Performance Level
Performance Level (PL) is a discrete level (a–e) specifying the ability of safety-related parts of control systems to perform a safety function under foreseeable conditions, as defined by ISO 13849-1. PL a represents the lowest reliability, PL e the highest. It depends on diagnostic coverage and hardware reliability, and is used to demonstrate control reliability under WorkSafeBC and ANSI B11 standards.
On the shop floor, PL is applied through functional safety design and verification. A risk assessment determines the required performance level (PLr) for each safety function, such as guard interlocks (often PL d/e) or emergency stops (commonly PL d). Safety-rated devices like interlock switches, light curtains, and safety PLCs are selected to meet PLr. Verification involves calculating achieved PL using MTTFd, diagnostic coverage, and architecture. Maintenance must preserve PL by using equivalent safety-rated parts and avoiding bypasses. Documentation supports compliance with WorkSafeBC Part 12 and audit readiness.
How does Performance Level relate to 'control reliable' machine controls under WorkSafeBC and OSHA/ANSI?
Control reliable means a single fault in safety-related parts will not prevent the safety function, and the fault is detected or the system fails safe. In ISO/IEC terms, this corresponds to Category 2 with PL d, Category 3 PL d/e, or Category 4 PL e. WorkSafeBC recognizes ANSI B11 standards, so demonstrating PL d or PL e for high-risk machinery is an accepted way to show compliance with control reliability expectations.
How is the required Performance Level (PLr) determined for a specific safety function?
Using ISO 13849-1 methodology, assess Severity (S1 slight, S2 serious), Frequency/Duration (F1 seldom, F2 frequent), and Possibility of Avoidance (P1 possible, P2 hardly possible). These are plotted in a decision graph to output PLr (a–e). For example, a power press with frequent operator access and severe injury potential (S2, F2, P2) typically yields PLr = d or e, requiring high diagnostic coverage and robust architecture.
How do hardware reliability and Diagnostic Coverage contribute to achieved PL?
Achieved PL is determined from MTTFd (Mean Time To Dangerous Failure) of components, Diagnostic Coverage (DC) percentage of detected dangerous failures, and Category (1–4) describing architecture. These are combined to calculate PFHd (Probability of dangerous Failure per Hour), which maps to PL a–e intervals. High PL (d/e) requires high MTTFd, medium to high DC, and redundant monitored architecture (Category 3/4).