/long).
- Possibility of Avoidance (P): P1 (possible in specific conditions), P2 (hardly possible).
Procedure:
- Identify all safety functions.
- For each, assign S, F, P based on context.
- Use the risk graph (ISO 13849-1 Annex A) to map S, F, P to PLr.
- Document PLr in the Safety Requirements Specification (SRS).
Risk assessment must be updated with system or operational changes.
Stepwise Determination of Required Performance Level (PLr)
Stepwise Process (ISO 13849-1):
- List safety functions (e.g., emergency stop, interlock, light curtain).
- Assess risk parameters (S, F, P) for each.
- Apply risk graph to get PLr.
- Document all results and rationale.
Risk Graph Example Table:
| Severity (S) | Frequency (F) | Possibility (P) | Required PLr |
|---|---|---|---|
| S1 | F1 | P1 | PL a |
| S1 | F1 | P2 | PL b |
| S1 | F2 | P1 | PL b |
| S1 | F2 | P2 | PL c |
| S2 | F1 | P1 | PL b |
| S2 | F1 | P2 | PL c |
| S2 | F2 | P1 | PL c |
| S2 | F2 | P2 | PL d/e* |
(*) PL d is typical; PL e may be needed for extreme risk.
Performance Level (PL) Scale and Probabilities
PL quantifies reliability via PFHd:
| PL | Probability of Dangerous Failure per Hour (PFHd) |
|---|---|
| a | ≥1 × 10⁻⁵ and <1 × 10⁻⁴ |
| b | ≥3 × 10⁻⁶ and <1 × 10⁻⁵ |
| c | ≥1 × 10⁻⁶ and <3 × 10⁻⁶ |
| d | ≥1 × 10⁻⁷ and <1 × 10⁻⁶ |
| e | ≥1 × 10⁻⁸ and <1 × 10⁻⁷ |
- PL a: Basic measures for low risk.
- PL e: Advanced, redundant systems for highest risk.
Application Examples:
| PL | PFHd (1/h) | Example Application |
|---|---|---|
| a | ≥1 × 10⁻⁵, <1 × 10⁻⁴ | Minor hazard machinery |
| b | ≥3 × 10⁻⁶, <1 × 10⁻⁵ | Light-duty machines |
| c | ≥1 × 10⁻⁶, <3 × 10⁻⁶ | Standard machines |
| d | ≥1 × 10⁻⁷, <1 × 10⁻⁶ | High-speed robots |
| e | ≥1 × 10⁻⁸, <1 × 10⁻⁷ | Presses, hazardous AGVs |
Standards Governing Required Safety Performance Levels
ISO 13849-1
ISO 13849-1 is the principal standard for machinery safety-related control systems, covering all technologies (electrical, hydraulic, pneumatic, mechanical):
- Requires risk assessment and assignment of PLr.
- Specifies system architectures (Categories B, 1, 2, 3, 4).
- Demands calculation of MTTFd, DC, and CCF.
- Mandates validation and documentation.
The 2023 update emphasizes “subsystem” terminology, expanded software requirements, clarified risk graph parameters, and integrated validation provisions.
IEC 62061
IEC 62061 focuses on electrical/electronic/programmable safety systems, using the SIL concept (1–3 for machinery). It addresses programmable logic, diagnostics, and electromagnetic compatibility. Both standards are harmonized under the EU Machinery Directive.
Other Relevant Standards
- EN ISO 12100: Risk assessment and reduction principles.
- ANSI/ASSP Z10: Occupational health and safety management.
- Sector-specific standards: (e.g., ANSI/ASSP Z244.1 for lockout/tagout, B11 for machine safeguarding).
- Aviation: ICAO Annex 19, Doc 9859, GASP.
Recent and Upcoming Changes (ISO 13849-1:2023)
Key updates include:
- Adoption of “subsystem” for SRP/CS.
- Enhanced treatment of software and documentation.
- Clarified requirements for risk graph and performance level assignment.
- Integrated validation procedures.
Conclusion
Safety standards and required levels of safety performance form the backbone of risk reduction in machinery, process, and high-reliability sectors. By systematically assessing hazards and assigning objective PLr/SILr values, organizations ensure that safety functions are robust, reliable, and compliant with international best practices. Continuous review and validation guarantee that safety performance adapts to changing technology and risk landscapes.
For tailored advice or support in implementing or validating required levels of safety performance in your organization, contact our safety experts or schedule a demo .