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 MIL-HDBK-419A
1.9.1 Design Practices.
a.
Sensitive data and instrumentation facilities should be located as far as possible from high voltage
(66 kV and above) transmission lines.
b.
The routing of data and signal lines should be perpendicular to main power lines wherever possible.
Where parallel runs cannot be avoided, maximum separation must be maintained. In many instances, routing of
the data and signal cables in ferrous conduit may be necessary.
c.
Distribution feeders to the facility should be routed perpendicularly to high voltage power lines, if
possible. In any event, long parallel runs between distribution feeders and the main power line should be
avoided.
d.
Where overhead distribution lines are necessary, pre-assembled aerial cable should be used in
preference to open wires. Since the conductors of pre-assembled aerial cable are twisted, the associated
magnetic field is greatly reduced.
e.
All internal distribution power conductors near sensitive test and measurement facilities and
carrying more than 5 amperes should be twisted. A suggested rate of twist is one complete twist for each
length equal to approximately 25 times the diameter of the insulated power conductor.
f.
Metallic enclosures should be used for power conductors wherever possible to take advantage of the
shielding they offer. In order of preference, the types of enclosures recommended are:
(1) Conduit. From the standpoint of noise reduction, rigid steel conduit is the most effective
enclosure for power conductors and should be used wherever practical. Electrical metallic tubing (EMT) and
rigid aluminum or copper conduit provide effective electrostatic shielding, but their magnetic shielding
properties are at least an order-of-magnitude poorer than rigid steel conduit.
(2) Cable armor. Armored cable is sometimes used in lieu of conduit and individual insulated
conductors. The armor provides an effective electrostatic shield but is not as effective as rigid steel conduit
for magnetic shielding. Steel armor is preferable to aluminum or bronze.
(3) Flexible conduit. Because of its construction, standard construction grade flexible conduit is a
poorer electrostatic shield than either of the above and provides considerably less magnetic shielding than rigid
steel conduit. It is recommended that the use of flexible conduit be restricted to short lengths and only where
required to absorb vibration or to permit position adjustment of the equipment or device served.
(4) Wireway or cable tray. Wireways, which are rectangular sheet metal duct-like enclosures, and
cable trays are not nearly as effective for electrostatic or magnetic shields as rigid steel conduit. Unless the
wireway or cable tray is made of a ferrous metal and all discontinuities are carefully bonded, its use for the
shielding of power conductors should be limited.
1-171
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