Don't Let Lightning Strike You Out

When you were a child, your parents always said the last place you should be during an electrical storm is under a tall tree because lightning would probably hit it, and it could fall on you. Now that cell towers climb higher than most trees, they are targets for lightning bolts. Good news for trees, bad news for wireless carriers.

A lightning strike to your cell tower is not always a big deal. Most of the time, you won't know it hit unless you have inadequate protection in place. Grounding schemes and lightning-protection equipment is critical to your sites' survivability, and with tower-mounted electronics becoming more popular, extra protection on the tower is necessary, too.

Lightning is a threat to any wireless carrier, but when your network is in one of the most lightning-prone areas of the county, it can be your worst enemy. While other carriers are worrying about an occasional strike, Powertel's towers are hit repeatedly by direct and indirect lightning. The company's network includes middle and north Florida and parts of lower Alabama, high-moisture areas built on lightning magnets such as sand and quartz, said Jim Coovert, Powertel vice president of technical operations.

"In these areas, lightning is a constant threat to multiple sites," he said. "A typical radio base station (RBS) costs 94,000 dollars. When you have grounding violations, it can result in major capital loss."

For this company, well-engineered grounding schemes are critical to network reliability. The company goes above and beyond the National Electronics Commission's (NEC) code requirements to protect the safety of its personnel and ensure its equipment survives some of the worst lightning attacks. Although manufacturers build a certain amount of surge protection into their equipment, Powertel has its own plan to protect it.

"The voltage spike comes in so fast that it will blow a circuit breaker and do a lot of damage to the component," he said.

Coovert explained that the key to protecting people and equipment is designing a grounding scheme into your company's standard site layout. Grounding provides a low reference point approximately equal to the potential of the earth for sensitive equipment. Powertel's elaborate site design goes into detail about everything from choosing a site to preventive maintenance. The process begins with testing the soil's resistivity, moisture and composition with an earth tester probe, or megger. Powertel's geo-technical consultants perform resistivity testing along a 30-foot traverse, with probes every 10 feet. In most cases, Powertel requires the soil resistance to be less than 5W before it will build on the land.

"If the resistance is higher, we can't get a good earth ground," he said.

If the megger test is above 5W, the company has some decisions to make. Ideally, it will choose another area for the site. However, Coovert said sometimes multiple site tests come back above 5W, so it ends up being a choice between the lesser of two evils. If the test is above 10W, Powertel will go with a chemical rod solution. Coovert said chemical rods bring down resistance and still give a quality ground, but they come with a much higher price tag than ground rods. Other times, the company must take the risk of a lightning strike.

"There are some cases where we just say, 'If it happens, it happens,'" he said.

In most cases, Powertel's sites are 5W or less, and the next step is to build a heavy ground ring. The number of grounding rods, their spacing and their placement greatly influence the company's ground-system design. Powertel requires sites to have 10-foot, copper-clad steel grounding rods, and it spaces them 10 feet apart from one another. The ground ring is a minimum of 18 inches outside the tower and the RBS pad. No sharp bends are allowed in the ring; any change in direction must be a gradual curve of at least 30 degrees.

Powertel's ground ring ties into almost everything on the compound, including the RBS ground bar, the power/telco box enclosure, the arrestor bracket, any metal fence post within six feet of the ground ring, the transmission line entrance hatch, metal fuel storage tanks, exterior ice shields, the RBS support frame, the monopole bottom ground and the insulated ground bar. Its coaxial cables are grounded on a 52-inch ground bar isolated from the tower and connected to the ground ring.

Powertel requires vertical coaxial cable that runs longer than 200 feet to have a grounding kit installed at the top and bottom of the tower, as well as a kit every 150 feet on the tower toward the ground until the cable is less than 150 feet from the bottom. Powertel uses a combination of Huber+Suhner and Andrew surge arrestors.

Technicians check Powertel's grounding system every time they visit a site, usually once a month. They check for corrosion, make sure everything is tied into the ground ring and ensure that co-locating companies did not damage its grounding scheme. In co-locating situations, other companies can create separate ground rings or tap into an existing ground ring. Powertel always keeps its ground rings separate from those of its competitors to avoid possible damage.

"We have to be very careful how other companies do their construction," Coovert said.

PROTECTION IN HIGH PLACESA well-engineered grounding system has been the traditional form of lightning protection, but as interest grows in placing electronic equipment on towers, carriers must take steps to protect it. Rod Nelson, AT&T Wireless senior vice president & CTO, said in the past antennas were the only equipment AT&T mounted on towers; all the electronics stayed on the ground. If they were damaged during a lightning strike, it was easier to change them out. Nelson currently is wrestling with new technology that requires carriers to mount electronics on towers.

"There are a lot of benefits to things like towertop low-noise amplifiers," he said. "We would like to be able to provide better quality service, but the reservation has been putting this bit of electronics at the top of the tower."

AT&T Wireless has placed electronics on towers in some areas. When it does, it requires redundancy from its vendors. The devices it installs are designed to fail in a certain predictable way when hit by lightning so the antenna system will continue to work, though not with the same quality. The signal still gets through the antenna down to the base station, but without the positive effects of properly working amplifiers.

"That is a rather crude form of redundancy, but you don't have to climb the tower to be back in service," Nelson said. "You have to climb the tower to be back with the same quality of service, but you might argue that if it takes a day to do that, it is not as bad as taking a day to get service restored."

In other areas where lightning strikes are frequent, the company has installed extra antennas. Technicians change antennas out at the bottom of the cell site without having to do any work on the tower.

Powertel's Coovert said protecting electronics on the tower from lighting is an issue with smart antennas, too.

"If you have electronics up in the sky, you are asking for it," he said.

If you use smart antennas, he suggested that you use a combination of surge arrestors at the top of the tower, just as you would at the bottom. Powertel does not place technology on the tower, Coovert said, but it is not because he worries about the equipment's reliability.

"Cell-site technicians don't climb towers, so if you have all your electronics up there, there is a maintenance issue," he said.

Marty Cooper, ArrayCom CEO, explained that the brains of smart antennas are down in the base station. Between two and 12 antennas sit on the tower in a smart-antenna system, and they are smaller and less susceptible to lightning damage than regular antennas. If one antenna should go down in a lightning zap, the rest will pick up and remain operative.

"Losing one, two or three elements is not at all destructive," he said. "It gives you a tiny reduction in performance."

Cooper noted that there are advantages to towertop electronics. You can save money in the cost of cables from the ground to the top, which could make up for losses from lightning hits.

Katharine Petersen, Metawave marketing communications manager, said Metawave's smart antennas have no active electronics at the top of towers. Its smart antenna is an add-on to the base station that increases capacity, so although it has different passive antenna panels, all the electronics stay at the bottom.

"If lightning does strike, it would be the same as with any other antenna," Petersen said. "It is a passive unit, so it would be protected by the lightning arrestor."

She said devices such as superconductors and towertop amplifiers are more likely to house electronics on the tower.

Blake Isaacs, Superconductor Technologies director of marketing, said his company has developed some prototypes for tower-mounted installations of its superconductor-based, receive-filter amplifier systems. The company includes two levels of lightning protection on units that go on towers. One is at the RF input level, and the other is at the power supply.

"If there is a lightning strike to the antenna, energy comes directly into the inputs of our product, so we put protection on those inputs," he said. "There also is a possibility that the energy of a strike anywhere on that tower could cause a surge into the power supply to the unit."

Isaacs said mounting electronics on towers is common internationally and is slowly picking up interest in the United States. The hesitation among carriers does not come from fear of lightning, but rather from insurance and liability risks, the hassle of trying to schedule tower crews, and the high cost of making equipment rugged enough to stay outdoors on a tower.

Sometimes, Isaacs added, electrostatic discharge can cause damage that will become a late failure. Things continue to work at first, but they break weeks later.

"It is generally good practice to have surge protectors and lightning protection in place to prevent damage from those kinds of strikes," he said.

Great lightning protection on the ground and on the tower may not make your cell site a safe place to go during a storm, but it will make your network reliable in some of the worst weather.

Andrew recently introduced two grounding kits for installation with 21/4" Heliax coaxial cable to protect radio equipment from lightning strikes. The SureGround grounding kit has a self-securing ground strap that eliminates the need for attachment tools and reduces installation time. The kit includes a pre-tensioned, solid-copper, clip-on grounding strap that ensures proper surface-contact pressure between the strap and the outer conductor and prevents damage from over-tightening. Once in place, the kit provides protection against lightning strikes up to 125kA. SureGround kits are available for 7/8", 15/8" and 21/4" Heliax coaxial cables.

The SureGround Plus grounding kit combines a wraparound grounding strap with a preformed rubber weatherproofing boot. The rubber boot replaces time-consuming taping and provides weatherproofing protection. The appearance of the finished system is suitable for areas with tough zoning requirements. The installer removes a short length of cable jacketing, snaps the wraparound grounding strap in place, slips the rubber boots into position and secures it with clamps. SureGround Plus grounding kits are available for 7/8" and 21/4" Heliax coaxial cables.

Now that your site is well-grounded, don't forget to consider how a lightning strike on the wired telecommunications network could affect your wireless network. When the wireless and wired networks connect, several circuit-protection issues come into play, said Gary Wiseman, Raychem marketing director for circuit-protection products.

Wiseman separates hazards into two classes: overvoltage hazards and overcurrent hazards. Overvoltage hazards result from direct or induced lightning. You can protect your network from overvoltage by placing combinations of thyristors or diodes between the telecommunications line and the ground. When they sense an overvoltage, they go from being resistive to being conductive, shunting energy to the ground instead of letting it flow to the equipment. Thyristors and diodes sit directly on the tip-end ring pair that connects the base station to the trunk line.

Overcurrent hazards are caused when the electrical power system and telecommunications system come into contact with one another. Overcurrent protection devices guard your network from a power cross by going from low resistance to high resistance, similar to a fuse, to stop the current flow and protect the devices on the other end. Wiseman suggested that you check into self-resetting devices to protect your network from overcurrent hazards. After these fuses become high resistance, they return to their normal operating state when the fault goes away. They don't require manual intervention, unlike fuses that need to be replaced.

"When a base station is remote from anything, the self-resetting nature reduces the number of service calls required and improves the reliability of the equipment," he said. "Rather than a fuse blowing and taking down the equipment, it would protect it and reset itself."