Machine safety

May 1, 2012
In this month's Design by Objective department, Mike Miller of Rockwell Automation discusses EN ISO 13849 and how this standard is having an impact on the way machine safety systems are designed.

ISO 13849 and beyond

ISO 13849 is a globally recognized standard that was originally released in 2006 and adopted by the European Union as a required standard in 2009 as EN ISO 13849. The standard categorizes safety levels, helping designers identify the level of safety necessary to effectively mitigate risk.

As of December 31, 2011, two safety standards — EN ISO 13849 and EN IEC 62061 — officially superseded EN 954-1, requiring machines shipped into or out of Europe to comply with one of these two standards. The implications of this change are global. Machine builders who proactively migrated to the international standards before the deadline have a competitive edge when serving global or multinational customers. However, even machine builders currently serving only North American manufacturers benefit from adoption, and are better prepared to pursue global customers in the future. For machine builders who haven't yet made the transition, they could be missing out.

Meeting mandates

Increasingly, manufacturers — especially global or multinational ones — buy machines adhering to internationally accepted standards. Beyond the fact that it's mandated in many countries, more and more manufacturers follow the standards to streamline safety processes, reduce liability, improve operating efficiency, and protect workers — with designs that can be used nearly anywhere in the world. International standards also require designers to document risks, and promote controls to reduce those risks.

The standards are designed to assess risk over prolonged periods and boost long-term safety-system predictability — so operators gain confidence in the machine and increase productivity. Ultimately, a more predictable machine is a safer machine, and safer machines are more productive.

Traditional machine safety standards are prescriptive, providing only guidance on structuring controls to meet safety requirements. With these older standards, a designer may have had difficulty understanding — or explaining — why a costly or seemingly sophisticated safety system was needed for a particular application.

The newer international safety standards are more rigorous in their design requirements and provide quantifiable methodologies for machine builders to identify and document the potential hazards associated with a machine and the risk levels to users:

  • To comply with ISO 13849-1, a machine builder must define and document the statistical probability of an unwanted occurrence or dangerous failure, or the calculated mean time to dangerous failure as part of the overall performance level (PL).

  • To comply with IEC 62061, a machine builder must describe the amount of risk to be reduced and the ability of a control system to minimize that risk in terms of a safety integrity level (SIL).

This documentation helps machine designers demonstrate actual risk reduction and justify the value of safety, including the costs of upgrades.

Safety standards deliver ROI

The combination of new international standards, technologies, and design approaches position safety as a core function that delivers business and economic value — including financial returns beyond the benefits of reducing costs associated with accidents and medical expenses. The following tips provide a starting point for design engineers to follow:

Utilize new standards

The functional safety lifecycle, as defined in standards IEC 61508, IEC 62061, and ISO 13849, is the foundation for a detailed, systematic design process. A key objective of the lifecycle is addressing failure modes early on — so safety functions can be tailored to the application. It also reduces complexity while boosting safety for each defined safety circuit.

Conduct a risk assessment

Conducting a risk assessment is the first phase of the safety lifecycle. It lays the framework for overall risk reduction and:

  • Eliminates hazards with inherently safe design concepts

  • Isolates danger with hard guarding and safety devices

  • Provides for complementary personal protective equipment

  • Improves working practices with safety procedures, training, and supervision

Take a hierarchical approach

A hierarchical approach is a qualitative safeguarding effectiveness rating that helps determine the required level of risk reduction. For example, it ensures that warning signs are not applied alone as mitigation for higher-level risks. Steps:

  • Design out or minimize the risk.

  • Use fixed enclosing guards or distance guarding to protect against or detect exposure to hazards.

  • Use interlocked guarding or other safety devices to protect against or detect exposure to hazards.

  • Incorporate awareness methods, such as lights and horns.

  • Develop safe working procedures, safety training, and operator awareness of residual risks.

  • Specify the use of appropriate personal protective equipment and policies.

Consider the entire system

Unlike previous standards, which dictated safety circuits only, the new standards require a holistic approach. Builders must identify and document all potential machine hazards, risks to which workers are exposed, and the ways in which a machine's safety functions and applications interact during operation, including under extreme temperature, humidity, voltage variations, and noise.

This month's handy tips courtesy of Mike Miller, Functional Safety TÜV Expert, global safety market development, Rockwell Automation.

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