Rockwell Automation Safety Guidelines for the Application, Installation, and Maintenance of Solid-State Control - Manual (Page 5)

Publication SGI-1.1 - August 2009
Section 2: General Authorized Engineering Information 5
Comments: 2.2 —Electrical Noise
Solid-state devices are generally more susceptible to electrical noise
interference than their electromechanical counterparts. The reasons are
straightforward. The operating mechanism for electromechanical devices
requires a deliberate input of electrical energy that can be converted into a
sustained mechanical force that is strong enough to close the hard
contacts and maintain the closure for the duration on the ON cycle. Most
random electrical noise signals lack the energy content to produce that
magnitude of mechanical force. The operating mechanism for solid-state
devices is totally different. The deliberate electric energy input is used to
disturb the placement of the electrically charged particles within the
molecular structure. This molecular displacement changes the electrical
characteristic from that of an insulator to that of a conductor or vice versa.
The required energy level is very low. In addition, a sustained signal is not
required for components such as SCRs, triacs, and logic gates because
these types are self-latching. Most random electrical noise signals are of
the momentary low-energy type. Since it is difficult to separate deliberate
signals from random noise, the devices are thereby more susceptible. This
is cause for special concern regarding the electrical environment and
possible need for noise rejection measures. See sections 3.4, 3.4.1, 3.4.2,
and 3.4.3.
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.
Comments: 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 that 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.
See also section 3.5.2.