Don't mess with Mother Nature

Dec 1, 2007 12:00 PM, By Matt Gauvin

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.