Parker H Series, Installation & Service Instructions Manual

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Safety Guidelines for the Application, Installation and Maintenance of Solid State Control E116P
8
3.1.3 Redundancy and Monitoring
When solid state devices are being used to control
operations, which the user determines to be critical, it is
strongly recommended that redundancy and some form of
checking be included in the system. Monitoring circuits
should check that actual machine or process operation is
identical to controller commands; and in the event of failure
in the machine, process, or the monitoring system, the
monitoring circuits should initiate a safe shutdown sequence.
3.1.4 Overcurrent Protection
To protect triacs and transistors from shorted loads, a
closely matched short circuit protective device (SCPD) is
often incorporated. These SCPDs should be replaced
only with devices recommended by the manufacturer.
3.1.5 Overvoltage Protection
To protect triacs, SCRs and transistors from overvoltages,
it may be advisable to consider incorporating peak voltage
clamping devices such as varistors, zener diodes, or
snubber networks in circuits incorporating these devices.
C.3.1.3 Redundancy and Monitoring
The normal operating mechanism for solid state
components depends upon a deliberate electrical signal
input altering the internal molecular structure of the
semiconductor material. Unfortunately, spurious input
signals may also alter the internal molecular structure
without any means for external detection that this has
happened. Therefore, solid state devices are subject to
malfunction due to random causes that are undetectable.
Because of this, redundancy and monitoring are the most
highly recommended means for counteracting this situation.
When redundancy is used, dissimilar components not
susceptible to common cause failure should be used for
the redundant elements if a common cause could produce
simultaneous failure of those elements in a dangerous mode.
A “safe shutdown sequence” can involve much more than
disconnecting electrical power for some machinery and
processes. Examples include machines with high inertia
and hazardous access points; processes that become
unstable at shutdown unless a specific sequence is
followed; etc. The control system for such applications
should be configured to deal with the particular hazard(s)
through use of special features such as automatic transfer
of control functions to redundant devices in the event of
failure of primary controls; alarm circuits and diagnostics
to signal and identify failures that require repair in order
to maintain redundancy; emergency power sources with
automatic transfer upon loss of primary power source; or
other appropriate features.
C.3.1.4 Overcurrent Protection
Even a closely matched short circuit protective device
(SCPD) will generally protect a solid state device only
against shorted loads, an accidental short to ground or a
phase to phase short. Depending upon the application,
additional protective measures may be needed to protect
the solid state devices against small to moderate
overcurrents. Consult with the manufacturer if necessary.
C.3.1.5 Overvoltage Protection
See section 2.7.