Trigger action emergency stops must be used
17 June 2009
With the Machine Safety Directive deadline just a matter of months away and the replacement of BS EN 418 by BS EN ISO 13850 Schneider Electric, the global energy management specialist is highlighting the need to use trigger action emergency stops and advising against using inferior products.
Peter Still, Schneider Electric’s standards manager
Peter Still, Schneider Electric’s standards manager is warning that despite years of legislation to the contrary, there are still some machine builders using potentially dangerous non-trigger action emergency stop devices.
Still explained: “For many years all our emergency stop products have had a trigger action, as this is the only way to ensure compliance with the legal requirements of BS EN ISO 13850, as this type of product allows the stored energy in a spring to make certain that both the contact opening and latching functions are performed. If the action isn’t sufficient to latch the button it can lead to extremely hazardous situations.”
The first European Machinery Directive (89/392/EEC) specified as far back as 1989 that the emergency stop control must latch and must trigger the stopping function before it latches. In 1993 this was amended to make it more precise, stating: ‘Once active operation of the emergency stop device has ceased following a stop command, that command must be sustained by engagement of the emergency stop device until that engagement is specifically overridden; it must not be possible to engage the device without triggering a stop command; it must be possible to disengage the device only by an appropriate operation, and disengaging the device must not restart the machinery but only permit restarting.’
Still continued: “In plain English, this statement means that if the stop command is generated (i.e. the contacts opened) the mechanism must latch, and if the mechanism latches then the contacts must open. This is partly so that on large machines with multiple emergency stop buttons, the position where the emergency stop was initiated can be found by checking which button has latched, and partly to reduce the chances of a machine restarting due to a fault.
“The same complicated paragraph is included in the revised Machinery Directive 2006/42/EC. BS EN ISO 13850 doesn't use exactly the same words, but has the same meaning and intent, as does BS EN 60947-5-5, the product standard for electrical emergency stop devices. In normal use, a sharp smack on an emergency stop button will both open the contacts and latch the mechanism. However a more gentle push of some buttons might stop the machine but not be sufficient to latch the button; this can lead to extremely hazardous situations.
“For many years my own company has insisted that all products sold as emergency stop devices are of the trigger action type, but there are still some machine builders using the non-trigger action type of button as emergency stop devices. The replacement of BS EN 418 by BS EN ISO 13850 has drawn attention to the need for trigger action, which some designers might not have been aware of.”
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