Review today’s machinery safety standards

Interrelated, and sometimes overlapping, safety standards have been developed as a result of the European Commission's enactment of the wide-ranging Machinery Directive. Safety standards include...

03/17/2009


Read Integrated Safety and Motion article

Control Engineering North American edition March 2009 Cover story covers integrated safety and motion.

Control Engineering North American edition March 2009 Cover story covers integrated safety and motion.

A number of interrelated—and sometimes overlapping—safety standards have been developed as a result of the European Commission’s enactment of the wide-ranging Machinery Directive. Different safety standards apply to manufacturers of machines and electronic control system and drives. Knowing these are important because of increasing efforts to

integrate safety and motion in machines and on the plant floor

.
For machines builders , EN 954-1 (European), ISO 13849-1 (International Standards Organization), and IEC 62061 (International Electrotechnical Commission) standards currently apply with one important change. EN 954-1 (“Safety of machinery, Safety related parts of control systems”) is in a transitional period that expires as of Nov. 2009, when ISO 13849-1 (with the same title) becomes the applicable standard. EN 954-1 has served as a sort of “gold standard” but its simpler deterministic approach to assessment of risk and safety system reliability needed updating to newer probabilistic methods and technology advancements.
For electronic control system and drives manufacturers , applicable standards include IEC 61508 (“Functional safety of electrical/ electronic/ programmable electronic safety-related systems”) and IEC 61800-5-2 (“Adjustable speed electrical power drive systems”). These newer standards cover the concepts of functional safety discussed in the main article, “Integrated Safety and Motion.” They define methods to assess probability of dangerous failure in machine and control systems through calculation tools such as safety integrity level (SIL) and performance level (PL).
ISO 13849-1 builds on EN 954-1, specifying system reliability in one of five PLs, based on a “hardware-oriented structure,” calculated mean time to dangerous failure, and diagnostic coverage of the safety function. This standard applies beyond electric/electronic systems to include hydraulic and pneumatic equipment.
IEC 61508 was the first safety standard to address failure in a probabilistic way. It defines safety system requirements using SIL 1 through 4 and is often cited as a test standard for safety devices. This standard is also the basis for three others: ISO 13849-1, IEC 62061, and IEC 61800-5-2.
IEC 62061 (“Safety of machinery–Functional safety of safety-related electrical, electronic, and programmable electronic control systems”) applies safety classifications only up to SIL 3. Required SIL is obtained using three factors—exposure frequency; hazard occurrence probability; and prevention possibility—the sum of which determines a harm probability class. The class number expressed in four ranges and a further parameter, seriousness of possible harm (levels 1-4), form a matrix from which SIL can be determined.
IEC 61800-5-2 (“Adjustable speed electrical power drive systems”) applies specifically to electric drives. It also expresses safety requirements as SIL 1-3 and defines the various integrated safety functions (STO, SS1, etc.) discussed in the main article.
U.S. standards transition : Involvement of the U.S. in machinery safety standards originated through parts of American National Standard Institute (ANSI) and Occupational Safety and Health Administration (OSHA) standards that dealt with “removal of power from a motion device.”
A more recent and significant development is the change in the 2007 revision of NFPA 79: “Electical Standard for Industrial Machinery,” issued by the National Fire Protection Association . NFPA 79 (section 9.2.5.4.1.4) allows a safety-relevant drive to be the final safety disconnect (or switching element) for machine safety operation. This eliminates the need for a separate, external contactor or safety switch that was previously required.
NFPA 79 2007 covers control systems for safety related functions and recognizes safety testing of drive systems in accordance with IEC 61508 and IEC 61800-5-2.
Portions of this article are based on information in Bosch Rexroth Corp . publication “Safety Onboard Functional Safety in Automation” (2008).

Related resources on machine safety follow.
- Safety: Which technologies safeguard the most machines?

- Safety white papers: Encoders, instrumented systems, light curtains, networks

- Safety Sensors Rise to New Heights

www.iec.ch
www.nfpa.org
www.boschrexroth-us.com
Frank J. Bartos , P.E., is Control Engineering consulting editor.





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