A Quick Guide to Lightning Arrestors
There are two basic ways electricity and electromagnetic energy can destroy a broadcast tower, microwave link, cellular base station, or any other wireless infrastructure: transient voltage spikes and lightning. Of the two, the former is the more frequent and the latter more damaging, and neither can be ignored. When lightning strikes a tower with inadequate protection, the results can be catastrophic, as a direct cloud-to-ground strike will deliver more than 100 million volts seeking ground, whether it goes directly there or through components it vaporizes along the way. In short, there’s a good reason why lightning arrestors are often called electromagnetic pulse (EMP) suppression devices.
Although no one keeps statistics, it is likely that a significant number of installations are under-protected, not well maintained, or both. That’s risky, because almost all towers will eventually experience either a direct or indirect lightning strike, as they’re in service for years or decades. And yes, lightning does strike the same place twice, sometimes repeatedly in just a few minutes, and especially if it’s a large, pointy structure. Infrastructure also does not need to suffer a direct hit to be severely affected, as lightning will travel to the nearest ground, including through the ground itself. In fact, 55% of all deaths are caused by ground currents, followed by side flashes at 35%. Direct strikes on people are actually rare.
Lightning has one goal: to find the least resistive path to earth, and if there’s no direct path it will find the next best route through the best available conductors. Once it gets inside of an equipment shelter, for example, lightning will travel through electrical, phone, plumbing, and coaxial cable, as well as metal wires or bars in concrete walls or flooring. Lightning can also travel long distances through phone, coax, and electrical wires, so surprises can come from unexpected sources.
And when it finds no points of entry, it may “side flash” to reach a better grounded conductor, sometimes traveling through a room to get there. There is no way to guarantee that a direct lightning strike won’t cause some damage because to achieve that the solution would have to divert nearly all the current from the strike, an impossibility considering that even if all but 1% is diverted, there will still be perhaps 100 A left to destroy equipment and even cause a fire.
Having posed the grim details of the damage lightning can cause, there are nevertheless ways to prevent equipment damage or destruction, but a single “solution” is rarely enough. And while many contractors and even electricians believe they have the knowledge needed to deploy an effective system, the task is better contracted to someone who specializes in lightning protection, most of whom are certified by the Lightning Protection Institute and UL.
The Basic Components
The basic components of lightning protection systems are air terminals (which Benjamin Franklin called lightning rods); hefty copper, copper alloy, or aluminum (low resistance) cables; multiple 10 ft.-long rods pounded into the ground; and surge protection devices.
All but the surge protectors are designed to be the first defense, routing a lightning strike away from the buildings, towers, and the other structures whose highest points the air terminals are mounted on, and as directly as possible to ground. Surge protectors are devices composed of at least one non-linear component that limits surge voltages to equipment by diverting it to ground. They are essentially the second line of defense and are a mandatory requirement for wireless installations whose components are sensitive to even modest increases in voltage and current. They must be placed at each point of entry.
Surge Protection for Coaxial Lines
There are specific guidelines for the use of surge protection devices (SPDs) used on coaxial lines, and they are specified in Underwriters Laboratories standard UL 497C, which covers the most widely-used types: air gap, gas discharge tube, quarter-wave, and solid-state, with or without fuses or other current-limiting devices. They are employed on transmission lines between devices, from antennas and amplifiers to radios and other RF equipment.
An air-gap arrestor (Figure 1) is the least expensive type as it is very simple: an inline device that basically looks like a male-to-female coaxial adapter. However, it introduces a small air gap between the center conductor and the grounded case. When a lightning strike occurs, a high voltage is induced in the transmission line, and the air gap will flash over between the center conductor and the case, which creates an arc that forms a low-voltage conductive path, diverting current to ground.
Most of these devices have a fixed gap but others have an adjustment screw that determines the flash-over voltage required to create the arc. These arrestors are very popular in the amateur radio community as they are inexpensive and are typically operable to about 500 MHz. Although they could be used for commercial applications, their ability to handle high transient voltages is questionable.
Gas discharge tube (GDT) arrestors (Figure 2) consist of two end plates connected by a ceramic or glass tube. The internal gap between the end plates is filled with one of several suitable gases that when normal voltages are present is a poor conductor. When a specified voltage is reached the gas ionizes (breaks down), becoming a very good conductor which produces a short circuit and the voltage is sent to ground. When the high voltage disappears, the gas discharge tube returns to its original state of high isolation and is ready to operate again. The gas tube is removable, allowing the housing to remain in service. GDT’s can operate at frequencies up to about 7 GHz.
A Metal Oxide Varistor (MOV) arrestor consists of metal oxide material between two semiconductors (varistors) that have variable, voltage-dependent resistance. When the voltage is below a specific level, the electrons in the semiconductors create a very high resistance and if the voltage exceeds this level, the semiconductors create a much lower resistance, diverting the current created by the excessive voltage to ground.
As the voltage decreases, the MOV returns to its high-resistance state. In some conditions, an MOV can dissipate a large amount of potentially damaging heat, so it is typically used in conjunction with a fuse. Other types of arrestors can also be supplemented by a fuse a backup device. A hybrid type of arrestor (Figure 3) employs an MOV and a GDT to provide the advantages of both.
Quarter-wave arrestors are three-port devices with one port acting as a short-circuit between the center and outer conductor and measures a quarter wavelength at the center frequency. These devices act like band pass filters, passing the desired frequencies and suppressing the lower-frequency lightning surges, diverting them to ground. Unlike GDTs, quarter-wave arrestors are simple mechanical devices, so they can withstand multiple surges without destruction.
Summary
Lightning strikes travel near the speed of light and pack a massive punch that causes more than $6 billion in insurance claims every year. However, even though it’s impossible to guarantee that even the most extensive protection systems will be completely effective, it is still possible to provide a high level of protection for the sensitive components used in these systems. All surge arrestors but the basic air-gap type are very effective, and considering their very low cost are the best insurance money can buy.
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