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## [TowerTalk] Tower grounding

 To: TowerTalk [TowerTalk] Tower grounding Red Sat, 15 Jul 2006 20:08:21 -0500
 ```TTrs; JC asked me a question, see following my response. I can answer regarding grounding for lightning protection, but will defer to others who know NEC much better than I do. JC Smith wrote: Hi Red, Just a quick question about those six ground rods. I'm told that the NEC requires a minimum of two ground rods located within two feet of the tower. This prevents having them spaced twice their depth. Are you talking about branching out from the rods at the base of the tower or are you just installing six rods equally spaced around and away from the tower? (Which would mean no rods within 2' of the base.) Thanks, JC Hi JC, The six rods spaced away from the tower are to dissipate lightning. That recommendation does not preclude other rods if NEC requires them. However, two rods within 2' of the tower will not prevent a large voltage rise on the tower in event of a lightning strike. We have to provide that lightning protection in addition to AC power safety. It becomes important at the tower because the tower and the antennas on it are typically the tallest structures in the immediate vicinity and are prime targets for lightning. A volume of earth approximately equal to a cylinder with a hemispheric bottom around a ground rod will, for want of a better word, saturate with charge from a lightning strike and the voltage will rise higher than what we want for protecting our electronic equipment. A charge accumulates in the capacitance of that volume and then leaks off through the conductivity. Earth has a dielectric constant greater than one; its conductivity is much less than that of the metal conductors we are accustomed to using. Think of that volume of earth as a leaky capacitor. The object of putting six rods equidistant around the tower is to achieve maximum capacitance; six capacitors in parallel. The seventh capacitor is the earth the tower base is in. Adding more rods inside that combined volume adds little lightning protection. If more protection is desired, it is necessary to add another ring of rods outside that volume. When using 8' rods, place the next ring of rods 16' beyond the first ring. They may be spaced 16' apart in that ring, which will accommodate 12 more rods, or fewer if one chooses to space them more widely in that ring. A reason for not spacing them at greater than 16' radius is to minimize the inductance of the radial wires connecting to the ground rods. This spacing is a widely accepted pattern to achieve minimum impedance, Z, from the tower to ground. That impedance is composed of resistance, inductance, and capacitance and the design must minimize the first two and maximize the third. An exact pattern might vary depending upon the wire used and the characteristics of the soil. The recommended pattern is generally good. Major exceptions are encountered in dry sand and on very rocky soil. There is an article in the latest QEX describing a method of measuring the conductance and permitivity of earth at RF frequencies. They vary with frequency. The article includes information about the physics of conductivity in earth. The conductivity is primarily an electrolytic conduction, not a metallic conduction. Dry or freeze the earth, and the conductivity drops to nearly nothing. The permitivity, on the other hand, is roughly the permitivity of sand, a principle mineral constituent of earth. Of course, that varies with different kinds of soil but it doesn't change much with drying or freezing. I am not aware of a NEC requirement for dual ground rods at the tower. I thought NEC was primarily concerned with AC power safety. I am aware that NEC requires all ground systems to be tied together. I am also aware that many communities and states require dual ground rods. My understanding is that they are trying to assure that there is one safety ground even if one fails, as ground rods or connections to them often do because they are typically installed with clamped connections and are never serviced or even inspected after that. There are several contributors who know the NEC much better than I. I post this to the list in hopes that one of them will provide additional information. 73 de Red _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
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