Ground Zero
Lightning and the millions of dollars of annual damage it causes to telecommunications facilities creates a highly visible mental image-there’s a big flash, followed by a loud noise and visible charring of unprotected equipment. One of the first steps in effective lightning protection, however, deals with the invisible-the geotechnical conditions below ground level at the site.
The same soil reports that are necessary for competent tower construction are equally applicable to competent grounding of the tower and the equipment that uses it. Required procedures include boring to appropriate depths, performing soil and rock analysis to establish resistivity, identifying water tables and climatic conditions and other factors. Just as the absence of this information can void a tower manufacturer’s warranty, lack of preplanning related to grounding can also void the warranty on the communications equipment.
Betty Robertson, E.E., president of Torrance, CA-based Lyncole XIT Grounding, has been designing and installing grounding systems for nearly two decades. Before starting her own company in 1985, Robertson worked as an engineer at Continental Telephone, which she credits as one of the leaders in developing site protection and grounding protection for telephony equipment and contributing to the creation of industry standards. After her work with Continental, Robertson joined Hughes Aircraft at its Torrance facility. Hughes was using XIT as its standard grounding system. The XIT system incorporates exothermic welding to avoid an increase in resistance over time that can occur with compression fittings that may corrode or loosen. (See photo on page 26.) The rod’s metallurgy is also designed to generate moisture to maintain contact.
“I said, ‘Wow! It answers every engineering question I ever had about grounding,'” Robertson said. “It makes its own moisture, it doesn’t need maintenance, it’s all-copper, and there’s no dissimilar metals.” When the XIT patent became available for sale, Robertson seized the opportunity and, supported by her degrees in engineering and business, formed Lyncole. The company now employs about 35 people, including 10 full-time engineers, and has been designing and installing systems for sites ranging from small government and service provider operations to those of Fortune 500 telecom companies. The company functions both as an engineering consultant and as a manufacturer of electrolytic grounding systems. Robertson noted that the engineering staff still outnumbers the sales force, “So you can see where my heart is,” she said.
Soil descriptions Robertson concurred that geotechnical surveys are crucial to developing an effective grounding system. “Generally, gravel or loosely packed soil is much higher in resistance and, particularly in the telecommunications market, companies like Lucent, Nortel and Motorola will tell their buyers ‘If you don’t have a 5V or less ground, we’re not going to warranty our equipment, and if you ask for a variance or a waiver on that, then we may go to 10V, or we may not insure it to the same level of replacement.'”
“It’s becoming extremely important to know and anticipate, or predict, what your ground is going to be,” Robertson said.
“The 5V or less can be predicted with calculations very closely. Our group, with good soil resistivity readings, can do that within about 2%. So we can tell you whether your installed system is going to be at 4.5V or 4.7V. So we’ve gotten pretty accurate at that over the last 15 years,” Robertson said. “It’s just now becoming extremely important to the carriers because all of a sudden the equipment manufacturers are recognizing that ‘When my ground resistance goes up, my equipment operates in a stressed mode, my equipment is noisier, it fails earlier and if there’s a lightning strike and the lightning gets into the equipment, it’s destroyed.’ So each level of problem gets higher with the incident.
“But the incident only gets into the equipment if the ground resistance is above 5V” Robertson said. “By looking at soil resistivity and the geotechnical reports, or the soil characteristics, you can calculate very closely the results of putting in one ground rod, or electrode, or 10. Sometimes it takes that many, depending on what the spacing should be. That’s where our consulting comes in. We do designs and tell them how many rods it will take, what spacing and how deep.”
Lyncole performs calculations assisted by computer programs that have been customized to reflect its grounding equipment standards and experience. “All of the standard grounding systems would assume a 3/4″ steel and copper-coated rod,” Robertson said. “For our system we have 30 years of test data. Because I bought the patent from the inventors, we have all of their research they had done to qualify the rods and get them through UL listing. I feel pretty lucky at having all of that history to draw on, and I can use it to literally save the client money.
“Say we recommend four rods, and you’ll get to 5V; very seldom could you get to 5V with three rods. The spacing is extremely important, and the depth, but you need soil resistivity and what I call a soil profile. It’s like a doctor’s exam: You need to know everything you can know about the soil and what you’re going to get when you auger that hole into the ground 10 feet, and 20 feet, and 40 feet down,” she said.
Robertson noted that compaction is generally better as depth increases, and better compaction allows current to dissipate more effectively, but moisture is also important.
“It [current] has to have a medium to travel through that soil,” she said. Unsatisfactory geotechnical reports should evoke the same business decision for grounding concerns that they do for inability to support a tower, Robertson said. If the report is not good-move the site. “Generally, if you can’t get the foundation support because of the soil, you probably can’t get a good ground. You may be able to drive pilings so your equipment doesn’t slide down the hill, but you’re not going to be able to get a very good ground because you need that compaction. It can’t be loose rock or a mudslide area that is going to destroy a grounding system or prevent a grounding system from working at its 5V-design criteria,” she said.
Robertson also noted that soil resistivity measurements should be taken in at least three different directions at several spacings on even small sites. More probe data allows designers to provide the most effective design and compensate for variations of soil resistivity that may vary from clay to adjacent limestone.
Safety factors In conjunction with the soil profile, the nature of lightning strikes where the site is to be located affects the design of an effective grounding system, Robertson said. “Basically, the frequencies of lightning are generally dissipated or reflected at somewhere between 18 and 24 inches down. So it’s the first few feet of your grounding system that works during lightning, or is most effective. There are many places in many countries that use five-foot ground rods because they feel that lightning is the only thing they need to protect from. … In the United States, eight feet is the minimum, according to the national electrical code. Generally we, engineers in general, have zeroed in on 10 feet because we have added in a safety factor. It’s about five feet below where there would be disturbed earth.
“In many cases we go to 20 feet deep because the soil is more conductive. You get a little closer to the water table so there’s a little more moisture. There’s more conductivity, and when you have to auger a hole to put the rod in place, you’re only setting up that drill machine up once to go down 20 feet, rather than going down 10 feet and using three 10-foot rods to do the same thing that one 20-foot rod would do,” she said.
Robertson said Lyncole prefers to do soil testing to a depth of 60 feet. “Generally, we like to do soil resistances at five, 10, 20, 30, 40, 50 and 60 feet deep. When a geotechnical firm does the soil resistance, we get the cores of dirt they dig out as deep as they will go. That’s what allows us to perform our best for the customer.”
Fill materials A standard part of Lyncole’s grounding systems includes fill material to surround the grounding rods, Robertson said. The company uses a proprietary modified version of bentonite clay (driller’s mud) as backfill material for the systems.
“In driller’s mud, there are polymers added to that to prevent water from seeping through it, but we want that backfill material to absorb the water as it would do naturally and also release it. As our rod produces moisture, we want the backfill to absorb that moisture and replace it if the area has become dry due to drought.
“Unlike driven rods, we can put the active XIT system in underneath a building-and it can be an old building, 20 or 30 years old. So we core through the concrete, and of course it’s very dry under there. So by putting the clay, in a slurry form, under there, it fills in all the cracks and crevices that either the rock or auger have left, so you get excellent rod-to-earth contact. It never fully dries out, and with the XIT system replenishing that moisture on a regular basis, you always have that a current-carrying solution to earth,” Robertson said.
Grounding design When planning rod spacing, Robertson noted that one grounding system should be located as close to the equipment as possible-for instance, at the entry of tower cables into the service structure.
“You want to have one rod directly under where they make that 908 turn,” she said. “The ground bus bar should be directly connected to earth. You want to have a straight path for that lightning surge to go to ground, rather than following some of that low-resistance cable into the building or into the equipment.
“The spacing between what you want to protect and the ground system should be as close as practical. The spacing between rods, to take the most advantage of the length of the rod, should be twice the length of the rod,” Robertson said. The total available area of the site should be evaluated to maximize the spacing to achieve proper resistance. (See photo on page 24.)
Evaluation The initial ground system test should be done before the system is connected to utility power, Robertson advised. “You have to have a benchmark.” There is one meter that will test the ground system after it’s energized, she said, but reliable readings are best obtained ahead of time.
Robertson noted a case where a single, 10-foot driven rod tested out at 65V with the neutral bond disconnected. With the utility neutral connected, the resistance dropped to 2.5V.
