Lightning protection systems
Static dissipation techniques avoid lightning strikes by dissipating the charges that lead to them.

By Bruce A. Kaiser

The Bottom Line__________

Lightning strikes can do more than damage equipment. They can also destroy profitability by robbing your station of revenue and market share during the time you are off the air. Mas! Lightning protection systems wait for lightning to strike. When it does, they provide a safe path that (hopefully) leads it away from your equipment. Static dissipation systems, on the other hand, attempt to prevent lightning by intervening in its creation. This approach can protect against direct lightning hits and secondary damages caused by near misses.
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Axiom:
The probability of a given structure or system being struck by lightning is directly proportional to its value.

Corollary:
If two structures or systems of equal value sit next to each other, the one that will be struck by lightning is the one in which replacement parts are no longer available.
 

Why is one structure more likely to be struck by lightning than another? More important, what can be done to make a structure less susceptible to lightning strikes?

To answer these questions, we need to examine the cause and propagation of a lightning strike. This article will attempt to explain lightning by exploring its mechanisms. It will then suggest practical methods for coping with this hazard.

 

Rub two clouds together

Various mechanisms within an electrical storm produce stratified charges in a storm cloud. (See Figure 1.) This results in a strong electrical charge at the cloud's underside, known as the cloud base charge. This charge induces a shadow of opposite charge on the surface of the earth beneath it. This is known as to the ground charge.

As the charged cloud moves through the atmosphere, it drags the ground charge along beneath it. If the charged cloud passes over a structure, it pulls the ground charge up onto it. This concentrates the ground charge on and around the structure.

If enough ground charge accumulates, the difference in potential between the cloud base charge and the ground charge overcomes the dielectric of the intervening air. This leads to an arc, or lightning strike.

A lightning strike begins with stepped

 

 

leaders that branch down from the cloud. These stepped leaders move downward in jumps of approximately 150 feet. Each set propagates down through the set before it, leapfrog fashion. Stepped leaders form tendril-like branches from the cloud down. These are often visible in a photograph of a lightning strike.

When the stepped leaders are approximately 500 feet off the ground, the electric field intensity becomes so strong that structures on the ground begin to break down electrically. They respond by shooting streamers up toward the stepped leaders. When a streamer connects with a stepped leader, the ionized path becomes the channel for the main lightning discharge. The other streamers and stepped leaders never mature.

For this discussion, it doesn't matter whether the cloud base charge is positive or negative, because it can vary. The entire process can occur in the opposite direction,

Lightning solutions
  given our current technology, nothing can be done to affect the cloud charge or affect the propagation of stepped leaders. To protect a structure from lightning, something must be done about the ground charge and the formation of streamers from the structure to be protected.

One of the primary tools for lightning control is the air terminal (a lightning rod). In most cases, this is mounted at the topmost point of a structure.

One of the primary
tools for lightning
control is the air
terminal. Alt air
terminals are not
created equal, nor do
they function alike.

All air terminals are not created equal, nor do they function alike. Three schools of thought include:

1.       A structure can be protected by installing air terminals that attract lightning. The energy of the strike can then be conveyed through a low resistance path to ground. This is generically referred to as early streamer-emitting air terminal technology.

2.        Conventional air terminal technology can be used that is designed to intercept nearby strikes and convey the energy to ground.

3.        Static dissipation technology can be used to attempt to reduce the likelihood of a strike.

A static dissipation array consists of many air terminals with small point radii. These discharge ground charge, and build up the corona to discourage streamers.

The first two approaches assume that strikes are inevitable. They attempt to handle them with minimum damage. Static dissipation technology attempts to reduce the incidence of strikes. However, a well designed static dissipation system can also handle direct strikes in the same manner as conventional systems.

Spare the rod?

Conventional air terminal technology is well understood. It is described in detail in Underwriter's Laboratories UL 96A and in National Fire Protection Association NFPA 78. For this article, we will focus on static dissipation techniques.
   This method of lightning prevention has two Thrusts. First, the buildup of the ground charge can be reduced to keep it from reaching the critical flash point. Next, the points on the structure from which streamers are most likely to originate can be identified, then their formation or their height can be delayed. Doing this would make these streamers less likely to be the first to reach a stepped leader. This means the arc would form elsewhere, effectively diverting the lightning strike static dissipation devices function because of the point-discharge principle. Point-discharge theory holds that electrical discharge from the point of an electrode (such as a broadcast tower) into a surrounding medium (such as the atmosphere) will follow predictable rules of behavior. The smaller the radius of a static dissipation electrode (actually, the 'smaller the radius of the point at the end of the electrode), the greater the resulting

 

A well-designed static
dissipation system can
handle direct strikes
in the same manner
as conventional
systems.

electric field intensity and flux density. As the dissipation electrode radius approaches zero, the electric field intensity approaches infinity. These strong fields bleed off some of the ground charge into the atmosphere surrounding the air terminal. This reduces ground charge accumulation. This high electric field intensity and flux density also retards the formation of streamers. It is difficult for the cloud charge or stepped leaders to pull a streamer thru the intense corona.

This is illustrated by comparing a sharp lightning rod with a blunt one. Assume a sharp rod and a blunt rod sit side-by-side. As the ground charge reaches the two rods, the potential rises on both. The sharp rod will tend lo break down into corona under a relatively low potential. The blunt rod will hold its charge, with ions accumulating on the blunt end.

As the ground potential builds, the corona builds around the sharp rod, while the blunt rod tends to hold its charge. When the ground potential becomes extremely high, such as when the stepped leaders are on their way down from the cloud, the sharp rod's corona will build in density and elevation. On the other hand, when the blunt rod finally breaks down, it does so catastrophically. The accumulated charge jumps off of the blunt rod in a streamer extending well upward toward the stepped leaders. Because the object that throws off the best streamer is the one most likely to be struck, the blunt rod is more likely to trigger a strike than a sharp rod.

All objects on the ground dissipate naturally to some extent. How they dissipate is also related to the point-discharge principle. The ground charge is first drawn to the top of the object (the ultimate point), and then to the corners or other points
 

All objects on the
ground dissipate
naturally to some
extent, flow they
dissipate is related to
the point-discharge
principle.

 

 

from which it bleeds off into the atmosphere.

These natural charge accumulation points also tend to be the points from which streamers originate. How are these points identified? Perhaps an oversimplified method is to imagine turning the structure upside down and dipping it into syrup. When it is lifted out, the points from which the syrup drips will be the charge accumulation and streamer formation points. Applying static dissipating air terminals or arrays lo these points will reduce the accumulation of charge and retard the formation of streamers. An array of dissipation elements is merely a series of air terminals on which the points' radii are reduced as close to zero as possible, with out harming the structural integrity.

Broadcast facilities Towers are a prime target of lightning. As the ground charge is pulled up the tower by the cloud base charge, it accumulates according to the point-discharge principle.

In spite of bonding and grounding wires provided for the purpose,
the ground charge will now, even
over insulators, and accumulate
on a structure according to
point -discharge principles.

The ground charge flows lo the top of the tower on the lower structure and the guys. Therefore, a dissipation device, or sets of dissipation devices, should be in­stalled on the tower above the lop guy cable of each set. II the tower uses a candelabra, a dissipation device should also be installed at the lower top. One should also be mounted at the top of any top-mounted, DC-grounded antennas. It can be mounted lo the lightning rod cage if one exists.

Different ground rules

Sometimes it is helpful to remember that the ground charge is not a free electron charge that moves around the structure on wires. It is an ion charge that (lows over a structure in a fluid (the atmosphere). In spite of bonding and

Although a satellite dish is not likely to be struck because of in rounded shape, it may still be venerable due to secondary effects of the ground charge rushing to a nearby strike. The dissipation array helps bleed off the accumulated ground charge.

grounding wires provided for the purpose, the ground charge will flow, even over insulators, and accumulate on a structure according to point-discharge principles.

This concept of ground charge motion helps address the question of the need (or a bonding wire from the dissipator to the grounding system. Obviously, the ground charge gets to the top of the structure all by itself (or else the structure would not have a lightning problem). Therefore, the Structure of The tower is sufficient to convey the ground charge to the dissipator
 

If the dissipator acts as a lightning rod and takes a hit (all static dissipation systems should for designed lo withstand this possibility of lightning will follow the path of least impedance lo ground. The tower structure will usually offer a path of sufficiently low impedance so that a separate ground wire is not necessary. Therefore, in either case it is unnecessary.

Other structure

This approach lo arranging dissipation devices works for all towers except AM towers. On a series fed AM tower, the dissipator should mount horizontally and symmetrically, slightly below the top, but above the top guy cables. This way, it will not change the length of the lower on a folded unipole. The dissipator should mount horizontally and symmetrically at the tower top. A properly designed and installed dissipator should not change the impedance or the tower's radiating properties.

Satellite dishes present unusual problems in a lightning environment; their shape tends to retain ground charge. However, because the top of a dish is usually a large, smooth arc, it tends not to emit good streamers. Dishes, therefore, rarely suffer direct lightning strikes. However, they are prone to secondary effect damage. When a strike occurs within the area

 

 

Figure 1. The charged storm cloud pulls a ground charge along beneath it a) when the ground charge encounters a tower or other structure, the cloud charge draws it up, concentrating the charge; b) as the dielectric of the air breaks down, the cloud sends down stepped leaders; c) when they are about 500 feet off the ground, the tower emits streamers; d) when a stepped leader meets a streamer, the ionized path becomes the course of an arc, or lightning bolt,____________
 

of ground charge occupied by the dish, charge may rush off the dish toward the point of the strike. This often adversely affects the dish's aiming mechanism. Installing dissipation equipment along the top of the dish allows some of the accumulated charge to leak into the atmosphere. This reduces the amount of charge available to cause harm due to secondary effects from a nearby strike.

No bolts from the blue

Lightning rods on a conventional system do offer a limited amount of protection if a lightning strike occurs. Static dissipation

 

Lightning rods on a conventional system offer a limited amount of protection if there is a lightning strike.

However, static dissipation systems do their work by avoiding strikes.

 

systems offer this same level of protection, but they also work to avoid strikes. This is done by dissipating the ground charges that accumulate on structures. They also delay the formation of streamers by promoting a strong corona. Therefore, such systems offer protection by decreasing the likelihood of a strike and by duplicating the action of a traditional lightning rod.

For more information on static dissipation technology, circle Reader Service
 Number 302.