02/28/2020
Performance Level Calculation
DEEPENING
Taking these parameters into account, following figure provides a graphical method for determining the PL, achieved by the SRP/CS. The combination of Category and DC determines which column is to be chosen. According to the MTTFD of each channel, one of the three different shaded areas of the relevant column shall be chosen. The vertical position of this area determines the achieved PL which can be read off the vertical axis.
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The ISO 13849-1 “Safety of machinery – Safety-related parts of control systems – Part 1: general principles for design” provides safety requirements and guidance on the principles for the design and integration of safety-related parts of control systems (SRP/CS), including the design of software. It applies to SRP/CS for high demand and continuous mode, regardless of the type of technology and energy used (electrical, hydraulic, pneumatic, mechanical, etc.), for all kinds of machinery.
ISO 13849-1 provides the calculation method for quantifying the achieved Performance Level of the various SRP/CS. However, it does not provide the minimum performance level required by the specific safety function of the machinery for which reference is made to the risk assessment.
Performance levels for ISO 13849-1 are defined in terms of probability of dangerous failure per hour. Five performance levels are set out, from the lowest PL a to the highest PL e with defined ranges of probability of a dangerous failure per hour as per following table:
ISO 13849-1 provides the calculation method for quantifying the achieved Performance Level of the various SRP/CS. However, it does not provide the minimum performance level required by the specific safety function of the machinery for which reference is made to the risk assessment.
Performance levels for ISO 13849-1 are defined in terms of probability of dangerous failure per hour. Five performance levels are set out, from the lowest PL a to the highest PL e with defined ranges of probability of a dangerous failure per hour as per following table:
| PL | Average probability of dangerous failure per hour (PFHd) 1/h |
| a | a ≥ 10 e-5 a < 10 e-4 |
| b | b ≥ 3 x 10 e-6 a < 10 e-5 |
| c | c ≥ 10 e-6 a < 3 x 10e-6 |
| d | d ≥ 10 e-7 a < 10 e-6 |
| e | e ≥ 10 e-8 a < 10 e-7 |
The main factors that contribute to the calculation of the Performance Level of the safety functions of the industrial machinery are:
- Category: classification of the safety-related parts of a control system in respect of their resistance to faults and their subsequent behaviour in the fault condition, and which is achieved by the structural arrangement of the parts, fault detection and/or by their reliability.
- Mean time to dangerous failure /MTTFD: expectation of the mean time to dangerous failure
- Diagnostic Coverage / DC: measure of the effectiveness of diagnostics, which may be determined as the ratio between the failure rate of detected dangerous failures and the failure rate of total dangerous failures.
Taking these parameters into account, following figure provides a graphical method for determining the PL, achieved by the SRP/CS. The combination of Category and DC determines which column is to be chosen. According to the MTTFD of each channel, one of the three different shaded areas of the relevant column shall be chosen. The vertical position of this area determines the achieved PL which can be read off the vertical axis.
Visit the ENGAL Services NewsRoom where you will find news regarding the Certification procedures, the Regulatory Updates, the news on the Interpretative Trends of the technical requirements and the Problems encountered in the matter of Conformity and Certification.
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