Don’t mess with Mother Nature
The issues surrounding lightning protection have changed little since Benjamin Franklin famously invented the lightning rod in 1752. While Franklin was looking to protect ordinary buildings from fire after lightning strikes, at 21st-century base transceiver station sites, the priority — apart from the obvious safety requirement — is to safeguard the sensitive electronic equipment housed in the site’s equipment building.
The consequences of a loss are not simply related to the cost of replacing the equipment, but also to the loss of service. When contemplating protection against these losses, the site’s transmission lines are a critical consideration, as they provide direct conductive paths from the top of the tower to the equipment below.
It is no surprise that electrical storms are so spectacular, as they are truly high-energy events. One-third of all recorded lightning strikes are rated between 20 and 100 kiloamps (kA). The effects of lightning have been well-researched and documented. It has been known to blast chunks of concrete from walls. Occasionally, a tower will be hit by lightning at its highest point — the antenna — which not only will “fry” the antenna but, without surge protection, can travel into the very expensive base station equipment housed in the equipment building.
Surge events are not only caused by hits to towers or antennas. At base transceiver station (BTS) sites, lightning strikes within a few hundred yards of a tower are just as dangerous as direct hits, as they may induce high-energy electromagnetic fields onto the feeder lines. The art then, is to design a system that protects the electronic equipment from a surge on the feeder lines, regardless of the source.
A properly grounded transmission system always will employ a minimum of three cable-grounding points. One is at the top of the main feeder line, within three feet of the top connector. The second is at the bottom of the tower-mounted section of the feeder line, and within three feet, but not on, the lateral bend away from the tower to the cable tray. The third is within three feet of the entry to either the radio cabinet or equipment building. Additional grounding points also must be added to the main vertical run of transmission line — one for each additional 200 feet of feeder cable.
The grounding kits used for this purpose can either clip onto the cable or be held in place with clamps and screws, depending on the requirements of the installation. Regardless of the method used, a good electrical connection is vital; otherwise the resistance between the ground wire and the feeder may limit the protection against a strike. Once correctly attached to the feeder, the grounding lead usually is connected directly to the tower metalwork or to a site grounding bus bar.
Fitting a grounding kit to a feeder line leads to a protection concern of a different kind — weatherproofing the grounding system. It is extremely important that the raw material in the cable is not exposed to the weather, as that could cause oxidation issues.
Further safeguarding of the sensitive electronics in the equipment room is provided by the insertion of electromagnetic pulse (EMP) protection devices close to the equipment, between the jumper cables and the feeder lines. Such devices often are quite compact — only slightly larger than the connectors. Together, the transmission line grounding kits and EMP protection devices minimize the risk of equipment damage after a lightning strike.
During normal operation, EMP devices — also referred to as surge protectors — do not influence the RF behavior of the antenna and cable systems; they activate only when there is a surge on the transmission line. Such devices fall into two broad categories: those that pass direct current (DC), and those that block it. The default choice is a DC-block device, but if power is required for tower-top components (such as boosters and amplifiers), a DC-pass device is selected.
Weatherproofing the EMP-protection devices is just as important as weatherproofing the feeder cable. All surge protectors that are mounted to the base of the tower, at the junctions, and at the jumper and feeder line exit and entry points should be weatherproofed.
Having selected the appropriate forms of protection to suit the base station’s configuration, designers need to consider the connections to the site’s grounding point. Every feeder run should have its own grounding. The grounding leads are connected to a bus bar at the base of the tower, which is connected in turn to a single ground rod, or a ground ring with multiple rods. The rods, which are generally 5 feet or more in length, are buried and disperse the energy into the ground.
A high-quality connection between the feeder’s outer conductor and the grounding kit is a basic requirement. High-speed fitting allows the installer to be productive, but this needs to be achieved without loss of quality, otherwise cable lifetimes can be reduced, or work may need to be redone.
Tools are available that make the grounding job both easy and fast. For instance, stripping tools are designed specifically to remove the jacket material for grounding so installers aren’t forced to use their knives to manually strip the cable off in the middle of the feeder run. This approach not only is quicker and cleaner, but it also takes the correct amount of material off without overstripping. Using these tools, installers don’t have to worry about scratching, denting or damaging the cable.
For the connectors and the EMP-protection devices, applying the correct torque at the fitting stage is important. Good mechanical connection and electrical contact are required, but they must be achieved without overtightening. Using a torque wrench will achieve the desired result.
Although lightning strikes at BTS sites always will occur, the combination of careful design, appropriate protection devices, good tooling and expert installation will minimize the risks. Proper grounding and surge protection are small investments compared to the system replacement cost following a lightning strike. When lightning does hit a tower, everything above the lightning protector usually will be “smoked” and replaced, but the big expensive racks of equipment down in the BTS will be protected.
Matt Gauvin is the area product manager for Transmission Lines North America at Radio Frequency Systems.