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Grounding standards for broadcast
Each of the standards refers to the concept of single-point grounding (SPG), which is absolutely essential. The variances in the standards arise over subtle nuances in how this is achieved. Regardless of small differences, the SPG concept is crucial. The rise in ground potential that results from a lightning strike is manifest as a wave or surge that flows outward as it is dissipated. With a single-point scheme, all metallic objects are forced to rise and fall in potential together. While protection of equipment is very important, the failure to minimize the ground potential rise risk can also subject personnel at a facility to a rise in potential. The adverse biological effects of such exposure are well documented.
With the adoption of Rev G, we find that the maximum ground resistance has been set at 10Ω, similar to Department of Defense requirements in Military Handbook 419A. Portions of Motorola's standard take this a step further and discuss maximum resistance at 5Ω. In the case of Rev G, towers and their associated structural integrity is the primary concern, while the R56 standard is more inclusive, also covering the associated transmission equipment located at the facility. Ideally we would like to have the resistance to earth at 0Ω, but since we are stuck in an imperfect world, a finite resistance will always result.
Certainly the most common earth ground scenario in practice today is a single ground rod driven by an electrician when electrical service is installed at a facility. From Military Handbook 419 we can determine the resistance to ground in ohms of a single ground rod by the following:
In this equation ρ is the soil resistivity in ohm-cm, l is the rod length in centimeters, and d is the rod diameter also in centimeters. As an example, consider clay soil, where the resistivity varies from about 200 to 15,000Ω-cm, a single 10'-¾" rod would result in an earth-to-ground resistance typically ranging anywhere from 0.7 up to 50Ω. If the structure is located on rock where the resistivity can range from 50,000 to 1,000,000Ω-cm, then the resistance to earth could be greater than 3,300Ω. So unless you are located in the ocean or a marsh, which of course breeds another set of issues, a single ground rod comprising your single point ground is really insufficient to protect your facility.
Under Rev G we see an increase in the number of ground electrodes over that specified in Rev F. The previous version specified three ground rods for a self-supporting structure, while Rev G specifies six. For guyed towers we have seen an increase from two to three rods at the base, and monopoles, addressed for the first time, require six symmetrically spaced rods around the base with a minimum 20' separation between them. Note that these are minimums, and under certain soil conditions may not be sufficient to address proper mitigation. The standard of course covers this by requiring tower owners to verify ground resistance at or below the 10Ω value already discussed.
While we are in the lull before thunderstorm season, be sure to take stock of your grounding situation and make repairs or upgrades. Yes I know it is a long walk out to the guy anchors, but include them in your inspection. Foundation work is always much more expensive than swapping out a stack of NIC cards.
While progress certainly has been made recently in addressing lightning strikes and resultant damage, standards typically specify minimum requirements. I am sure everyone agrees that each individual station is distinct. While standards provide an easy cookie-cutter approach, you should in no way limit yourself to the absolute minimums. The small investment in time and expense now could possibly reduce heartburn in a few months.
Ruck is a senior engineer with D.L. Markley and Associates, Peoria, IL.
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