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Risk Assesment
The Idea of Performance Level
ISO 13849-1 Revision
PL(Performance Level)
PL Parameters
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PL (Performance Level)

PLr and PL (From ISO 13849-1:2006)

Performance Level
(PL)
Probability of Dangerous
Failure per Hour (PFHd) 1/h
a ≥10-5 and <10-4
〈0.001% to 0.01%〉
b ≥3 × 10-6 and <10-5
〈0.0003% to 0.001%〉
c ≥10-6 and <3 × 10-6
〈0.0001% to 0.0003%〉
d ≥10-7 and <10-6
〈0.00001% to 0.0001%〉
e ≥10-8 and <10-7
〈0.000001% to 0.00001%〉

The performance level (PL) is a value used to define the ability of safety-related parts of control systems to perform a safety function under foreseeable conditions. On the other hand, the required performance level (PLr) is used in order to achieve the required risk reduction for each safety function. Therefore, the performance level (PL) of safety-related parts of a control system must be equal to or higher than the required performance level (PLr).

Determination of Required Performance Level (PLr)

For each safety function to be carried out by a safety-related parts of a control system, the required performance level (PLr) must be determined, which is based on the following three parameters:

SSeverity of injury
S1:Slight
S2:Serious (such as irreversible injuries and death)
FFrequency and/or exposure to hazard
F1:Seldom-to-less-often and/or exposure time is short
F2:Frequent-to-continuous and/or exposure time is long
PPossibility of avoiding hazard or limiting harm
P1:Possible under specific conditions
P2:Scarcely possible

Determination of Required Performance Level (PLr)

Evaluation example
The PLr is determined to be “c” for the following case.
Severity of injury: Serious (S2)
Frequency: Seldom (F1)
Possibility of avoiding: Possible (P1)
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