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 MIL-HDBK-419A
1.3.3.5.9 Protection of Underground Cables.
Protect against direct lightning strikes to buried cable by installing a guard wire above the cables or
a.
cable duct. A 1/0 AWG bare copper cable laid directly over the protected cables as shown in Figure 1-35(a) is
recommended. At least 25.4 cm (10 inches) should be maintained between the protected cables and the guard
wire.
b.
For a relatively narrow spread of the cables, 0.9 meters (3 feet) or less, or for a duct less than 0.9
meters (3 feet) wide, only one guard wire cable is necessary. For wider cable spreads or wider ducts, at least
two 1/0 AWG cables should be provided as illustrated in Figure 1-35(b). (Since the guard wire and protected
cables are embedded in the earth, the applicable cone of protection is not known.)
1.3.3.5.10 Buried Guard Wire. Experimental use of a buried guard wire embedded in soil above and parallel to
buried cable runs not enclosed in metal conduit has provided effective attenuation of lightninginduced
transients. Use of the guard wire is recommended for protection of buried equipment lines not enclosed in
metal conduit. Bare 1/0 AWG copper wire has provided the most effective protection during experimental use.
To be effective, the guard wire must be embedded in the soil a minimum of 25 cm (10 inches) above and parallel
to the protected cable run or duct. When the width of the cable run or duct does not exceed 0.9 meters (3 feet),
one guard wire, centered over the cable run or duct, provides adequate protection. When the cable run or duct
is more than 0.9 meters (3 feet) wide, two guard wires should be installed. The guard wires should be spaced at
least 30 cm (12 inches) apart and be not less than 30 cm (12 inches) nor more than 45 cm (18 inches) inside the
outermost wires or the edges of the duct. To be effective, the guard wires must be bonded to the earth
electrode subsystem at each terminating facility. Exothermic welds provide the most effective bonding. The
requirement and need for underground guard wires shall be determined by the project and civil engineer and
shall be determined on a case and location basis dependent upon the priority of the circuit and the degree of
lightning anticipated.
1-57
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