By connecting expansion relays to a
safety relay it is easy to increase the number of safe
outputs. This means that an unlimited number of dangerous
machine operations and functions can be stopped from
one safety relay.
Safe Soft Stop
When a gate is opened a program stop
is first given to the machine's PLC/servo which brakes
the dangerous operations in a soft and controlled way.
The safety outputs then break the power to the motors,
that is, when the machine has already stopped. Normally
between 0.5 and 1 second is needed to brake a dangerous
machine operation softly. Soft stop ensures many advantages:
·The
machine lasts longer.
·Parts
being processed are not damaged.
·Restart
from stopped position is enabled and simplified.
A safe soft stop is achieved by means of a safety relay
which gives the program stop, and an expansion relay,
E1T, which gives safe delayed stop signals. The drop
time delay on a E1T can as standard be selected from
0 to 3 seconds. By connecting several E1Ts in series
even longer times can be achieved.
When are delayed safe stops
used?
Delayed safety stop signals can be used
for emergency stops according to EN418 § 4.1.5.
Stop category 1, i.e. a controlled stop with power to
the actuator(s) available to achieve the stop and then
removal of power when stop is achieved.
Stop category 1 may also be permitted
when it is not possible to gain physical access to the
machine before the safe stop is affected e.g:
·Gates,
access time is normally over 1 sec.
·Covers
and gates which are locked until dangerous operations
and functions have been stopped.
·Long
distances between a safety device and a dangerous machine
function.
Safety Level
The E1T has twin stop functions, that
is, two relays with mechanically operated contacts.
A monitored stop function is achieved by connecting
the test output (terminals X1 and X2) to the test or
reset input on the safety relay which is being expanded.
One condition for a safe delayed stop
is that the delay time cannot increase in the event
of a fault. The E1T complies with this requirement.
