Determining the Performance Level to EN ISO 13849-1
The Performance Level (PL) states how reliably a safety-related control and its sensors carry out a safety function. This guide shows how to derive the required PLr from the risk graph using S, F and P, then prove the achieved PL through category, MTTFD and DC.
View safety sensorsWhat is the Performance Level and why does it matter?
The Performance Level to EN ISO 13849‑1 measures the reliability of a safety function in five steps from PL a (low) to PL e (high). Each step maps to an average probability of dangerous failure per hour (PFHD), roughly 10⁻⁵ down to below 10⁻⁷ at PL e. It rates the whole chain of sensor, logic and actuator, not just a single part.
The starting point is the required Performance Level, the PLr. It is derived from the machine risk and defines how good the later circuit must be at minimum. An emergency stop or a light curtain with its evaluation unit must safely reach or exceed the PLr.
How does the risk graph set the required PLr?
The PLr is set with the risk graph in the standard using three parameters: severity of injury (S), frequency and duration of exposure (F) and possibility of avoidance (P). Their combination points directly to a letter from a to e.
Example: a serious irreversible injury (S2), frequent access (F2) and avoidance that is hardly possible (P2) drive the risk graph to PLr e. Lower the frequency or make the injury reversible and the target shifts towards PLr c or b.
- S2 + F2 + P2 usually gives PLr e (highest demand).
- S2 + F1 + P1 typically leads to PLr c.
- S1 with low frequency often needs only PLr a or b.
- When in doubt, pick the more unfavourable branch and document it.
How is the achieved PL verified?
The achieved PL follows from the structure (category B, 1, 2, 3 or 4), the component reliability MTTFD, the diagnostic coverage DC and measures against common cause failure (CCF). These values are found per channel and combined into a PL using the standard's bar chart or a tool such as SISTEMA.
- Category B and 1: single channel, PL a to c depending on MTTFD.
- Category 2: single channel with test, up to PL d possible.
- Category 3: dual channel, single fault detected, PL d to e.
- Category 4: dual channel with high diagnostics, PL e.
Frequently asked questions
What is the difference between PLr and PL?
PLr is the target level required by the risk assessment. PL is the value achieved by the real circuit. The proof is valid only when PL is at least as high as PLr.
Which parameters set the PLr in the risk graph?
Three parameters: S for severity of injury, F for frequency and duration of exposure and P for the possibility of avoidance. Their combination gives a letter from a to e.
How do I reach Performance Level e?
PL e usually requires category 3 or 4 with dual-channel, redundant sensors, high diagnostic coverage and good MTTFD. Single-channel structures are not enough.
What is the SISTEMA tool for?
SISTEMA from the DGUV brings together category, MTTFD, DC and CCF and calculates the achieved PL. It documents the proof to standard and is free of charge.
Looking for safety sensors with the right PL?
We supply light curtains, safety switches and evaluation units for categories up to 4 - matched to the required Performance Level to EN ISO 13849-1.
Standard-compliant
Components with PL rating to EN ISO 13849-1.
Verifiable
Values for MTTFD, DC and category documented.
Up to PL e
Dual-channel solutions for the highest demands.
Expert advice
Specialists support you with the PLr determination.