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sensitivity of the solenoid valves
decreases. In the event of
activation, the behaviour is
reversed. In summary, operating
our solenoid valves on failsafe
outputs does not always comply
with the intended use of our
solenoid valves. The minimal
movements caused by the test
pulses could result in aging of
the magnetic system. This, in
turn, can adversely affect the
service life of the solenoid valve.
What are the alternatives for
safe operation of solenoid
valves?
• In any case, you must ensure
that the system complies with
the specifications in the
technical data and operating
instructions.
• If possible, switch off the test
pulses. Incorporate the MTTF
values of the failsafe output
when calculating the failure
probability of the safety-
related part of the controller
(SRP/CS). Check whether the
safety level of your SRP/CS
is still reached despite the
deactivation of the test pulses
of the failsafe outputs. The
MTTF of the entire control
chain must comply with the
required MTTF. This solution
is simple, practical and, in
particular, can be implemented
without taking additional time.
• Actuate the solenoid valve via
a non-pulsed output of a
standard PLC. For example,
connect a normally open
contact of a safety shutdown
relay between the solenoid
valve and the output, which
guarantees the safety function
when needed.
• Disconnect the solenoid valve
from the test pulses by
actuating it via a relay contact,
which is supplied by a non-
pulsed supply voltage. The
relay is actuated from a safe
output (even here, the test
pulses must be observed).
• Use filter clamps, as close as
possible to the solenoid valve,
to filter out the test pulses.
• The cable length or the cable
diameter used (like a
capacitor) has a damping
effect on the test pulse
reaction of the solenoid valve.
A short cable has a negative
effect (the test pulse reaches
the coil of the solenoid valve in
an attenuated state), a long
cable has a positive effect (the
test pulse is unattenuated
when it reaches the coil of the
solenoid valve).
Where can I find the maximum
pulse length of a solenoid
valve?
During the design phase of
a safety-related part of a
controller, always contact the
manufacturer of the solenoid
valve, and ask for the maximum
pulse widths for test pulses.