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 MIL-HDBK-419A
Cost considerations. When a buried cable run is 90 meters (300 feet) or less in length, it is
a.
economically feasible to enclose the cable run end-to-end in metal conduit. When the cable run exceeds 90
meters (300 feet), it is normally more economically feasible to provide transient suppression at building
penetration and equipment level than to install the conduit. However, use of metal conduit provides effective
protection against induced transients, regardless of the length of the cable run. The conduit must extend from
building penetration to building penetration, or building penetration to exterior equipment termination.
Grounding of conduit. To be effective, the conduit must be electrically continuous and effectively
b.
bonded to earth ground at each end. If building entry plates are available the conduit should be peripherally
welded. No. 2 AWG bare copper stranded cable is suitable for the earth ground connection, and exothermic
welds provide effective bonding underground. Approved pressure connectors are suitable for use above ground.
For runs over 90 meters (300 feet), the conduit should be connected to earth ground at each end and every 30
meters (100 feet). The structural steel of antenna towers may be used to effectively ground the conduit
provided the total bond resistance from the conduit to the earth electrode system is 5 milliohms or less.
c.
Transient suppression for lines in metal conduit. Only one level of transient suppression is required
for exterior line/equipment interfaces to provide effective protection against induced transients conducted by
lines in metal conduit. The one level of suppression may be located at building penetration or designed as an
integral part of the applicable equipment. The one level of suppression may consist of a single suppressor
connected line to ground, or two resistors connected in series with the external line input and a silicon
avalanche diode connected between the junction of the two resistors and earth ground or equipment case
ground, depending on location of the transient suppression.
Amplitude of transients on external lines enclosed in metal conduit. The number of lightning
d.
generated transients occurring on external cables will not change as a result of enclosing cable runs in metal
conduit. However, the voltage and current amplitudes will decrease a minimum of 90%. Therefore, Table 1-16
can be used to determine the number and amplitude (voltage and current) of transients that are projected to
occur on externally exposed lines, enclosed in metal conduit, in high-lightning incident areas.
1.3.3.5.22 Transient Suppression. In order to provide effective equipment protection against lightning
generated transients, externally exposed lines must have transient suppression installed on each end where the
line directly interfaces electrical/electronic equipment. This requirement applies in all cases when the
withstand level of the interfaced equipment is below the transient levels projected to occur at the
line/equipment interface. As previously noted, transient amplitudes projected to occur on lines enclosed end-
to-end in electrically continuous, ferrous metal conduit are only 10% of the transient amplitudes projected to
occur on lines not enclosed in metal conduit (Table l-16). Primarily because of insertion losses and impedance
mismatch, transient suppression is not currently available that is satisfactory for installation on externally
exposed rf coaxial lines at building penetration when the lines carry signals above 3 MHz in frequency.
Therefore, all protection for these line/equipment interfaces must be designed as an integral part of the
equipment. The most effective design for equipment protection is provided by installing a high energy level
transient suppressor at building penetration (on all lines that carry signals 3 MHz or less in frequency)
connected line to earth ground, and including low-energy suppression as a part of integral equipment design.
Figure l-40 depicts typical transient suppression at the facility and equipment level for both coaxial cables and
single wires or pairs. Suppressors installed at building penetration should be located in the junction box that
first terminates the externally exposed lines after building penetration.
1-75
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