Schneider warning triggers debate

London - With the deadline for the new Machine Safety Directive just a few months away and the replacement of BS EN 418 by BS EN ISO 13850 Schneider Electric is highlighting the need to use trigger action emergency stops. Some machine builders, it said, are still using potentially dangerous non-trigger action devices.

Trigger action designs are the only way to ensure compliance with the legal requirements, 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, said Peter Still, Schneider’s standards manager. If the action isn’t sufficient to latch the button it can lead to extremely hazardous situations, he warns.

Existing EU machinery rules require that if the stop command is generated, 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.

In normal use, a sharp smack on an emergency stop button will both open the contacts and latch the mechanism, the Schneider's standards manager explained. However, he said, 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,” stated Still.

However, Ray Maybury of Fortress Interlocks believes this advice is misleading, particularly in relation to Clause 4.1.6 of EN 3850, which, he said, does not mention any requirement for an instantaneous braking & latching action.

The wording of the standard is: 'Once active operation of an emergency stop device has ceased following an emergency stop command, the effect of this command shall be sustained until it is manually reset,' Maybury pointed out.

In response, Still agreed that there is no requirement for emergency stops to have an ‘instantaneous braking and latching action’ to comply with BS EN ISO 13850. However, he added, Schneider's guidance does not claim otherwise and simply discusses the importance of trigger action emergency stops; where the opening of the contacts always results in the latching of the mechanism.

"The two actions are in no way ‘instantaneous’ but the emergency stop operation starts both the breaking and latching processes," said Still. "Some emergency stops, when pushed lightly, would not complete both actions; the contacts could open without the mechanism latching and this is the cause for concern.

"With trigger action stops, one action cannot be activated without the other. Once the contacts are opened, the mechanism will latch – not instantaneously but inevitably."