This article was posted 11/29/2005 and is most likely outdated.

New grounding standard set for Antennas
 

 
Topic - Grounding and Bonding
Subject - New grounding standard set for Antennas

November 28, 2005 

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New grounding standard set for Antennas

Source: http://www.wirelessestimator.com

With the introduction of Revision G of the TIA/EIA 222 standard for antenna supporting structures and antennas, effective January 1, 2006, the standard for protective grounding will increase the minimum number of ground rods required and set a maximum number of ohms of total ground resistance. 

Ground rod quantity increases
In Revision F the minimum number of ground rods specified for a self supporting structure totaled three; the new standard requires six grounding electrodes. Guyed structures saw an increase from two to three rods at the base. As in Revision F, the new standard requires a ground rod at each anchor.

Monopoles were added to Revision G and require six ground rods installed symmetrically around the base of the structure with a minimum 20' spacing between each one. A minimum of three leads must be attached symmetrically to the base.

Ten ohms set as maximum
The new standard also addresses ground resistance values, stating that the owner will verify that the total resistance will not exceed 10 ohms. A resistance level had not been previously identified. Some site specifications require a maximum of 4 ohms. The total resistance of the structure's primary grounds as referenced to remote earth should be measured or calculated in accordance with the Institute of Electrical and Electronics Engineers (IEEE) Standard 142-1991, the standard states.

Ten foot rods set as minimum
The previous revision identified that the minimum ground rod should be a 5/8" diameter galvanized steel rod with a lead of not smaller than #6 tinned bare copper wire. The new standard requires the electrodes, as a minimum, to be 5/8"x10' metal rods constructed of copper, copper clad steel, galvanized steel or stainless steel alloy. The minimum embedment depth of the rods must be 10'. All electrodes must be electrically connected to the structure; however, all electrodes need not be interconnected, according to Rev G.

The standard cautions that for soils with resistivities less than 50 ohm-m, copper or copper plated grounding electrodes may contribute to galvanic corrosion. Under these conditions it is stated that grounding electrodes may be replaced by grounding anodes or other corrosion control methods. The grounding standards also identifies that special considerations shall apply for AM tower installations.

Minimum lead wire size increases to 2/0 solid
The new grounding standard requires connections between a structure and grounding electrodes or grounding anodes or connections between electrodes to be compatible with the electrodes and be accomplished by leads not smaller in surface area than 2/0 solid. This would allow the use of 2/0 or 4/0 tinned concentric strand. Some carriers already require 4/0-19 tinned.

According to Curt Stidham, Manager, Product Development & Application Engineering for Harger Lightning & Grounding, "When you are designing a ground electrode system for a wireless site you should not only be concerned with achieving a low resistance ground to remote earth, but most importantly you want to provide potential equalization between all the equipment and non-current carrying structures. You also want to try and divert energy from a lightning stroke away from the equipment shelter; tower ground radials are often used for this purpose."

The most commonly used electrodes, according to Stidham, are 5/8"x8' copper clad ground rods. He says that the lead and ground ring wire is most often #2 tinned solid copper. Some tower owners specify ¾"x10' rods and #2/0 or #4/0 stranded bare copper ground wire.

Depending upon the site's soil conditions enhanced electrolytic ground rods or plates are oftentimes specified. When designing the lightning protection ground electrode system, Stidham says, it is important to remember that other systems buried in the earth may be affected or damaged by the energy from a stroke.

Most grounding specifications will call out for exothermically welded connections to provide the lowest inductance path for high frequency lightning surges; they also eliminate the concern of deterioration due to corrosion.

The new standard states that alternate or special grounding systems or special grounding requirements should be included in the owner's procurement specifications. It also requires all electrically active equipment and appurtenances supported by a structure to be connected to a secondary ground.

When available this August, ANSI/TIA/EIA222-G can be obtained through www.tiaonline.org . It's a wise investment for companies that earn their living from the design, manufacture, erection and maintenance of communications towers and accessories.
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Comments

  • "According to Curt Stidham, Manager, Product Development & Application Engineering for Harger Lightning & Grounding, "When you are designing a ground electrode system for a wireless site you should not only be concerned with achieving a low resistance ground to remote earth, but most importantly you want to provide potential equalization between all the equipment and non-current carrying structures."

    And this isn't necessarily so easy at the higher frequencies lightning often exhibits. Low impedance at 60Hz, which is what we're concerned about in substation grounding practice is not the same as low impedance at lightning frequencies. That's it's important, as Mike has so often pointed out, to think in terms of diversion and bonding.

    "You also want to try and divert energy from a lightning stroke away from the equipment shelter; tower ground radials are often used for this purpose."

    Yes, but without the benefit of a controlled experiment to test the efficacy of such a measure, it's really tough. I wonder if one of these days we'll find that radials conduct stroke current from lightning hits outside our area of protection back up into the equipment we're trying to protect. That's why multiple measures are so important: we're kind of working blind, here.

    I understand that this isn't a useful comment for the guy who has to design the system and sell it to the owner, but it's the sort of thing that theorists think about.

    "When designing the lightning protection ground electrode system, Stidham says, it is important to remember that other systems buried in the earth may be affected or damaged by the energy from a stroke."

    The question, I suppose, is that it's not so clear just what should be done about this. My own research showed that, well, IEEE Standard 4 lightning impulses through sand to a buried piece of half-inch EMT conduit takes little peck marks out of it, sort of like you'd banged the pipe on the point of a nail. I think most direct-buried-type telephone and electric lines as presently made would have no trouble withstanding this sort of damage completely unless the line is located within a very short distance (we were working in the range of a few feet) of a ground electrode or conductor. Damage to a gas or water line would be more subtle, because a peck from a lightning stroke could poke through a thin layers of corrosion protection, and then you could get a hole rusted through the thing. But poor backfill is probably more likely to cause such difficulties.

    There are two exceptions to this comfortable state of affairs. One is a surprise that you might get if you're protecting a high power transmitting antenna (thousands of watts, not cell phone equipment) or a high-voltage power line. The line or antenna gets hit, lightning flashes over the insulators, goes into the ground electrode, and thence to the nearest good ground like a buried pipe. Pipe withstands a peck out of its periphery, which is no problem.

    But the lightning stroke may also ionize a conductive path from the protected antenna or power line through the air, over the insulators, through the ground, and then right up to the pipe. If substantial rf or AC is available from that antenna or power line, it may find a path along this ionized path and into the side of the buried pipe. And it can continue to chew away at the side of the pipe as long as the ionized path is maintained by this fault current.

    I don't know if this could occur with rf currents from high-power antennas--we didn't try it--but we know that with high-voltage AC power lines, the results can be devastating, with a big smoking hole burned through the side of the pipe.

    I forget what the other exception is. Mercifully.


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