In brief
The performance level (PL) under EN ISO 13849-1 describes, in five levels from a to e, how reliably a control system performs a safety function.
Contents
What is Performance level?
The performance level is the central parameter of EN ISO 13849-1 for safety-related parts of control systems on machinery. It ranges from PL a to PL e, with PL e denoting the highest reliability. The standard applies regardless of technology, i.e. to electrical, electronic, pneumatic, hydraulic and mechanical control systems.
First, the required performance level (PLr) is determined for each safety function, usually by means of a risk graph with the parameters severity of injury, frequency and duration of exposure to the hazard, and possibility of avoidance. The PL achieved results from the category of the control architecture (B, 1, 2, 3 or 4), the mean time to dangerous failure (MTTFd) of the channels, the diagnostic coverage (DC) and measures against common cause failures (CCF). For wearing components such as valves or contactors, the B10d value is taken into account.
For maintenance and procurement, this means: safety components such as emergency stop buttons, safety switches, safety relays or valves may only be replaced by parts whose parameters continue to satisfy the assessment. The text of the standard and the manufacturer’s specifications are authoritative.
In practice: maintenance and procurement
For safety components, ask not only for the order number but also for parameters such as PL, category, MTTFd or B10d, and compare them with the safety assessment of the machine. A different model may require recalculation; after the replacement, the safety function must be tested.
Synonyms and abbreviations: PL · PLr (required performance level) · EN ISO 13849-1 · ISO 13849
Frequently asked questions
What is the difference between PL and PLr?
PLr is the required performance level, i.e. the target that results from the risk assessment for a safety function. PL is the performance level actually achieved by the implemented control system. The safety function is adequately designed if the PL achieved is at least as high as the PLr. Both are stated on the same scale from a to e.
How is the performance level determined?
For the implemented safety function, the category of the architecture, the MTTFd of the components involved, the diagnostic coverage and measures against common cause failures are determined. The standard contains a simplified procedure from which the PL can be read off. In practice, calculation software and the parameter libraries of component manufacturers are often used for this.
What does the B10d value mean?
The B10d value indicates the number of switching cycles after which, statistically, ten per cent of the components have failed dangerously. It is used for components subject to wear, such as contactors, relays, valves or position switches. Together with the switching frequency in operation, it can be used to calculate the MTTFd, which feeds into the assessment of the performance level.