Glossary · Functional safety
Required Performance Level (PLr)
Also known as: PLr
In functional safety of machinery, the required Performance Level (PLr) is the Performance Level required for a specific safety function based on the risk assessment. It is the target that the achieved Performance Level of that function must meet or exceed.
- Functional safety
- Verification
In one sentence
The required Performance Level (PLr) is the target set for each safety function from the risk assessment under ISO 13849-1.
Example
For access to the point of operation of a press brake, the risk graph gives severe injury, frequent access and scarcely possible avoidance, which leads to PLr e for the protective function.
How it applies
- Risk assessment: ISO 13849-1 offers a risk graph that derives the PLr from severity of injury, frequency or duration of exposure, and the possibility of avoiding the hazard. Other justified methods are permitted.
- Functional safety: The PLr is set per safety function, before the design. It is then compared with the achieved Performance Level during verification.
- Technical documentation: Record the parameters chosen and the reasons. Reviewers often question the choice of "possibility of avoidance", so the justification should refer to real speeds, distances and operator positions.
- Evidence: If the use of the machine changes, for example higher cycle rates or a new operating mode, the parameters and the PLr may no longer hold.
PLr vs. PL
The PLr is the requirement, derived from risk. The PL is the property the implemented control system achieves. Documents should never mix the two.