Parker H Series, Installation & Service Instructions Manual

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Safety Guidelines for the Application, Installation and Maintenance of Solid State Control E116P
4
2.3 Off-State Current
Solid state controls generally exhibit a small amount of
current flow when in the off-state condition. Precautions
must be exercised to ensure proper circuit performance
and personnel safety. The value of this current is available
from the manufacturer.
2.4 Polarity
Incorrect polarity of applied voltages may damage solid
state controls. The correct polarity of solid state controls
should be observed.
C.2.3 Off-State Current
Off-state current is also referred to as leakage current in
the literature. A solid state “contact” is a solid block of
material which is switched from ON to OFF by a change
internally from a conductor to an insulator. Since a perfect
insulator does not exist, there is always some leakage
current present as long as voltage is applied to the device.
The presence of leakage current indicates that OFF does
not mean OPEN. The reader is warned that simply turning
a solid state device OFF does not remove the possibility
of a shock hazard.
Solid state
and electromechanical devices, used as inputs
to solid state controls, must be compatible with the solid
state equipment with which they are used. Solid state
devices have inherent off-state current, as explained in
the preceding paragraph. Electromechanical devices may
also permit a small amount of current to flow when the
device is in the “open” position due to poor insulation
characteristics, which may be subject to further
deterioration with age and use. An example is a switching
device that employs a carbon brush in contact with an
insulating segment of the switch in the off-state, such that
a conductive film may be deposited by the brush on the
insulating segment. Any input device that could produce
an erroneous signal of sufficient magnitude to cause a
malfunction of the solid state equipment, such as
unintended turn ON or inability to turn OFF, should not be
used with solid state controls.
Also see section 3.5.2.
C.2.4 Polarity
In some instances incorrect polarity can cause damage
to controlled equipment or unintended actuation of outputs.
This could result in personal injury due to an unexpected
response of the controlled equipment or process.
Also see section 3.3.2.