Functional safety Part 1 of 2

Oct. 1, 2010
New machine safety standards are ushering an era of better design flexibility and safety.

Keeping up with changing safety standards is nothing new for machine builders. However, recent changes to the European Commission's new Machinery Directive, which took effect December 29, 2009, will reshape how designers approach machine safety.

As it relates to functional safety, EN 954-1, the standard that categorizes safety levels, is being superseded by two standards that will coexist: Machine builders and integrators can choose to conform to the requirements of either EN ISO 13849-1 or EN/IEC 62061 to demonstrate compliance with the machinery directive. On a related note, the European Commission recently extended the deadline for transition from EN 954-1 to EN/ISO 13849-1 until December 31, 2011. However, this extension should not be viewed as an extension of EN 954-1, but as an additional transition period.

With adoption of new functional safety standards, designers will need to assess the reliability of safety components by adding quantitative calculations to controls safety design. While this requires more steps and procedures, it is beneficial: Namely, these standards result in a methodical approach that culminates in more predictable and reliable machinery with higher investment returns.

EN ISO 13849-1 (Safety of machinery, Safety-related parts of control systems) builds on EN 954-1, specifying system reliability in one of five performance levels or PLs based on a hardware-oriented structure; calculated mean time to dangerous failure; and diagnostic coverage of the safety function. A significant revision in the standard requires definition of the statistical probability of an unwanted occurrence or failure. In other words, it forces designers to validate that a control system does what is required. This standard applies beyond electric and electronic systems to include mechanical, hydraulic, and pneumatic safety-related control parts.

EN/IEC 62061 (Safety of Machinery - Functional safety of safety-related electrical, electronic and programmable electronic control systems) describes both the amount of risk to be reduced and the ability of a control system to reduce that risk in terms of safety integrity levels or SILs. The machinery sector uses three; SIL 1 is the lowest and SIL 3 is the highest. A SIL applies to a safety function, so subsystems comprising a system that implements safety functions must have an appropriate SIL capability.

Less complexity, better reliability

Historically, standards were mostly prescriptive and simply provided guidance on control structures for satisfying safety requirements. Categories based on redundancy, diversity, and diagnostics helped ensure that safety functions were performed. However, a very important element was missing — time. In fact, some consider EN 954-1 to be overly simplistic because it requires no time-related assessment of safety-component reliability.

In contrast, functional safety standards define reliability in terms of time, for more concrete assessments. As we'll discuss next month in Part 2 of this article series, accounting for time requires more work from safety-component suppliers during initial design work, because of increased product testing and performance documentation requirements. However, in the long run, it reduces work for machine operators and safety system designers, as it allows quantification of circuit reliability.

For more information, visit and click on the Safety portal link.

Machine safety continually evolves, and the latest functional safety standards are a giant leap forward. Ultimately, they will provide safer controls and more flexible and cost-effective safety designs. Though the deadline is months away, machine builders should evaluate the latest directive's impact on their equipment and become compliant before the new directive becomes mandatory.

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