Position Paper About Electronics Engineering

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STRUCTURAL LIGHTNING PROTECTION OF AIRPORTS:

COMPARISON OF FRANKLIN AIR TERMINAL AND MULTIPOINT DISCHARGE

SYSTEMS

The art and science of aviation consists of coordinating, communicating and

cooperating as much as flying; and airplanes are safe only as long as airports are. A huge

threat for an airport is a lightning strike and the consequences of a lightning strike hitting an

airport must be carefully examined, for it is highly probable; since an airport is presumably

built on a plain topography, with a spatial configuration consisting of empty fields

accompanied by a pointy tower and stocked aircrafts. These aspects of airports, especially the

tower provides lightning attachment points that are preferentially struck by the lightning.

Additional to that fact, a lightning discharge is theoretically able to damage the electronic

systems of the tower, the electronic and power systems of the aircrafts that are landing/taking

off and power lines [1]. Furthermore; even a random strike to a runway is a high risk for

airplanes, since it is able to open a dangerous crater of 0.6 by 0.5 m with a depth of 0.15 m as

it happened in India in 2002 [2]. That is why controlling and directing the energy of lightning

to protect humans, airport facilities, runways, aircrafts and electronics equipment is a vital

obligation that must be studied and exercised very seriously. At that point, there are two main

approaches to that problem: Franklin Air Terminal (FAT) and Multipoint Discharge (MD)

lightning protection systems (LPS). A critical review of these two approaches reveals that the

FAT approach is the more promising solution; because even though the MD system is

commercially available it is not reliable and the tests of Federal Aviation Administration

(FAA) reveals no quantitative results that supports the theory behind this approach [3],

whereas; FAT, the implemented and practiced approach, is the conventional system for nearly

200 years of structural lightning protection and thus supported by experimental validations

and empirical data [4].

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Figure 1: Formation of lightning strike:

Downward leader meets the upward leader

provided by FAT. -[6]

Figure 2: Multipoint Discharge System: Dissipation

Array illustration – [8]

If the two approaches are critically analyzed in terms of

readiness and reliance, the MD system’s lack of reliability is a

certain drawback. The MD system is a proactive device, whereas

the conventional FAT is a passive system. This means that MDS

is an energy consuming device, which may simply close down

because it depends on a certain source. When it is turned off, it

transforms into an ineffective conventional LPS. Basically, the

MDS technique provides a corona discharge by the elevated arrays

of sharp points built on top of a structure and that corona

discharge either discharges the overhead thundercloud, and

thereby dissipates any possibility of lightning; or discourages a

downward moving leader from attaching to the structure by

reducing the electric field around the array [5]. This procedure is

sustained by a generator, which continuously charges and ionizes

the air surrounding the array. On the other hand, The FAT

technique provides lightning attachment points called air terminals

(or rods) on top of the structures and lightning currents interacts

with those points before any point of the zone of protection. The

current follows a path beginning with the attachment

point and flows across the down conductor into the

ground counterpoise without damaging the protected

structure [7]. This system is highly reliable because of

its passive and independent configuration that becomes

integrated into the structure and acts as the most

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Figure 3: Zone of protection: Rolling Sphere Model – [9]

attractive point of attachment of the protected structure without any sources.

If the two approaches are reviewed within their theoretical frameworks and its

applicability in practice, the FAT system is clearly a better solution (it is the conventional

system), since the MD system has not yet been

fully supported by any practical study.

Theoretically, the MD system provides ionized

air molecules that produce an electric current

flow that weakens the electric field around the

dissipation array to a value lower than 1.5- 2.0

kV-m -1

(breakdown field value) [10]. This

eliminates the formation of upward moving leader and thus eliminates the lightning strike to

the protected area. However, the MD system tests that were done by FAA revealed that in

practice dissipation medium prevents only about 50% of potential strikes and when they fail,

they act like an ineffective conventional system and attract even the more damaging strikes

because of the mislead electric field manipulations, which is another failure [11]. Besides that,

because MD system has an umbrella like configuration as it is shown in Figure 2, another

sharp point at the same height intercepts with the lightning strike before the MDS [12]. That is

why, it must be practically higher than a FAT, if they are to be compared with each other;

which is another practical drawback. In FAT system, the placement of air terminals is

determined by electro geometrical model (EGM) as it is qualitatively discussed by Mousa

[13]. A simple method that is required for practical implications of this framework is the

Rolling Sphere Model (RSM) [14] and even though it provides 99% of protection in practice,

it has a theoretical drawback because this method does not cover the real complexity of

electrodynamics theory and functions properly within statistical ranges and practical limits of

architecture. In other words, any extraordinary lightning current, striking distance or

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application height may cause a malfunction and a deeper EGM analysis may be necessary for

the installation. However, it is not a disadvantage against MDS; because in theory the MDS

also depends on EGM analysis and in practice it is not even possible to apply RSM for MD

systems.

When the two approaches are compared with each other about their feasibility, the

conventional one is practicable and efficient. In theory, the whole airport can be covered by

both of these approaches. However, the air terminal rod (FAT) configuration is much easier to

configure than the dissipation array (MDS) thanks to its passive and simple structure. An

umbrella shaped array of the MDS is an array of barbed wire with about 700 points and an

area of about 3 m 2 [15]. This is not a feasible way to protect especially the runways.

Moreover, considering that hundreds of each of these systems would be required to cover all

the runways and buildings; the fact that MDS being an active electronics device reveals how

complicated and consuming this system would be. On the other hand; in FAT system, the

proper installation of configuration of wires, down conductors, counterpoises and air terminal

rods would be enough to protect any required area as it was reported as an airport LPS case

study at Camp Blanding, Florida [16].

At last, the durability of a system is important under challenging meteorological

circumstances. Wind is one of the natural consequences of a thunderstorm, and it is a

drawback to MDS, whereas non-effective to FAT. The MDS provides corona-produced

charges in order to decrease the electric field around the dissipation array and eliminate the

formation of upward moving leader. The corona-produced, ionized air shields the array and

therefore reduces the electric field that causes the upward moving leader. A vertical or a

horizontal wind sweeps away the light, charged ions and as the ions move away from the

array, their shielding effect reduces and the electric field may increase as a result. This may

cause a resultant upward moving leader to escape the space charge cloud and intercept the

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downward moving leader [17]. This drawback does not occur in FAT approach, and it

functions as long as it is not physically damaged (in those extreme cases both systems would

be down so it is not arguable).

The multipoint discharge system (MDS) is a modern, controversial idea. However, it

has not yet become a feasible and reliable solution for a very demanding practical purpose

like protecting an airport. On the other hand, the conventional method of Franklin Air

Terminal (FAT) has been used for over 200 years. With empirical data and theoretical

framework it is proven to be valid and effective in practice. Standards like RSM have been

developed and the configuration methods have been codified. There are still a lot to be

understood about lightning strikes and their associated forces, but for practical engineering

purposes Franklin Air Terminal (FAT) provides 99% protection. In case of airports, it is

clearly a lifesaving achievement of electrical engineering history.

REFERENCES:

[1] Tarimer, I. and Kuca, B., “An Overview on the Protection Measures for Air-Port

Protection in High Density Lightning Regions”, Elektronika Ir Elektrotechika, 19, 2013, pp

29-32.

[2] Gopalan, T.V., “Lightning Protection of Airport Runway”, Journal of Performance of

Constructed Facilities, 19, 2005, pp 290-294.

[3] FAA, 1990: 1989 Lightning protection multipoint discharge systems tests: Orlando,

Sarasota, and Tampa, Florida. Federal Aviation Administration, FAATC T16 Power Systems

Program, Final Rep. ACN-210, pp 48.

[4] Federal lntaregancy Lightning Protection User Group. The Basis of Conventional

Lightning Protection Technology: A Review of the Scientific Development of Conventional

Lightning Protection Technologies and Standarts

[5] [6] [7] [17] Uman, M.A. and Rakov, V.A., “A Critical Review of Nonconventional

Approaches to Lightning Protection”, American Meteorogical Society, 216, December 2002,

pp 1809-1820

[8] [9] [10] [12] Zipse, D., 1994: Lightning protection systems: Advantages

and disadvantages. IEEE Trans. Ind. Appl., 30, 1351–1362.

[11] “Lightning strike protection criteria, concepts and configuration”

Lightning Eliminators & Consultants, Boulder, CO, Rep. LEC-01- 86.

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[13] Mousa. A.M. and K.D. Srivastava, "A Revised Electrogomt~cM odel

for the Termination of lightning Strokes on Ground Objects''. Pruc. Inter.

Aermppocc and Ground Conference on Lightning and Sinric Electricity,

Oklahoma City. OK, 342-352. 1988.

[14] Lee, R.H.. "Prote~yt our Plant Against lightning", IEEE Trurrr. On Indur-

19 Applicarions. Vol. /A-IS. pp. 236~240. 1978.

[15] Rison, V., “Experimental Validation of Conventional and Non-Conventional Lightning

Protection Systems”, IEEE, 2003, pp 2195-2200

[16] Bejleri, M. and Rakov, V., “Triggered Lightning Testing of an Airport Runway Lightning

System", IEEE Transaction on Electromagnetic Compatibility, 46, pp 96-101