Broadcast Writer

By Vicki W. Kipp
Site Management & Technology magazine, Sep 1, 2002

While humans have 1 in 6,000 odds of being struck by lightning, towers have 1 in 1 odds of being struck. It’s basically inevitable.

How lightning works

When the equilibrium of electrical charges between the atmosphere and the earth becomes unbalanced, nature uses lightning to restore the balance.

The atmosphere is composed of atoms. Warm air moving upward and atmospheric turbulence from storms cause atoms to dissociate into separate groups of charged ions. Negatively charged ions accumulate at the base of the clouds in the lower atmosphere while positively charged ions ascend to the upper atmosphere (Figure 1). Normally, the surface of the earth has a negative charge.

However, when negative charges build up in the lower atmosphere, they repel the negative charges on the surface of the earth.

Consequently, the earth takes on a large positive charge.

Since opposite charges attract, the negative ions in the lower atmosphere are now attracted to the positive surface of the earth. Negative ions are very light so they can move towards positive charges with speed and ease. The negative charges move swiftly toward the earth, creating a phenomenon known as lightning. As the negative ions head toward the ground, positive ions on the surface of the earth are drawn upward slowly. Initially, the ions flow slowly because air is a poor conductor. However, the attraction between the negative and positive ions becomes so great that they overcome the resistance of the air.

When negative ions move down through the air, their flow is called a ‘step leader’ or ‘pilot streamer’ because of the erratic path that electrons take as they seek the earth. The negative ions flow downward until the resistance of the air becomes too great, and then they travel horizontally, followed by further downward movement.

Finally, the downward moving negative ions are met by the upward moving positive ions. When negative and positive ions connect, a conductive path from the cloud to the ground is formed (Figure 2). Negative ions hurry down the path creating an observable stroke. New negative ions flow into the void left by the discharge of negative ions. These new negative ions rush along the path.

Additional negative ions come from neighboring clouds. Negative ions continue to flow until equilibrium returns between the atmosphere and earth.

There is a long-standing argument about whether lightning strikes up or down. Although the negative charges are moving downward, it is the fast-moving charges that create the light. Hence the visible lightning stroke in fact moves upward.

Lightning seeks towers

Observing an NTSC antenna from the Candelabra as it lay on the ground during tower work, I noticed that the antenna grounding rods were covered with sizzle marks where they had been branded by the tips of lightning bolts. According to Winton Wilcox of ComTrain, “Towers are struck by lightning more than any other man-made structure.” Towers are frequent targets for lightning because they are so high above ground level. For an optimum coverage area, broadcast towers are intentionally designed to be taller than neighboring buildings.

Besides the height factor, towers attract lightning because they are built of conductive steel. Positive ions from the earth can travel up a steel tower much easier than they could travel up through air alone. The highest point at the top of the tower is where the positive charges will accumulate.

Lightning damage

When lightning strikes a tower, various types of damage can occur. Under certain circumstances, a lightning strike could lead to collapse of the tower structure. Lightning can melt the insulation on the guy wires or cause cracks in the concrete guy anchor. Transmission lines and voltage sensitive devices can be damaged by large peak voltages from lightning. Electrical current from lightning can generate heat and transfer energy.

Guyed towers can tolerate lightning better than self-supporting towers because guyed towers deflect the lightning charge down the guy wires to the ground. Assuming that the guy anchors are grounded properly, a great deal of energy is dissipated into the ground away from the base of the tower.

For proper grounding, grounding components should be attached to the guy wires above the preforms, turnbuckles, and anchor heads.

Minimizing damage

Grounding allows some control of where energy will go when lightning strikes a tower. Experts remind us that grounding is meant to be a lightning protection system, not a lightning prevention system. Grounding involves applying a system to allow an electrical surge to pass through a conductor rather than lingering at and causing damage to the conductor.

Grounding also shields tower structures, such as a fence or site building, from the antenna’s radiation pattern. This prevents the tower accessories from absorbing and then re-transmitting RF, causing a skewed signal pattern.

Believe it or not, there are some people who are opposed to grounding systems. They argue that installing a grounding system provides a path to the top of the tower for positive charges to climb. The anti-grounding faction feels that grounding almost guarantees that the tower will be struck by lightning.

Grounding advocates point out that if a tower is struck by lightning and a grounding path has not been provided, the tower will be subjected to the excess charges. They claim that it is easier and more cost-effective to build lightning protection and grounding into a tower site than to repair lightning damage.

Grounding system

A successful lightning grounding system needs to rapidly disperse large quantities of electrons from a strike over a broad area. A tower grounding system must meet the specifications set in the 1996 TIA/EIA-RS-222-F standard. To be effective, the grounding system requires a low impedance path to earth, and a low resistance interface with earth ground.

A tower grounding system (Figure 3) usually includes a lightning rod or lightning dissipater, secondary ground, primary ground, and ground rods.

Lightning rod

A lightning rod, or collector, is placed at the top of a tower to extend at least two feet above all other tower hardware. The purpose of the lightning rod is to receive a strike and pass it through to the next element of the grounding system. The rod is usually made of copper clad steel.

Lightning dissipator

An alternative to placing a lightning collector on top of the tower is to place a lightning dissipater on top. A dissipater acts as a shield by reducing the potential between the tower and a storm cloud. Performing controlled leakage of the positive charge, it transfers the positive electrical charge to nearby ionizing air molecules. In theory, this action reduces the likelihood of a strike.

If the electric charge accumulation rate at the top of the tower significantly exceeds the dissipation rate and lightning strikes, the dissipater will redirect the lightning away from equipment toward a safe, planned path to earth.

Secondary ground

A conducting connection should be run between any tower appurtenance such as an antenna, bracket, or platform and the tower. For transmission line, a grounding connection should be made at the top of the tower, bottom of the tower, at the entry port to the building, and at every 200 feet of run.

This connection is called the secondary ground. The secondary ground provides a low resistance path to ground. It discharges static charges, lightning, or other electrical phenomena away from the tower structure. The term “down lead” is often used to describe the wire that runs between tower attachments and the primary ground. Copper wire is often used for the secondary ground.

Unfortunately, rain can cause a reaction between the copper strap and the steel tower that leaches away the copper.

Primary ground

The primary ground is the link between the tower and the earth or a conducting element used in place of earth ground.

Grounding straps (Figure 4) run as radials between the tower structure and the ground halo.

Flat wire is more effective than round wire for grounding straps since it has greater surface area.

Bus bar

A bus bar is a piece of highly conductive copper or copper-clad steel that collects energy from numerous sources and conducts it down a common path to ground. With dimensions of ¾ inch thick, 4 inches wide, and 18 inches long, a bus bar is connected to the ground with a ground strap.

A bus bar should be mounted to the exterior of the building where transmission lines enter the building and to the interior of the building just below the entry ports. The exterior bus bar is insulated from the building and grounded to the ground halo. Transmission lines are grounded to the exterior bus bar.

The bus bar that is mounted inside the building is called a ground window. The repeater equipment; entry hatches for transmission line (if they are a conductive material); door frames, window frames, ventilation louvers, and any other sheet metal surfaces; cable trays; AC power line and breaker panel box; telephone lines, blocks and related parts; any peripheral conductive item within 6 feet of any other conductive surface; metal battery racks; utility conduit and pipes; transmitter combiner; receive multicoupler; and any surge suppressor equipment should all be grounded to the common collection point of the ground window.

Ground halo

The purpose of a ground halo is to allow single point grounding. Single point grounding directs all charges down one path to one exit point. A ground halo is often built around a site building and is also built below ground to connect the ground rods. The underground ground halo connects to and transfers energy to all of the ground rods.

Foundation grounding

Controversy surrounds the premise that reinforcing bar in the foundation of site buildings should be grounded. Some argue that rebar is insulated inside the concrete, and does not need to be grounded. The debate centers on the conductivity of concrete.

Under normal circumstances, concrete is not conductive.

However, when the ground is wet and lightning strikes, rebar that is close to the surface could collect energy. There is a risk that the energy passing through concrete could turn the water portion of the concrete into steam, cracking the concrete.

Ufer grounding, named after the engineer who originated the concept, can protect against this risk. With Ufer grounding, the rebar is grounded inside the concrete block, and a ground strap is run along the underside of the foundation to a ground rod. Charges in the concrete are dissipated down into the earth.

Ground rods

Ground rods are conductive metal poles placed in the ground for the purpose of dissipating electric charges to the soil. They are made of steel and coated in a stainless cover of copper cladding or galvanized coating. The coating on the rod prevents rust. This is important since rust is a poor conductor of electrical charges.

A typical ground rod has a diameter of one-half inch to one inch, and length of eight to ten feet. Most ground systems contain at least four ground rods.

The successfulness of a ground system is influenced by the depth of the ground rods, conductivity and resistivity of the soil, and distance between the rods. Ground rods are inserted horizontally underground at a depth of at least two to six feet below ground level. Moist clay bearing soil is desirable for setting up a grounding system. The conductivity of the soil can be improved with soil treatment techniques such as electrolyte fill.

Installation of ground rods requires that the rods be driven into the ground forcefully instead of placing the rods in pre-drilled holes. Pressure must be used when inserting the rods so that the soil will be compacted to form a connection with the surface of the rod.

When ground rods are installed, the correct distance between the rods must be determined for proper placement. Traditionally, the minimum separation between rods should be greater than the sum of the lengths of two adjacent rods. The “sphere of influence” (Figure 5) of a rod is the amount of soil used in dissipating the charge from one rod. The area of the “sphere of influence” has a radius and depth equivalent to the length of the rod.

For example, the sphere of influence of a 10-foot ground rod would have a diameter of 20 feet around the rod and would be 10 feet deep.

It is essential to determine the correct separation distance between rods. When rods discharge they will saturate the soil in their immediate area. Inefficiency will result if a rod tries to dissipate charge in soil already saturated by another rod.

If the charge being dissipated by a ground rod is too great for the soil to absorb, the rod could actually fuse into glass. A glass ground rod makes a great insulator, and a poor conductor of charges.

Conclusion

Humans face a relatively slim risk of being struck by lightning. If such misfortune should occur, there is a lightning strike survivors support group that they can join. Towers have an extremely high risk of being struck by lightning. There isn’t a support group for towers, but there is a multitude of grounding hardware available to make a lightning strike less harmful.

Kipp is a broadcast engineer.

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