With increasing emphasis on safety in every part of industry, it is
essential that users of machinery employ a reliable emergency stop
device. Jeff Stewart looks at some key considerations when specifying.
The main purpose of an emergency stop function is to quickly and easily
shut down equipment likely to damage or threaten the safety of operators,
machinery or products, through a simple action - such as hitting a
button. These devices are not a replacement for machine safety equipment
but should be used as a back-up feature and should in no way interfere
with the effectiveness of normal safety procedures.
Current legislation, including machine safety standard EN 418 and
EN/IEC609457-5-5, specifies the colour, size and even shape of the push
buttons. European standards also state that push button emergency stops
must be 'self-latching', meaning that after a button is pressed and the
contacts open, equipment can only be restarted after a deliberate reset
action. Moreover, it is important that safety devices meet all major
international standards, including CCC approval for the rapidly growing
Chinese market, as well as complying with the Restriction of Hazardous
Substances in Electrical and Electronic Equipment (RoHS) Directive.
The new Allen-Bradley 800F from Rockwell Automation surpasses all current
safety standards and features an innovative Self-Monitoring Contact Block
(SMCB) for increased emergency stop protection. The last thing that
should happen in an emergency is the manufacturing process continuing
even after the push button has been pressed. However, whether it is
because of improper installation, physical damage or extreme vibration,
if the contacts separate from an actuator that is exactly what will
happen with traditional emergency stop units. The SMCB function instantly
cuts power to equipment thanks to an integral fail-safe mechanism that
ensures the circuit is broken even if the block is separated from the
actuator. It comprises a set of self-monitoring, normally open contacts,
which close automatically when the block is installed on the actuator.
The 800F range can accept three of the SMCB contact blocks per actuator.
The QuadCONNECT contact block also makes the 800F ideal for low voltage
switching. Unlike conventional contacts, this is extremely sensitive and
responsive, and suited to low voltage applications from 17V, 5mA down to
5V, 1mA - particularly for direct interfacing with state-of-the-art
electronic devices.
For extra reliability, the self-cleaning contacts include four 'fingers'
on either side of a flexible, corrosion resistant contact bridge,
allowing several possible current paths. Each finger is capable of
carrying the full load of the contact. The stationary contacts are wiped
by the bridge each time the circuit is opened or closed, removing any
debris or non-conducting film that may have formed.
As well as being quick and easy to assemble, there are a variety of
accessories available for modern push buttons, from earthing kits to
anti-rotation washers, and legend plates to replacement caps and lenses.
For additional safety, many applications also require labelled or
illuminated selector switches and push buttons so that they can be easily
identified. Integrated LED modules provide maximum flexibility, using
less power and emitting less heat than filament lamps, as well as being
resistant to shock and vibration. Available in 24V ac/dc, 120V ac and
240V ac, the LEDS have an extended lifespan of around 100,000 hours
compared with only 5,000 hours for traditional filament lamps.
You can't put a price on safety - but while the market is becoming
increasingly competitive, the introduction of the 800F push button
provides OEMs with an intrinsically safe emergency stop device that meets
all worldwide standards. The innovative SMCB and QuadCONNECT features
help m