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FM 11-487-4/TO 31-10-24

Chapter 7
INSTALLATION PRACTICES FOR LIGHTING PROTECTION SUBSYSTEMS

Section 1: Introduction

    Chapter 7.1 Components of the lightning subsystem:
  1. Air terminals (lightning rods)
  2. Down conductors to the earth electrode ground system
  3. Surge arresters (transient voltage surge suppressors)
    Chapter 7.2 Direct-strike protection
  1. For antenna towers 100-500 ft, the critical area is a circle with a radius 3 to 4 times the height of the tower. The fact that a facility is within the zone of protection of an air terminal does not negate the other grounding requirements for the facility or equipment.
    Chapter 7.4 Air Terminals
  1. The points are tapered and the diameter is at least 1/2". Air terminals will extend a minimum of 10" above the highest point of the structure. Air terminals that extend more than 24" above the mounting base require a brace.
    Chapter 7.6 Down conductors
  1. For buildings or towers less than 75' high #2 AWG copper cable is used. Towers higher than 75' should use 2/0 AWG copper cable. Bends will not have a radius less than 8" or an angle sharper than 90 degrees.
  2. Down conductors are electrically continuous from air terminals to ground connections.
    Chapter 7.7 Surge Protection
  1. Lightning surge arresters are part of a series of devices and techniques that provide progressive protection against lightning-caused transients.

Section II. Specific Applications

    Chapter 7.8 Lightning protection for towers and antennas
  1. Air terminals will extend at least 18" above the top of the tower or top of the highest antenna mounted on the tower, whichever is higher.
  2. Use at least two down conductors to connect each air terminal to the earth electrode ground system. Avoid bends sharper than 90 degrees or with a radius less than 8".
  3. A self-supporting tower will be bonded to the earth electrode ground system at each leg of the tower.
  4. If a tower is within 20' of another structure, a common earth electrode ground system is required. Two bare 1/0 AWG copper cables using separate routes will be used to bond the tower's earth electrode ground system to the earth electrode system of buildings and structures that have an electrical interface with the tower. Any metal object within 6' of the lightning down conductor will be bonded to the down conductor.
    Chapter 7.9 Lightning protection for waveguide and cable
  1. Each run of waveguide is bonded to one of the down conductors leading from the air terminal to ground. This bond is required at the following points: 1) At the top of the tower where the waveguide bends to meet the antenna. 2) At the bottom of the tower, immediately above the vertical to horizontal transition point. 3) At intermediate points if the tower exceeds 200'. 4) Waveguide is also grounded at the entry port of the equipment building. 5) All waveguides within 6' of each other are bonded together. 6) Cables that enter a facility are grounded using the general procedures in figures 7-37 through 7-39 ….. Coaxial Cables, Control Cables, Power Cables, Telephone Cables
    Chapter 7.10 Lightning protection for outside plant
  1. The shields of all buried cables entering a facility are bonded at the building entry plate and to the earth electrode ground system. TO 31W3-10-13/FM 11-372-1 Outside Plant Splicing, TO 31W3-10-14/FM 11-372-3 Outside Plant Cable Termination.
  2. At each service entrance, all neutral and fault protection conductors, cable shields, and metal enclosures are bonded to the earth electrode ground systems.
  3. The basic grounding of a pole for lightning protection is shown in figure 7-44. Notes on figure 7-44 state: Drive ground rod and connect wire to ground rod at least 18" below surface.
  4. Figure 7-48 Section AA describes the positions of coaxial cable, telephone cable, power cable and ground conductors on a pole.
    Chapter 7.11 Lightning protection for perimeter fences
  1. Ground rods are required on metal fences at intervals of 100 to 500 feet. Additionally, ground rods will be installed on each side of every gate and at points 150' on each side of high tension crossings. A #6 AWG copper wire will be woven horizontally through the individual links of a fence. Fences that come within 6' of the site's earth electrode ground system and will be bonded at a minimum of two points.
  2. Ground rods are driven 2' of more from fence posts.

Additional information and site survey information can be obtained by contacting

POC: Mike Helms mike@lightningmike.com
Copyright © 2009 Lightning Mike