Electrical Noise and Wiring Practices

DOE-HDBK-1132-99

Consider use of " ark board"concept for management of alarms.

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.

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