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Wronged, a safety switch breaks right
By Design News Staff -- Design News, October 11, 2004
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| Safe at Home: Energy stored by this emergency stop ebbs when the contacts are open. According to the manufacturer, other E-stops reverse the energy flow--a potential hazard if the switch breaks. |
Emergency stop switches form the final defensive unit in a machine's safety roster. After other operator safeties have left the arena, E-stops offer a last means to bridle a machine gone wild. If you've built, say, a packager, parts handler, or injection molder in accordance with the latest safety standards such as IEC60947-5-5, the E-stops are where they ought to be. Any operator in peril can reach one in time to ward off, or minimize, injuries. A question, though: How do you know the switches will work when they have to?
That's what engineers at IDEC Corp. asked as they surveyed the field of existing E-stops. They discovered that most emergency stops could fail closed, according to Lanny Schuberg, director of product marketing. In technical terms, most E-stops are at their lowest energy state when their contacts are touching, what engineers call normally closed. With its contacts contacting each other, a machine is able to run.
Actuating one of these E-stops opens the contacts, but also sets the switch at a higher energy level, he says. In lay terms, it's a relaxed compression spring that holds the contacts closed and a compressed spring that waits to close them while they're open.
IDEC's new switch addresses this potential hazard by reversing the spring to maintain the contacts opened during the low-energy state, Schuberg says. Now, if a falling control pendant should break the button off its E-stop actuator, or a lapsed mechanic should re-install a contact block incorrectly, or a stray machine vibration should loosen a contact block, the contacts stay open, disabling the equipment until someone fixes the switch. IDEC dubs the idea "Safe Break Action."
IDEC representatives are proposing that the standards committee make the design a directive of the next IEC code revision, Schuberg says. The switch exceeds current requirements, he adds.
As for convenience, the switch can be operated push/pull—the North American convention—or push/twist, the European way, Schuberg says. Machine makers can supply E-stops appropriate to a region's custom without actually knowing the equipment's destiny.
Contact:Lanny Schuberg, Idec Corp. Tel (408) 747-0550; lanny@idec.com; http://rbi.ims.ca/3856-500
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