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Alarm Management
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Design Considerations - index
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Lightning Protection for Instruments


DOE-HDBK-1132-99
Consider use of " ark board"concept for management of alarms.
d
4.5
Electrical Noise and Wiring Practices . When designing wiring systems, consider the
following:
Electrostatic interference occurs when a conductor acquires an unwanted
electrical charge from an adjoining electrical field. An electrostatic shield, such
as braided copper wire or aluminum foil wrap, is effective in reducing interference
coupling. Because the shield has high capacitance to the wiring it encloses, the
shield should be properly grounded or it will become a means of coupling
interference to the signal. Conduit normally should be grounded at support
points because the conduit is likely to be at a lower potential with respect to
ground than any external wiring. The thickness of the shield is less important
than shield discontinuities, such as seams and holes. For this reason, flexible
conduit with openings between segments is not desirable.
Electromagnetic interference is created when a conductor is located within a
varying magnetic field. This may occur when a conductor carrying an electrical
current is adjacent to another current-carrying conductor. Signal circuits are
generally in the 420 milliamp range and are susceptible to distortion. Of
particular concern is the interference caused by inrush current to a solenoid
conductor's capacitance. The inrush current appears as a high frequency spike.
This spike is magnetically coupled into all adjacent conductors via inductive
coupling.
Twisting of conductors is effective in rejecting interference induced by magnetic
fields. Twisted pairs should be used not only on vulnerable circuits for
protection, but on offending circuits as well, for prevention. As the strength of
the magnetic field near a wire varies inversely with the distance from the wire,
physical separation of signal circuits from control or power circuits is
recommended. To a lesser degree, steel tray or conduit provides protection from
magnetic interference. The steel used in rigid conduit and tray construction is
not normally of sufficiently low reluctance to be very effective.
II-46


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