23-07-2012, 01:44 PM
THE EMERGENCY STOP the button of last resort
THE EMERGENCY STOP.pdf (Size: 307.31 KB / Downloads: 527)
EMERGENCY STOP ACTUATOR
When required the emergency stop must be accessible, recognisable and
must work, reliably and safely.
It may not be a button. It could be a grab wire, rope, bar or handle and in
some specific applications, a foot pedal without a protective cover or a
combination of devises.
Whatever actuations are used they must be accessible to all who may have to
operate them and their location should be obvious.
Location
They should be positioned for easy access by the operator or anyone who
may need to use them, however, they should not be located where their use
could endanger the user. Remember that the person using the emergency
stop may not necessarily be the person in danger! It may, therefore, be
prudent to position an emergency stop near an adjacent machine, or machine
zone in the case of a complex system, giving the neighbouring operator the
opportunity to stop the machine if the operator gets into trouble. Where this is
done, the zone of effectiveness of the Emergency Stop must be clearly
indicated to avoid confusion.
Presentation
The colour, and action of the emergency stop actuator is clearly defined in BS
EN 418. It should be red and, as far as is practicable on a yellow background.
An emergency stop button must be of a mushroom style.
Action
Emergency stop devices should meet the requirements defined by BS EN 418
and BS EN 60947-5-5. In common with all other actuators the emergency
stop operation should result in it mechanically “latched in” and not “delatching”
until the device itself has been reset. Without exception operation of
the emergency stop should result in the “de-energisation” of the emergency
stop control circuit. This must be achieved through opening of the contacts
and “positive mode operation” where the contact separation must be as a
direct result of the movement of the switch actuator. Emergency Stop buttons
using detachable contact blocks should be configured such that the contact
will open should the contact block become detached ensuring Fail Safe
operation.
The resetting of the emergency stop device itself must not allow the machine
to a restart.
THE EMERGENCY STOP SYSTEM
The design of the emergency stop system should take into account that,
hopefully, it will be used very infrequently but it must be available and ready
for operation at all times.
Operation in an emergency
The nature and operation of the machine must be considered.
· Is it safe to have the emergency stop system cut the power to the
machine drives and actuators? This may result in the hazard
“freefalling” leading to a more dangerous situation.
· Should the system actuate a brake or clamp?
· Would stopping the machine in position result in a worsening of an
injury?
· Should the system allow the machine to continue on or reverse to a
safe position?
ADDITIONAL PARTS OF THE EMERGENCY STOP SYSTEM
When designing the Emergency Stop safety system there is a tendency to
think only in terms of the electrical controls. It is vital to take into account all
the sources of power used on the machine, pneumatic, hydraulic, etc., and for
the safety system to encompass them. It is important to remember that, unlike
most standard ac electrical systems, pneumatic and hydraulic systems may
retain significant amounts of energy even when the primary supply source has
been isolated. From a safety perspective this retained energy may be a
hazard or on the other hand the retained energy may be used to retard the
hazard. These should be significant considerations in the safety design.