When NFPA 79 changed by removing requirements for hard wiring machine safety devices in 2002, many automation suppliers introduced new safety rated devices for machine guarding. Domestic standards also have updated requirements to provide direction for use of safety rated devices. Do the new devices represent new hazards, such as lighted emergency stop buttons?

JB Titus

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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? 

Your comments or suggestion are always welcome so please let us know your thoughts. Submit your ideas, experiences, and challenges on this subject in the comments section below. Click on the following text if you don't see a comments box, then click into and scroll down in: Machine Safety: Does new technology create new hazards? 

Related articles:

ANSI B11.19-2010, Performance Criteria for Safeguarding

NFPA 79 – 2012, Electrical Standard for Industrial Machinery

E-Stops and Your Compliance

E-Stops Aren’t Safety Devices 

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