EN ISO 13849-1 simplified procedure for assessing PL

EN ISO 13849-1 Safety of machinery - Safety related parts of control systems - Part 1: General principles for design has replaced EN 954-1, though in some circumstances it is more appropriate to use another functional safety standard, EN (IEC) 62061.

One of the arguments sometimes levelled against EN ISO 13849-1 is the complexity of the calculations. However, the following simplified process can be used to ascertain the PL of a safety-related control system if the control design is based on one of the architectures shown in section 6 of the standard. In addition, the following typical assumptions are also made:

• Service life of 20 years
• Constant failure rates within the service life
• For category 2: request rate less than 1/100 of the test rate
• For category 2: mean time to dangerous failure for test equipment (MTTFd, TE) greater than half of the mean time to dangerous failure for the logic (MTTFd, L)

To apply this simplified procedure, these parameters are required:

• Category (in accordance with the architecture)
• MTTFd (mean time to dangerous failure)
• DCavg (average diagnostic coverage)

The combination of category and DCavg determines which column is to be selected in the graph below. The respective shaded area is then determined in the column in accordance with the MTTFd of each channel. The resulting PL can then be read from the vertical axis.

Relationship between category, DCavg, MTTFd and PL for each channel:

In this graph the blue section of each column corresponds to an MTTFd that is low, green corresponds to a medium MTTFd and yellow to a high MTTFd.

Validation

It is important to validate the design of each safety-relevant control function to demonstrate that the design of each safety function satisfies the corresponding requirements. EN ISO 13849-2 Safety of machinery - Safety-related parts of control systems - Part2: Validation contains information on fault consideration, maintenance, technical documentation and notes on usage.

More machinery safety resources

More information about designing safety related parts of control systems to EN ISO 13849-1 is available free of charge on the Leuze website in the Safety Know How section. In addition, this portal has information about the Machinery Directive, Use of Work Equipment Directive (usually referred to as PUWER in the UK), the hierarchy of European standards for machine safety (Types, A, B1, B2 and C), risk analysis and risk assessment, and IEC/EN 62061 Safety of machinery - Functional safety of safety-related electrical, electronic and programmable electronic control systems. There is also practical advice, together with Calculation Wizards and selection tools, to aid in the specification of the correct safety devices for particular applications.