Grounding the Shelter
Learn how to protect your shelter from lightning
By Bruce A. Kaiser

"Single-point grounding" is a new buzz-word used to describe the protection of communication sites from the ravages of lightning and general ground transients.

What is single-point grounding? Actually the term can be misleading. Single-point grounding has nothing to do with the grounding system itself. The grounding system is determined by the soil conditions and other factors at a specific site. Single point grounding is actually the connection of all site equipment to the grounding system at a single point or more precisely, bonding all communications site equipment to the grounding system in such a way that all of the equipment samples ground potential is at only one point.

A change in potential in and of itself does not damage equipment. It is a difference in potential across or within a piece of equipment that causes damage. If one radio is connected to both of the coaxial cables from the tower and to a power supply and there is a difference in potential between those two services, that difference in potential equalizes within the radio causing damage or accelerated wear. The same applies to two pieces of equipment communicating with one another through data lines. If there is a difference in potential between the two pieces of equipment, that potential equalizes through the data lines within one or both of the pieces of equipment.

When we refer to the communi-cations site equipment it is important to note that we are referring only to electrical or electronic equipment. Communications site equipment does not include door frames, air conditioning ducting or the tower.

To perhaps oversimplify the concept, envision an imaginary plane at or just below the floor level of the site. All of the site equipment is appropriately bond-ed together above this plane and a grounding system is established below this plane. Those two systems are bonded together at only one hole through that plane. Therefore all equipment within the site is at the ground potential of that single-point. For this discussion the single-point ground is defined as the point at which the unified ground passes through this plane.

In a typical communications site we must be concerned with several different ground potentials. The first set of ground potentials is associated with the services to the site, such as ac power, telco and transmission lines from the tower. The second set of potentials is associated with the various electrical and electronic equipment chassis. Although there are other sources we will limit ourselves to addressing these two areas in this discussion.

Changes in ground potential across a site may be affected by a variety of factors but the most dramatic and familiar is lightning. During a thunderstorm the charge at the base of the storm cloud induces a shadow of opposite charge on the earths surface beneath it. As the storm cloud blows along through the atmosphere this ground charge or more accurately earth surface charge is dragged along the surface of the earth beneath it. Cloud-to-ground lightning results when the difference in potential between the storm cloud charge and the ground charge exceeds the dielectric of the intervening air. That air breaks down in a series of steps and the lightning strike occurs. When lightning strikes a particular point on earth the ground charge at that point is vacated relative to the surrounding ground charge. The surrounding charge rushes to the point of the strike.

Looking at the distribution of ground charge immediately after a strike it is apparent that the ground charge potential will change with distance from the strike. A point close to the strike will be at a different potential than another point at a different distance from the strike. If a ground rod is driven at each of these points, during a lightning strike there will be a dramatic and almost instantaneous difference in potential between these two ground rods. If a power supply is grounded to one of the rods and a radio powered by it is grounded to the other during a strike the radio will see a difference in potential between its power input and chassis. This potential difference will equalize within the radio. At many sites the ac power service entrance is located at one end of the equipment shelter, the telco service entrance at one side and the transmission line service entrance at the end opposite the ac power entrance. (See Figure 1.)

Each service is grounded where it enters the shelter. When lightning strikes in the woods near the site, the ground potential changes across the site due to secondary effect. The propagation rate of the ground charge causes a difference in ground potential (dashed lines) at each service entrance ground. The equipment within the shelter therefore sees a difference in potential between the RF input at the potential of ground #1, the power input at the potential of ground #3 and the telco input at the potential of ground #2. That difference in potential is equalized across and within the equipment causing damage and/or wear.

When looking at the ground potential of the services to the site, the most difficult service potential to control is the transmission line from the tower. This is in part because the tower is located several meters away from the site and the potential of the tower is constantly changing.

You can do one of two things to control transmission line ground potential. You can attempt to limit it relative to the rest of the site or you can make the transmission line potential the reference potential for the remainder of the site equipment.

Based on our experience with cell sites in Florida, the second solution is much easier and is actually more effective. We make the transmission line potential the site reference potential (single-point) by establishing a large copper buss bar on the outside wall of the equipment shelter just below the transmission line service entrance. We then install a grounding kit on the transmission line (in addition to the grounding kits already installed at the top and bottom of the tower and bond it to that buss bar. We then run a large lead from the buss bar to the site grounding system. This means that the RF side of the site equipment will be at the potential of this buss bar. (See figure 2 ) Then we route the ac service to the site so it meets the equipment shelter just to one side of the transmission line service entrance. At the main disconnect, the ac neutral and ground are bonded together and a ground lead extends from that box. This ground lead should be attached to either the transmission line service entrance buss bar or to the large ground lead from that buss bar above the point at which it attaches to the grounding system. This assures that the ac power side (and low voltage power side) ground of the site equipment will be at the same potential as the transmission line buss bar, and therefore the same potential as the RF side of the equipment.

Then we route the telco service to the site so it meets the equipment shelter just to the opposite side of the transmission line service entrance. We then take the ground lead extending from the telco service box and also attach it to either the transmission service entrance buss bar or to the large ground lead from that buss bar above the point at which it attaches to the grounding system. This assures that the telco side of the site equipment will be at the same potential as the transmission line buss bar, and therefore, the same potential as the RF and power sides of the equipment.

Any difference in ground potential of the different services to the site will be equalized at the single-point we have established and the site equipment will sample all ground potentials at one and only one point, the single-point ground. The same lightning strike in the woods produces the same step potential across the site. However, because all site equipment is attached to services sampling ground potential at only one point and there is no other ground reference potential, the equipment sees no current flow across or within it to equalize potential.

This arrangement is effective and easy to design into a new site. But what about an existing site? The internal distribution routing of ac power and telco lines is already established which is difficult and expensive to change. The answer is to leave the existing internal distribution routing intact and just move the service entrances. With the ac power, move the main disconnect to a location near the transmission line service entrance and ground it to the single point. Then run a primary jumper internally back to the existing distribution panel. This way none of the distribution routing needs to be changed. The same technique should be employed with the telco service, moving the demarcation or service entrance to the transmission line service entrance, and establishing its ground at the single point. Although it is expensive to move the service entrances, it is less expensive than replacing damaged equipment or equipment that wears out prematurely. And at least you do not have to re-route the internal distribution of the ac power and telco lines within the shelter.

The next area of grounding concern is chassis grounding. In many sites particularly older sites, an internal halo ground was installed around the perimeter of the shelter near the ceiling or floor and bonded to ground at all four corners of the shelter. Site equipment was then bonded to the halo at the closest or most convenient location so that piece of equipment sampled ground potential at the closest grounding point. Again, during a lightning strike in the woods near the site the ground potential at one corner of the site may be vastly different from the ground potential at any other corner. The chassis potential of each piece of equipment will be closest to the potential of the ground at the nearest corner of the building.

Because many pieces of equipment communicate with one another through data lines and all share power, potential differences will again equalize within and through the site equipment. The solution to this problem is to bond the chassis of each piece of equipment to ground at the same point. The easiest method is to install a copper buss bar inside the shelter on the opposite side of the same wall as the external transmission line buss bar. Each piece of site equipment is then bonded back to the internal buss bar, preferably with a bonding wire run from each chassis directly to the buss bar. The internal buss bar is then bonded through the wall to the external buss bar. Ideally the equipment ground leads should be arranged across two internal buss bars according to how they produce or absorb surges and whether they require non-isolated or isolated signal ground references. But the point is all equipment should be chassis grounded to one potential.

Again the now-familiar lightning strike in the woods produces the same step potential across the site. However because all site equipment chassis sample ground potential at only one point and there is no other ground reference potential the equipment sees no current flow across or within it to equalize potential.

It should be noted at this point that the real purpose of the external buried counterpoise around the shelter is really to protect personnel and the site from the deleterious effects of step potential across the site during a nearby strike. Therefore, it is important and should be part of any site grounding scheme. But its purpose is not to provide a convenient buss to which equipment grounds should be tied.

Again, this method of single-point chassis ground is effective and easy to design into a new site. But what about existing sites with internal halos and multiple point grounding of that halo to the external buried counterpoise? Fortunately many of the problems at an older site can be solved with a good pair of wire cutters. First, cut the internal halo at the end of the building away from the transmission line service entrance, eliminating the large ground loop. Then cut each wire grounding the internal halo to the external buried counterpoise and remove them from the walls. This leaves each equipment chassis grounded only at the single-point buss bar. During this process be certain that you do not inadvertently leave any piece of equipment ungrounded. One of the ironies in this case is that it is less expensive and uses less material to ground the site properly than it is to multiple-point ground.

We have referred to the above lightning strike in the woods as the culprit in producing lightning damage. A direct strike to the tower equipment shelter or utility service pole on the site produces an even more impressive change in ground potential across the site. With a properly designed and installed single-point grounding system along with effectively designed and installed high quality surge suppression, your site should survive the ground potential change of even a direct strike.

A few points become apparent when considering single-point grounding of all services and equipment. The most obvious is that it is much easier to design a new site to this standard than it is to modify an existing site. Equipment shelters can be specified and ordered from the manufacturer with all service entrances at the same end of the shelter and adjacent to one another. Site layout can specify that all services are routed to the same end of the shelter and have effective, high quality surge suppression installed. By the way, if the site has water from a commercial service or from a well the water pipe should also enter the facility at this location and be bonded to the single point. If the shelter has a structural lightning protection system, one of its grounding points should also be bonded to the single point.

The use of all-metal equipment shelters is also eliminated by single-point grounding design because a metal shelter provides an unlimited number of ground loops between service entrances and equipment chassis. If electromagnetic shielding is a concern, it can be effectively accomplished in non-metallic structures without creating ground loops.

You may ask, if single-point grounding does manage to get the potential of all equipment chassis and service grounds to the same potential, what about the hot or line sides of those services? Any excess difference in potential between the line side and the now unified ground will still cause equipment damage or accelerated wear. Single-point grounding does not, and cannot address that issue. That is why it is important to install effective, high quality transient voltage surge suppression equipment in a "staged protection" design, with the primary TVSS device at the service entrance to prevent damage to equipment caused by an excess difference in potential between the ground and hot or line sides of the services.

Again, effective lightning protection is a 3-part process involving effective bonding and grounding, transient voltage surge suppression and structural lightning protection. Without all three elements you are not protected.

Additional information and site survey information can be obtained by contacting

POC: Mike Helms mike@lightningmike.com