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Machine Safety – does new technology create new hazards?
November 07, 2012
Since 2002, when NFPA 79 changed by removing requirements for hard wiring machine safety devices, we’ve seen automation suppliers introducing lots of new safety rated devices for machine guarding. Also since 2002, additional domestic standards have likewise updated their requirements to provide direction for use of safety rated devices. Don’t some of these new products introduce possibilities for new hazards? Take for example – lighted emergency stop buttons?
Just think for a moment about how a red mushroom button on a safety rated emergency button might work if the red palm button lights up when it’s been activated? Wow, if that’s how it works, the lighted red palm button would surely differentiate it from other red buttons on any operator panel. Right? Has anybody already used any of these devices? Is this how they all work? Or, do lighted emergency stop buttons all work the same way? Aren’t they all regulated by industry standards?
Let’s see? For the design and testing of emergency stop devices we have IEC 60947-5-5:2005, Control circuit devices and switching elements – Electrical emergency stop device with mechanical latching function. This standard provides detailed specifications relating to the electrical and mechanical construction of emergency stop devices with mechanical latching function and their testing. For applications of emergency stop devices we have NFPA 79 – 2012, Electrical Standard for Industrial Machinery, ANSI B11.19 – 2010, Performance Criteria for Safeguarding, ISO 13850:2006, Safety of machinery -- Emergency stop -- Principles for design, and IEC 60204-1:2005, Safety of machinery – Electrical equipment of machines. All four of these standards regulate the applications of emergency stop devices in machine control systems. Only IEC 60947-5-5 regulates the design, build and test of an emergency stopping device, however, I don’t believe it addresses the design/application of a lighted button.
So, if we have a lighted button in a safety rated e-stop device is the light function part of the safety rating? Why is this important? Well, if the device is applied in a Cat 3 or 4 (EN 954) safety circuit does the light need to be redundant in the event the first bulb burns out? Is it a safety critical hazard if the bulb doesn’t even work? Or, is the bulb’s only purpose to aid in reducing the trouble shooting time for resetting the device? Do all lighted e-stop devices from all suppliers function the same way regarding the light function? Can anybody help us out here?
Do lighted button e-stop devices create a new hazard?
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Contact: www.jbtitus.com for “Solutions for Machine Safety”.
For more than 30 years, J.B. Titus has advised a wide range of clients on machine functional safety solutions, including Johnson + Johnson, Siemens, General Motors, Disney, Rockwell Automation, Bridgestone Firestone, and Samsung Heavy Industries. He holds a Bachelor of Business Administration degree from Oklahoma University in industrial management and an MBA from Case Western Reserve University in marketing and finance. He is a professional member of the American Society of Safety Engineers and is OSHA-certified in machine guarding. Titus is also TUV-certified as a Functional Safety Expert and serves on several American National Standards Institute, National Fire Protection Association, and National Electrical Manufacturers Association national safety and health standards committees. Reach him at jb(at)jbtitus.com and via www.jbtitus.com.
Saturday, 12-01-13 13:29
Despite a rough machine translation, I understand your interesting article. Thank you for sharing.
Saturday, 10-11-12 09:04
Heinz - Thank you for your very accurate accounting of treatment by industry standards and engineering practice. What is your opinion about the very distinctly different human behavior issue? Do all lighted buttons light when the e-stop is actuated? Are some lighted e-stop buttons illuminated all the time during run mode to help distinguish the e-stop from other red buttons in a reduced lighting atmosphere? Are there any specifications regulating the above states for an illuminated e-stop button for absolute consistency? Having said all this, will human behavior be consistent so as to not degrade the level of risk for personnel? The human behavior issue is always a concern to standards writers.
Friday, 09-11-12 09:49
The illumination of the Estop does not in itself a perform a safety function. The light will not appear in the Safety Block Diagram as a Series Element. The standards are clear that the RED color identification of the Estop Operator may not depend on illumination. So, the illumination is an information feature for either operational function (EX: On a conveyor line, where is the device stopping the line located?) or a maintenance aid for the same reason. Loss of the indication of the state of the Estop device does not reduce the ability of the Safety Related Part of the Control System to eliminate the hazard/risk.