Non-Conventional Lightning Protection System – Part Four


In Article Types Of Lightning Protection Systems LPS ", I list the main types of Lightning Protection Systems as follows:




Types of Lightning Protection Systems LPS

Lightning protection systems for buildings and installations may be divided into three principal types as follows:

1- LPS for Protection for buildings and installations against direct strike by lightning, which includes:

A- Conventional lightning protection system, which includes:

  1. Franklin Rod LPS,
  2. Franklin/Faraday Cage LPS.


B- Non-Conventional lightning protection system, which includes:

a- Active Attraction LPS, which includes:

  1. Improved single mast system (Blunt Ended Rods),
  2. Early streamer Emission System.


b- Active Prevention/Elimination LPS, which includes:

  1. Charge Transfer System (CTS),
  2. Dissipation Array System (DAS).


2- LPS for Protection against overvoltage on incoming conductors and conductor systems,

3- LPS for Protection against the electromagnetic pulse of the lightning.





And, I explained the Conventional Lightning Protection System parts and components in the following Articles:




And in Article " Non-Conventional Lightning Protection System – Part One ", I explained the first type of Non-Conventional Lightning Protection System which is Active Attraction LPS, which includes:

  • Improved single mast system (Blunt Ended Rods),
  • Early streamer Emission System.


Also, in Article " Non-Conventional Lightning Protection System – Part Two ", I explained the following points:
  • Differences in Lightning Protection Technologies,
  • Principle of Operation for Active Prevention/Elimination LPS,
  • Types Of Lightning Elimination/Prevention Systems.

And in Article " Non-Conventional Lightning Protection System – Part Three ", I explained Types and Components of Dissipation Array System.



Today, I will explain the Design considerations for Dissipation Array System (DAS).





1- General Information About
Non-Conventional Lightning Protection System

  • In the 1970s, two types of unconventional air terminals had been commercially reinvented and introduced in the world market under a variety of trade names. They are:


1- The lightning Active Attraction air terminal: 

the lightning attracting air terminal is claimed to be able to attract the lightning to it (and hence away from the building) in order to protect the building that it was installed on.

 2- The lightning Active Prevention/Elimination air terminal:

 the lightning prevention air terminal is claimed to be able to prevent lightning from occurring and hence protect the building.

  • Some of the trade names for of unconventional air terminals are as follows:



Product Name
Country
Dynasphere
Australia
Prevectron
France
EF
Swiss
St. Elmo
France And Italy
Pulsar
France
DAT Controler
Spain
Paratonerre
France
Preventor
France And UK
EF33
Australia


  • In reality, the inventors of these un-conventional air terminals have never been able to provide any scientific basis for their invention. None of the “scientific papers” that they have published in the last 30 years have been independently verified by the scientific community.
  • In addition to this, these inventors have never been able to provide any independently validated proof that their inventions work. However, they have provided plenty of anecdotal (i.e. hearsay) evidence which had been obtained from “satisfied customers” and some insurance carriers will accept them as equivalent for the conventional techniques.
  • These Non-conventional air terminals are claimed to be superior to the conventional lightning protection but neither experimental data nor theory supports these claims.

  • For these reasons, these inventors and manufacturers have not been able to get their unconventional air terminals approved by the standards bodies like:

  1. NFPA,
  2. IEEE,
  3. IEC,
  4. US Military,
  5. UL.


  • Hence the LPS that used these Non-conventional air terminals have been classified as Non-standard LPS by academics, scientists and the various standards bodies around the world.
  • The Non-standard LPS are usually easier and cheaper to install when compared to the conventional system but the protection that it provides is very limited i.e. equivalent to that of a single Franklin rod! Hence these vendors had to rely on some very creative marketing to sell their non-scientific and unproven products.








Important Notes

  • The volume or zone of protection afforded by the air termination system shall be determined only by the real physical dimension of the air termination system. Typically if the air rod is 5m tall then the only claim for the zone of protection afforded by this air rod would be based on 5m and the relevant Class of LPS and not any enhanced dimension claimed by some non-conventional air rods.
  • In this Article, I will discuss the Active Prevention/Elimination LPS from the point of view of their proponents and in next articles; I will discuss the arguments against the claimed advantages of Active Prevention/Elimination LPS by their proponents.





2- Protected Area Considerations in Dissipation Array Systems


Fig.1: Protected Area by DAS


Because of the DAS  operational concept, there are three factors that influence the size and shape of a protected area (see Fig.1). These are:

  1. The number of branches from an incoming leader,
  2. The distance between the individual branches,
  3. The proximity between the DAS and the closest descending branch.


1- The number of branches from an incoming leader:


Fig.2: Many Braches from Leader


The number of branches from the leader will determine the random probability of one approaching the DAS site. A typical leader will start out and develop a few branches; however, by the time it reaches a few hundred meters from earth, it will have many branches as in Fig.2, up to 20 or more. Therefore, the chance of terminating to any point is less than one in twenty.


2- The distance between the individual branches:

The distance between branches seems to vary from tens of meters to well over a kilometer at near ground level.


Note:

The above two factors will influence the risk of termination on the site since they are subject to random probability. However, they both establish alternate paths for a termination if the DAS delays the progress of a branch approaching it. Therefore, these factors do influence the protected area. By delaying the termination of one branch the alternate termination point could be as close as 100 meters from the DAS or as far away as several kilometers. Again, this is a random variable; therefore, there is no way to predict the next closest termination point.


3- The proximity between the DAS and the closest descending branch:

The proximity factor is the measure of the potential distance between the DAS and the closest potential termination point that would not be influenced by the DAS. This would also define the radius of the “protected area”.







3- DAS Selection Criteria


Fig.3: Different Types of DAS Ionizer


The selection of a DAS ionizer from available different types (see fig.3) is a semi-arbitrary decision. That is, there is no hard and fast rule, but rather, it involves a review of the influencing factors and a selection based on a tradeoff between those parameters. The factors that influence this decision are:

  1. The geographical location of the site and its related isokeraunic number,
  2. The height of the facility structures, and their related increased risk of high peak currents,
  3. The distribution of peak return current risk,
  4. The geography of the site,
  5. Radius of dissipator electrode cross section,
  6. Dissipator construction material,
  7. Number of dissipator electrodes,
  8. Density of dissipator electrodes,
  9. Configuration and height of dissipator on structure to be protected,
  10. Dissipator Design flexibility crucial,
  11. Dissipator Performance and effectiveness.








3.1 The Geographical Location Of The Site And Its Related Isokeraunic Number

The isokeraunic number (K) is related to the geographical location and the number of potential strikes (N) for a given square kilometer per year, where:

N = 0.04K 1.25

So, if the isokeraunic number is 100, the average number of direct lightning strikes to each square kilometer in that area is expected to equal 12.7 strikes per year. (This equation is taken from IEC 1024-1-1.)

 the isokeraunic number or level for a specific location or country can be estimated from one of the following Maps:


Isokeraunic Number or Level World Map



Isokeraunic Number or Level World Map






3.2 The Height Of The Facility Structures, And Their Related Increased Risk Of High Peak Currents

  • The height of the structure will determine the strike collection risk. Heights (H) of up to about 80 meters on flat land will collect the strikes within a radius of about 2H.
  • Therefore, if the facility has an area of 0.1 square kilometers, and the expected number of strikes per square kilometer is 20, then the facility will collect about (0.1 x 20 = 2) strikes per year.
  • However, structures of over 100 meters will initiate more strikes than the simple collection rate estimate. Further, the higher the structure, the larger the number of strikes initiated.







3.3 The Distribution Of Peak Return Current Risk


Fig.4: The distribution of peak currents in a return stroke


  • The distribution of peak currents in a return stroke is presented in Fig.4 which is based on worldwide data. So as an example, if a given location received 100 strikes per year, 50 would peak at 30 kA or less and one may peak at 100 kA. Unfortunately, this is a worldwide average.
  • For mountain tops, the curve will skew to the right and peak currents in the range of 120 to 200 kA will make up 25 to 35 percent of the strikes. These will be a positive polarity and, therefore, require more ionization to delay the counter leader launch.







3.4 The Geography Of The Site

The geography of the site and its location will influence the risk of a strike termination. Only general rules can be used to deal with this parameter. These include:

  • Mountain tops require maximum ionization to prevent strikes.
  • Flat land and valleys require much less ionization to prevent strikes.
  • Waterfront areas are very vulnerable to storms approaching from the sea; however, the peak currents are not as high as mountainous areas. The number of strikes is higher than that expected for that area based on the isokeraunic number.







3.5 Radius of Dissipator Electrode Cross Section


  • Static dissipation arrays work, as the name implies, by dissipating static charge. The radius of the dissipator electrode cross-sect ion is critical because the process which enables dissi­pation of static ground charge to the atmosphere is related to electric field intensity (and flux density) surrounding the dissipator. The laws of physics indicate that a sphere one centimeter in radius has a maximum charge breakdown of about 30,000 volts, depending on air pressure, temperature and humidity. At this point Static dissipation arrays provide, in effect, a "low resistance" route for static ground charge to reach the atmosphere. As the radius is reduced the amount of charge potential also reduces preventing the buildup of ground charge, thus preventing a build up of the ground charge to the value necessary to trigger a strike.
  • Point discharge theory holds that electrical discharge from the point of an electrode to a surrounding medium will follow predictable rules of behavior. That discharge creates an electric field around the electrode. The theory, as it applies to this discussion, can be described by these basic formula: 


  • Since the above formula tell us that electric field intensity will increase as electrode radius decreases, it makes sense to use the smallest radius electrodes possible consistent with structural integrity. 
  • In fact, by not using the smallest possible electrode cross-section, one would entirely miss the point of point discharge. For instance, a dissipator electrode of .015 inch is not merely three times less efficient as a dissipator electrode of .005 inch. As the above formulae indicate, the radius is squared, hence the factor is not three, but nine.







3.6 Dissipator Construction Material

  • Conductivity and durability are vital qualities in the materials used in static dissipation arrays. Obviously the system must have a long life and give good service. Ionization brushes are typically made of stainless steel. A good conductor must provide maximum discharge of current during operation. A properly constructed dissipater is constructed to absorb a lighting strike, in the rare case that occurs.
  • The best dissipaters conform to UL and NFPA codes for air terminals. Often, UL-listed dissipater air terminals replace conventional “lightning rods” in code-compliant building and structure protection systems.
  • At the same time, the dissipator must provide a long and trouble-free service life, combining light weight and low wind resistance with durability.







3.7 Number of Dissipator Electrodes

  • The number of dissipater electrodes or ionization brushes also plays a vital part in choosing the dissipater array.  Calculating the required number of dissipator points is not an exact science. One must not only dissipate the structure charge to be protected; one must also dissipate the ground charge, a around the structure. The ground charge is a function of the strength and speed of the storm. Therefore dissipation requirements are determined not only by the structure, but also by that ground charge, i.e. the absolute difference in potential which must be reduced through dissipation and the rate at which that dissipation must occur to prevent a strike.
  • Since a static dissipation array must provide a low resistance path to the atmosphere, it seems logical to provide as many discharge points as reasonably possible. By using a large number of points one can compensate for any loss of efficiency from a theoretical maximum, and spread the dissipator elements over more of the cross-section area of the structure.







3.8 Density of Dissipator Electrodes
  • The density of dissipater electrodes or ionization brushes also plays a vital part in choosing the dissipater array. The density is critical as they must not be too close to one another causing inter-point interference.
  •  If the dissipator electrodes are held too close to one another, the points interfere with one another's ability to dissipate. Experimentation indicates that the smaller the radius of the dissipator electrodes, the closer they can be arranged without interference.

  • Given moderately close spacing, this interference only affects the dissipation capability of any given point: not of the system as a whole. Moderately close spacing of extremely small radius electrodes may lead to some inter-point interference and limited loss of efficiency by individual points. However, it is more than offset by providing a greater overall number of points and greater overall dissipating capacity.
  • At the extremes, too close spacing results in the array under heavy discharge approaching a solid surface, be it a cylinder, plane or toroid. On the other hand, if the dissipator points are too widely spaced, the result is unnecessary supporting structure with resulting excess weight, wind loading and cost. If dissipator points cease to interfere at a given distance, there is nothing to be gained by increasing that distance.
  • Assume, for a moment, that there is no problem of interference between dissipator points located in close proximity to one another. Another limiting factor arises; the ability of the volume of atmosphere surrounding the dissipator points to accept the charge. Therefore, the points must not only be separated to prevent interference, but also be separated to provide a sufficient volume of surrounding atmosphere to avoid "saturating" that surrounding volume of atmosphere with charge.
  • Of course, this does not take into account the effect of wind, usually present in abundance during the conditions under which peak dissipator discharge occurs. Wind presents constantly renewing surrounding volumes of atmosphere, and, if the dissipator electrodes are sufficiently flexible, continuous movement of the points in relation to one another providing momentary increasing in spacing.







3.9 Configuration and Height of Dissipator on Structure

  • All objects have natural dissipation points. The natural dissipation points typically occur at the top and corners of the structure or antenna.  Enhancing these natural points on the structure is the most effective way to support the charge dissipation function.  This is done by supporting the dissipator from the structure itself at these natural dissipation points, and to take advantage of any existing grounding and bonding provisions, particularly if the structure is a building. In other words, the dissipator should be tailored to the structure, not vice versa.
  • It was once believed critical to effectiveness that a dissipator be the absolute highest point on a structure. Practical experience has proven it need not be. Indeed, mounting a dissipator too high above the structure in an effort to clear all appurtenances can reduce the level of protection by allowing charge to continue to accumulate at the structure's natural dissipation points. 







3.10 Dissipator Design Flexibility

  • A dissipator, if designed in such a way that it need be the highest point on the structure or mounted in any other specific manner, interferes with available space and, by limiting mounting flexibility, may inhibit use of the structure for its intended purposes. Therefore, dissipator design should offer maximum mounting flexibility, be adaptable to any existing structure, take advantage of existing grounding and bonding, and should not preclude any utility application of structure space.
  • Particularly in tower applications where weight and wind loading can be critical, any weight and wind loading contributed by a dissipator reduces the amount of capacity available for revenue producing items, i.e. antennae, etc. Therefore, it is important to keep the weight and wind loading of the dissipator as low as possible, consistent with performance.






3.11 Dissipator Performance and Effectiveness

  • The stated goal of static dissipation should be to cost-effectively reduce losses due to damage caused by lightning strikes. It should be remembered that installation of a system is not a stand-alone solution which means that a lot of points must be taken into consideration before selecting a dissipation array system from specific manufacturers, these points can be listed as follows: 
  1. Availability and helpfulness of the manufacturer in answering questions and providing needed technical information, 
  2. The quality of the dissipator and installation material, and availability of alternate installation material to accommodate requirements arising from the structure upon which it is to be mounted or from the surrounding environment, 
  3. Convenience and ease of installation, 
  4. Installation scheduling, 
  5. Cost of static dissipation array, 
  6. Cost of installation, 
  7. Manufacturer service, follow-up and responsiveness to any problems.







Important Note

There are some rules that must be considered mandatory. Where 100% strike prevention is required:
  1. The DAS ionizer must be correctly sized to protect the desired area and/or structure.
  2. That ionizer must present a smooth surface without any discontinuities.
  3. Any ionizer of lesser size is based on accepting some level of risk acceptable to the customer.







4- SBI and SBT Application Criteria

4.1 Using the SBI and SBT in Standards-Based Systems:

  • Standards such as NFPA-78 and UL96A are based on the use of a single point lightning rod known as the air terminal or the stroke collector.
  • However, since UL has listed the SBI and SBT, these assemblies can be used in place of the single-point terminal. In most cases, they can be used as a direct replacement. The SBT is designed to fit into the conventional lightning rod mounting plate.



4.2 Location and Spacing Criteria:


Fig.5: Building with Hybrid Stroke Protection System


  • Fig.5  illustrates a typical NFPA-78 building protection system that has been converted to a Hybrid Stroke Protection System. Model SBT-24 hybrids are used in the required locations around the periphery. The model SBI-48 hybrids are used in the required locations down the middle of the building.


  • Location and separation distance between SBTs or SBIs are a function of two factors:

  1. The percentage of strokes to be eliminated.
  2. The potential strike zone physics.


  • A properly completed standards-based system that is based on the use of the Spline Ball Ionizer hybrid system will provide two modes of protection:

  1. A stroke prevention mode that reduces the risk of a strike to the protected facility using the size and number of SBI/SBIs in proportion to the size of the facility.
  2. A stroke collector-diverter system that is far superior to any system now in use because it collects strokes entering the “protected” area from any direction and angle.


The general criteria for the location and spacing of the SBT for successful operation as an air terminal is as follows:



1- When considering a system that must satisfy UL96A and/or other standards:

the SBT-24 is 24 inches high and may be used as described in the following:

  • Space the SBTs 25 feet apart, but no more than 2 feet from the edge of the facility.
  • Space each row of SBTs no more than 50 feet apart.
  • Interconnect rows at no more than 50-foot intervals.
  • Ground each row at least every 100 feet.



Standard Spline Ball Terminal (SBT)

  • The collector efficiency is related to its ability to produce streamers that will propagate toward the downward moving leader as it approaches the Point of Discrimination. As illustrated in Fig.6, the strike zone for that leader is of a discrete volume, dependent entirely on the energy in the leader. High-energy leaders have strike distances of up to 180 meters for the common negative polarity and as short as about 15 meters. Positive stroke distances are much longer.

Fig.6: Strike Zone

  • From these data it is evident that, to provide a 100 percent effective collection function, the SBT must be within the strike zone of the lowest energy leader, and it must be the most prominent streamer generator within that zone.
  • The design problem is related to the site situation as well as the facility. Since the strike will occur at the point of highest potential between the leader tip and the closest streamer generator (point or sharp edge), the closest streamer generator will be the stroke terminus. This premise is true only if there are no other streamer generating objects close enough to compete as a stroke terminus.
  • Based on the foregoing and the desire to collect the weakest stroke, it would appear that a separation distance equal to the strike distance would be the maximum safe separation.
  • Therefore a spacing of no more than 15 meters or approximately 50 feet should be safe. However, a 40-foot separation would offer a margin of safety.



2- For a non-standard collector-diverter concept based on the SBT

the following rationale may be applied:

  • The objective of a collector-diverter system is to provide an efficient collector and a safe diversionary path. The diversionary path parameters are defined by UL 96A and Ground each row at least every 100 feet.



Note:

  • A non-standard system is one based on the premise that a standard per se does not need to be satisfied. It also follows that when the number of SBTs required is reduced, the dissipation capability is reduced and the major protection mode is that of a collector-diverter concept.





In the Next Article, I will explain Some Examples of Typical Designs of DAS, SBT and SBI. Please, keep following.






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