Lightning Protection Systems
Udaya Kumar
Chapter Outline
Charge Structure of a Thundercloud
Electrical Discharges above Thundercloud
Events in a Cloud-to-Ground Lightning
Lightning Protection System – Components of External Protection System
Lightning Protection System – Internal Protection
Lightning Launch Commit Criteria
Introduction to Lightning
Lightning is a spectacular transient high current electric discharge in the atmosphere. It is estimated that over the whole globe on average 50–100 lightning events occur per second. For all practical purposes, the thunderclouds (cumulonimbus) are the main source of lightning. Consequently, the lightning threat to space vehicles is not limited to the vehicle on the launch pad, but it exists until the vehicle clears the thundercloud. A brief account of general charge distribution in a thundercloud is given below.
Charge Structure of a Thundercloud
Shape and size of a thundercloud depend on several environmental factors and they basically consist of water droplets and ice particles. Lightning can be produced by ordinary summer thunderstorms, severe thunderstorms, hurricanes, winter snowstorms, oceanic convection and mesoscale convective systems. The lightning producing thunderclouds are usually 3–20 km in vertical extent and the horizontal extension ranges between 3 km to greater than 50 km.
The charge separation in a thundercloud is a complicated phenomenon. The major contributing processes are understood, but it is still difficult to provide a very quantitative picture. The charge structure is assessed by various types of measurements including multi-station ground based measurements.
Generally a simplified tripole charge structure is considered involving a small pocket of positive charge at the cloud base at about 2 km height, a main negative charge distribution at about 5–7 km height and a main positive charge distribution at about 7–12 km height. However, as the formation of cloud is governed by the meteorological environment and geographical location, the height of the cloud base can vary from less than 1 km to about 4–5 km. Consequently, the charge structure and its location can get altered and can differ from the one quoted above. In contrast to the main negative charge, the main positive charge is more diffuse. The total charge in main charge centers is in the order of tens of coulombs or more, while that in the lower positive charge distribution is in the order of a few coulombs. The potential of the charge center in the cloud can be estimated to be in the range of tens of millions of volts or more. The Earth acting like a good conductor carries image charges, which are of opposite polarity and located at a depth equal to the height of charges above the ground. Fair weather electric field indicates that the Earth is at about –300 kV with respect to the ionosphere and lightning seems to be responsible for maintaining this voltage difference.
Types of Lightning Discharge
The lightning discharges of engineering interest can be broadly classified as: (a) intra-cloud discharges, where an electric discharge occurs between two opposite charge centers of the cloud; (b) cloud-to-ground discharges, where the discharge occurs between a charge center in the cloud and ground; and (c) air discharges, where the discharge does not bridge the gap and is mostly from a charge center in the cloud to air. The cloud-to-ground discharges can be further qualified as positive or negative depending on the polarity of charge lowered to the ground. The cloud-to-ground flashes, as shown in Figure 1, are subcategorized into downward negative, upward negative, downward positive and upward positive lightning. The ground flashes are dominated by negative in most of the places except winter lightning in selected places like Japan. Many estimates indicate that the negative cloud-to-ground lightning forms 90% of strokes to ground.
FIGURE 1 Classification of cloud-to-ground flash (a) Downward negative lightning, (b) Upward negative lightning, (c) Downward positive lightning and (d) Upward positive lightning.
There is also another type of classification of lightning, which is based on its visual appearance. Such a classification is not of much engineering interest.
For a launch pad or a spaceport, it is mainly the cloud-to-ground discharges that are of serious concern. For launch sites located at seacoast or at sea, it is only downward lightning that needs attention. In these cases, the effective height of the pad including the vehicle is not high enough to cause significant upward discharges. On the other hand, for the vehicle, all the above listed lightning activities are of concern, because it is not only during the launch preparation but also during the flight that the lightning threat prevails. The next section will discuss this issue in more detail.
Vehicle Triggered Lightning
An ascending vehicle causes significant distortion of the ambient electric field produced by a thundercloud, with appreciable amplification at its nose. This under appropriate conditions can initiate a bipolar discharge from either ends of the vehicle, possibly leading to a lightning flash.
The following two incidents are examples of lightning strike to launch vehicle in flight. They were basically instances of vehicle-triggered lightning, which occurred shortly after successful lift-off.
Apollo 12 was launched on November 14, 1969, from Kennedy Space Center in Florida. A vehicle-triggered lightning strike was observed at 36.5 and 52 seconds into the flight. Nine non-essential sensors with solid-state circuits were permanently damaged. There were numerous temporary problems like loss of communication, flashing and sounding of various warning lights and alarms, disconnection of three fuel cells from the power bus, loss of altitude reference by the inertial platform, and disturbances to the timing system, clocks, and other instruments. Fortunately, all critical system problems were subsequently corrected (by the crew with support from the mission control center) and the mission was continued successfully. Of course the amount of damage and disturbances are dependent on various aspects including the amount of sensitive electronic systems employed in the vehicle control and guidance. Therefore, the experience of Apollo 12 cannot be extrapolated to modern vehicles, as will be evident from the next section.
Another incident of lightning strike to vehicle in flight was the Atlas-Centaur 67 rocket, carrying the FltSatCom (Fleet Satellite Communications) satellite launched on March 26, 1987, from Cape Canaveral. There was no cloud-to-ground lightning within 9 km of the launch site in the 42 minutes prior to launch, and only one strike occurred within 18.5 km during this time. Otherwise, the weather conditions were similar to those that existed at the time of the Apollo 12 launch and the field mill closest to the launch site read the electric field intensity at –7.8 kV/m. After the launch, a lightning strike caused a memory upset in the vehicle guidance system; this upset then caused the vehicle to commence an unplanned yaw rotation. The stresses associated with the rotation caused the vehicle to begin breaking apart. About 70 seconds after lift-off, the range safety officer commanded the vehicle’s self-destruction. A clear evidence of lightning attachment could be seen on the fiberglass-honeycomb structure that covered the front 6–7 m of the vehicle, which was subsequently recovered from the Atlantic Ocean.
It is worth noting here that, the sensitive electronics employed in modern day control and guidance system are highly susceptible to electromagnetic interference. The energy level thresholds they can withstand are quite small. In other words, the increased use of digital systems in the modern launch vehicles has further intensified their vulnerability to lightning electromagnetic threat.
Electrical Discharges above Thundercloud
Electrical discharges from top of clouds to the lower ionosphere with a visual extension ranging from 10–90 km have been observed. These upward propagating discharges have different manifestation and are called blue starters, blue jets, red sprites and elves. However, their impact on a vehicle in flight or even to the ground based systems is not clearly known and at present they are judiciously not classified as threats.
Occurrence Probability
Studies have shown that the lightning activity over land is roughly a factor of three higher than that on the ocean. On average, the lightning flash density is shown to be highest at the equator with a decreasing trend with the increase in latitude. Also the ratio of cloud-to-ground flash is highest at tropics and tends to decrease with increase in latitude.
Electric power transmission engineers have to protect their huge system from lightning and hence have put in considerable efforts towards understanding of lightning termination on ground based structures and on studying suitable protection measures. The early data available to them was the keraunic level, defined as the annual number of thunderstorm days TD, as detected by a human observer. It is based on the number of days in a year on which the thunder is heard at least once in a day. The practical range over which thunder can be heard is about 15 km and the maximum range is about 25 km.
Several uncertainties in the above approach were largely overcome by deploying lightning flash counters, whose working principle is based on sensing of the electric or magnetic field produced by a lightning flash. Based on the lightning flash counter measurements, an empirical relation was proposed between the thunderstorm days and ground flash density Ng in km–2 yr–1 as Ng = 0.04 TD1.25. For a range of TD from 4 to 80, the range of Ng was about 0.2 to 13 km–2 yr–1. Subsequently, it was found that the annual number of thunderstorm hours TH is more closely related to lightning incidence than TD. Another equation was suggested to relate the ground flash density to the number of thunder storm hours Ng = 0.054 TD1.1. However, caution needs to be exercised in using the above relations as they could be specific to the region at which the data employed in obtaining these relations were collected. Lightning location systems based on either magnetic direction finding or time of arrival technique and sometimes both have been used in the recent past. These lightning detection networks employ sensors distributed over vast geography along with global positioning and data communication. The accuracy achieved may not be 100%; nevertheless, the lightning detection network data are commonly employed in various countries for several purposes like meteorology, providing early warning system, etc. The satellite based lightning detection, which detects both cloud flashes and cloud-to-ground flashes without any distinction, has provided systematic data on worldwide lightning activity. Extensive data from such measurements is available at the NASA website.
Events in a Cloud-to-Ground Lightning
The lightning discharge called the lightning flash is not a single event but usually consists of set of discrete events whose time separation is inadequate for a human eye to differentiate.
Only a sensitive fast time resolved photography or video recording can depict the set of events contained in a flash. A streak camera image of the event is sketched in Figure 2.
The ground flash is initiated by an electrical breakdown process in the cloud charge center. It leads to a good conducting channel of air carrying charge, which propagates towards the ground in steps and hence is called a stepped leader. Several branches can develop on the stepped leader all of them propagate mostly towards ground. Due to lower magnitude of current carried by these stepped leaders, they are very faint. The stepped leader lowers electric charge towards ground and as a consequence, the electric field intensity on the ground, as well as on top of grounded objects, increases. Depending on the field intensity, several types of upward discharge can develop on the ground and grounded objects like corona, corona streamers and upward leaders. At a particular instant, the electrical bridging takes place between the descending leader and the ground or upward discharge from ground/grounded objects, which initiates a return stroke. A large magnitude of current ranging in tens of kilo-amperes or more propagates upwards along the stepped leader channel with an approximate velocity of one third the velocity of light. The temperature of the core of the channel increases to 20,000–30,000 K within a few microseconds leading to a bright light and thunder. In fact, thunder is a manifestation of initial shock wave, which due to radial expansion is converted into a sound wave within few meters from the channel.
The energy in a cloud-to-ground flash is roughly 108–1010 J, while the estimated power density in the channel is impressively high, which is about 180 GW/km.
The return stroke current neutralizes the charge deposited along the channel and part of the charge in the cloud. It can last for tens to a few hundreds of microseconds. After a pause of tens of milliseconds, another leader called the dart leader can be initiated from the cloud, which propagates faster than the stepped leader, following mostly the path traced by the former. The dart leaders usually do not possess any branches. When it is close to the ground, it initiates another return stroke, called the subsequent return stroke. This process comprising of dart leaders and return strokes can repeat several times. Generally, but not necessarily, the amplitude and duration of current in the subsequent return strokes are lower than that of first stroke. However, the rate of rise of current can be higher. After a return stroke, continuing current may flow for tens of milliseconds with current amplitude in the kilo-ampere range.
Parameters Involved
The main sets of parameters that are of general interest are peak amplitude of current, duration of the current, total charge transferred and rate rise of current.
As with any natural phenomena, the parameters of lightning flash are stochastic in nature. Therefore, only representative values can be quoted. These values are based on extensive measurements, which cannot be however be proclaimed to be very exhaustive. Table 1 presents some of the relevant statistics for downward flash. The relevance of different entries will be made clear in a subsequent section. The measured data on peak return stroke current is found to reasonably follow a log-normal distribution with probability of occurrence of strokes with low currents, as well as high currents, being lower.
General Threats
A lightning stroke can induce damage in two distinct ways. First, during the incident of a direct hit where the object/system under consideration becomes a part of lightning discharge path, different kinds of damage can occur. Second, coupling to the electromagnetic fields produced by a nearby lightning can lead to various types of damage, especially in sensitive electronic systems. The medium and low voltage power distribution lines and telecommunication lines more frequently suffer from such indirect means. It is worth noting here that the kind and magnitude of damage, in addition to lightning current parameters, are also dependent on the nature of the system.
The following is a summary of possible damage and most contributing parameters.
(i) Peak current: The peak voltage rise at the struck point on a purely resistive system, long transmission lines and peak voltage gradient or the step voltage along the ground near the stroke termination point are all dependent on the peak amplitude of stroke current. The peak amplitude of the mechanical force generated by lightning discharge current is proportional to the square of the peak current. This force assumes importance when the conductor carrying the stroke current has bends, joints, and when the discharge current is shared across parallel paths formed by the neighboring conductors.
With regard to a launch pad, the mechanical force developed by lightning stroke current is insignificant for this massive steel structure.
For the payload fairing of a rocket, like the nose of an aircraft, when made of non-metallic material, the peak current dictates the amount of mechanical damage and hence it is of serious concern.
(ii) Current integral: The damage at the lightning attachment points on metallic surfaces is dependent on the energy delivered, which is approximately proportional to the charge transferred. The charge transferred during a stroke is given by the time integral of current. The large burn through holes and firing of wood are mostly due to continuing current components, which have smaller current amplitudes but long durations, leading to large current integrals.
Once again, current integral is of concern for the payload fairing of rockets.
(iii) Action integral: The time integral of the product square of the current and the resistance of the struck object is a measure of joule heating and hence it is also an indicator for heating, melting and explosion of poor conductors. Due to heating and skin effects, the resistance offered by even the good conductors like metals is not really fixed for the duration of stroke current. For poor conductors like trees, composite materials, etc., the issue gets further more complicated due to non-linearity in their electrical conductivity. Due to these reasons and further noting that the lightning current is not affected by the resistance of the struck object, the joule heating effect is quantified in terms of action integral, which is the above-said integral for 1 ohm resistance. The explosion in a non-flammable atmosphere is normally due to building up of internal gas pressure. Apart of heating effects, along with the peak amplitude of the current, the action integral quantifies the total mechanical force exerted by the lightning current.
Even though the local overheating of the conductor at narrow paths and joints can lead to ignition in a flammable atmosphere, such overheating effects are very unlikely at launch pads and rockets. However, attention must be paid to the thermal insulation of fairings and fuel supply network. For the latter, electrical continuity is essential to avoid sparkovers.
(iv) Electromagnetic effects: As indicated before, the electromagnetic effects can be of two distinct types. When the system/object under consideration is directly struck, then it becomes a path for the large return stroke current. The large magnitude of the current along with the associated large rate of rise leads to several electromagnetic effects. The electric voltage difference between the system and remote earth can rise to hundreds of kilovolts or more. Also across different parts of the struck system/object, large transient potential differences develop. This can lead to the failure of the electrical system due to damage in the electrical insulation, sparkover across different parts and melting of electrical circuits. The electrical circuits connected to the struck system can be made to carry significant currents either due to sharing of small fraction of the stroke current or due to strong induction, which can lead to irrecoverable damage.
The indirect effect is due to the coupling of lightning electromagnetic fields to the system. Depending on the distance between the strike point and the system under consideration, either the induction term or the radiation term of the lightning electromagnetic field presumes prominence. The indirect effects get augmented when there is an electrical connection from a remote point to the system under consideration. In such situations, the induction all along the external electrical connection is conveyed to the system under consideration. The electric power and wired communication lines serve as the best examples of such a connection.
Modern day electronics is far more sensitive to any such electromagnetic disturbances. Inherently, unlike the electrical system, they are very susceptible to both direct and indirect effects. Every care is necessary to shield them from lightning electromagnetic fields and protect them with suitable surge absorbing devices.
A launch pad is a complex structure involving a great number of circuits, including electrical circuits. It contains external connection to control centers, fuel storage units, water tanks, etc. When the vehicle is not on the pad, threat due to a direct hit to the launch pad would mainly affect the internal electrical circuits. The external connection significantly contributes to this threat. On the other hand, when the vehicle is on the pad, umbilicals would carry induced currents, which, in spite of being orders of magnitude lower than the incident lighting current, can cause serious interference and damage. In particular, this is of serious concern to the vehicle equipment bay carrying flight guidance and control equipment and telemetry. The sensitive electronic circuits of these equipments have a low damage threshold. Another important concern arises out of pyro-technique or electro-explosive devices employed in the rocket. These devices employed in separating stages, as well as self-destruction, are triggered by an electrical signal. Therefore, extensive care is necessary to prevent any untowardly incident.
The lightning current in the event of a strike normally propagates along the outer metallic skin of the vehicle and appropriate electrical bonding between stages ensures minimal induction to internal circuits. However, the nose fairing and electrical window for telemetry or vehicle equipment bay, serve as apertures for penetration of electromagnetic fields, which results in electrical induction to internal circuits.
Lightning Protection System – Components of External Protection System
External lightning protection system consists of three elements working in cohesion, namely, air termination system, down conductor system and earth termination system. In order to minimize the possibility of secondary or back flashover and in order to minimize the risk posed by resulting electromagnetic fields, very critical systems generally have isolated external protection schemes. In such schemes the lightning protection system (LPS) is a physically independent structure. However, there could be exceptions, which will be evident in a later section.
Air Termination System
This is the topmost section of the LPS, which performs the task of stroke interception. It is generally composed of conducting masts commonly known as lightning rods and shield wires (catenaries). As described in the section on events in a cloud-to-ground lightning, the stepped leader descending from cloud lowers a fraction of cloud charge, and thereby enhances the electric field on ground and grounded structures. Tall slender objects like towers and also the catenaries, by virtue of their geometry, enhance the background electric field by more than an order of magnitude at their top. When this resultant field reaches a critical value, upward electrical discharge is initiated, which propagates towards the descending leader. The critical field level for corona inception, which is the first discharge and is very localized, is about 26–28 kV/cm at standard temperature (20°C) and pressure (1 atmosphere). Subsequent increase in the ambient or the background field can at appropriate conditions lead to the inception of a continuously propagating upward leader. As compared to the corona streamer discharge, the leader discharges can propagate to much larger distances. When the gradient between the tip of downward and upward discharges reaches critical breakdown value, an electrical bridging occurs and a return stroke is initiated. In other words, a stroke that would have terminated on the ground is now attracted towards the object and when this is performed by the LPS, it is called stroke interception.
Generally, for a given geometry, the taller the object, higher will be the field intensification and hence the stroke interception capability. As the upward discharge can also be initiated from the structure to be protected, it becomes inevitable that LPS must be taller than the structure. For the design of air termination system for launch pads, electrogeometric models are employed.
Down Conductor System
This part of LPS electrically interconnects the air termination system with the earth termination system. It is basically intended to safely convey the stroke current between the air termination and the ground and to prevent any secondary electrical flashover between the LPS and system to be protected. The resulting electromagnetic field in the protected volume is minimal. The down conductor system can be formed by the tower supporting the mast and catenoids or as in the insulated mast scheme it can be formed by catenaries running from air termination system to the remote earth.
Earth Termination System
It is generally formed by a network of vertically driven rods and horizontally lied counterpoise in soil. Earth termination system has the function of safely dissipating the stroke current into the soil. It has to ensure that the rise in soil voltage and coupling to neighboring earths and buried conduits are kept minimal. Electrically, there are different types of earth defined in soil like the power earth, signal/switching earth and the lightning earth. They interact with each other through the common medium the soil, and hence additional measures may become necessary to ensure that lightning earth while performing its function does not induce damage by strongly coupling to other earths.
Lightning Protection System – Internal Protection
The basic safety measures of internal lightning protection system are discussed here. The existing protection philosophy has not really been verified to ensure a total safety against a natural or triggered lightning. Incidentally, a launch operation is not permitted if there is a possibility of lightning.
Vehicle
With regard to the vehicle, the following general rules apply. With the metallic main structure, all tank sections must be welded and bonded for electrical continuity. This will not only provide low impedance path to lightning, but is also necessary as the body of the vehicle will serve as reference ground for internal electrical equipment. Any metallic structure larger than about 30 cm should have discharge path to the structure.
The top part of a rocket where the spacecraft is housed is typically a non-metallic construction comprising of fiberglass and aluminum honeycomb. Similarly, for the two-way communication to the external world, the vehicle equipment bay needs to have apertures or a window for electromagnetic signals, which is necessarily non-metallic. All non-metallic segments and external body parts made of composite materials need to have a well-secured conducting path that extends throughout. Incidentally, for non-conducting external surfaces of the vehicle, a conducting paint may become necessary for draining out charges collected by tribo-electrification process.
Coupling to internal cables are possible only through apertures and composite skin. The main coupling mode is the common mode. Suitable shielding and grounding throughout in accordance with the general guidelines of electromagnetic shielding is the common solution. This applies to power, radio frequency, digital and ordnance circuits and their connections.
Pad and Vehicle on Pad
The final ground operations with vehicle on pad are not only very important, but can prove to be quite dangerous as well. Strokes threats due to both bypass/shielding failure, as well as nearby strokes need attention. In order to limit the damage by lightning and consequential high current transients, the following is recommended.
All ground support equipment like umbilical tower, service tower, etc., must be adequately grounded and bonded. The individual equipment must be grounded to facility structure. All electrical connections between ground support equipment and payloads must be reliably grounded. Lightning surge suppression devices and other local measures should be employed at interfaces of critical circuits. Shielded twisted pair type wiring is recommended for umbilical cables. For connection to critical systems, insulated cables with double shields are recommended with outer shield grounded at both ends. For long length connections, the outer sheath be grounded at regular intervals of 10–20 m. If the existing electrical connections do not have shields, or wires are without braid, etc., separate grounding strips grounded at regular intervals of say 10–20 m should be installed.
Lightning Launch Commit Criteria
The final countdown and subsequent launch operation is dependent on prevailing weather conditions. Based on various considerations, suitable weather launch commit criteria have been developed and employed. The Lighting Launch/flight Commit Criteria (LLCC) form an important subset of weather launch commit criteria. It is formulated to ensure that no lightning strike, either natural or triggered by the vehicle, would occur during the entire phase of launching and transit of the vehicle. The rules are summarized here.
• Tanking shall not be started, if the forecast predicts a greater than 20% chance of lightning within 9.25 km (5 nautical miles) of the launch pad during first hour of the tanking.
• Further, even the umbilical connections may have to be taken out if the electric field in the area exceeds a certain level.
• During the process, if by chance the ground electric field in the pad area exceeds 5–10 kV/m, the prevailing safety rules in certain cases demand that the personnel shall evacuate the launch site.
• It is not permitted to launch (and fly) within 18.52 km (10 nautical miles) of any type of lightning or any cloud that has produced it, within the past 30 minutes. An exception is allowed if the cloud has moved beyond 18.52 km and if an electric field within 9.25 km of the flight path is lower than 1 kV/m for last 15 minutes.
• Launch is not permitted if electric field within 9.25 km of the flight path has exceeded 1–1.5 kV/m in the past 15 minutes. Exceptions are permitted for the rules under certain restricted conditions.
Apart from these, there are about 6–10 rules related to the type and condition of the clouds that are around the flight path. Many of these rules appear to be rather conservative but presently they are deemed safe.
It is evident that the main line of defense against lightning is LLCC and therefore the real efficacy of lightning protection systems has not been subjected to critical tests. Nevertheless, it is known that the LPS at many sites have successfully intercepted strokes and the system withstood the consequential electromagnetic fields. Unfortunately, details regarding the associated stroke currents and the operation being performed on the pad are not readily available.
Protection of Other Important Structures
The spaceport has several other critical structures that are essential for launch operations. They also need to be protected against natural lightning. Due to their physical disposition, it is necessary that each of the structures be provided with their own LPS. The detailed guidelines given in IEC 62305 for the lightning protection of structures must be followed in their design. For short structures very close to the launch pad, the LPS of pad itself would provide adequate protection from a direct hit. However, strong internal protection is necessary for any tall structure. Often for a reliable shielding against the lightning electromagnetic fields of critical control and equipment buildings next to the pad, a complete metal mesh or metal sheet lining of the building (either inside or externally buried in cement), with solid multipoint connection to the ground, is recommended.
A large number of cables of different categories run from the main control center to the control building next to the launch pad. The lengths involved are a few kilometers. Therefore, these cables are more susceptible to a lightning threat due to a nearby strike, both direct galvanic coupling when the strike is close by and induction due to lightning electromagnetic fields can lead to significant surges. Damage to those cables would be laborious to trace and replace. It is preferable that they run on a conduit lined with electrically connected metallic sheets. If this option is expensive, then the galvanized iron strips must be run with grounding at regular intervals of 20 m through driven rods. These strips must be distributed over the periphery of the cable duct. An optical cable without any inbuilt electric supply line is immune to electromagnetic interference and hence, whenever feasible, would be an ideal choice.
Additional Issues
There are non-conventional lightning protection systems for structures, which can be broadly classified into: (a) early streamer emission systems; and (b) lightning elimination systems. These are sold under various names in different countries. Even though manufacturers of these systems claim an impressive list of satisfied customers, the world’s scientific lightning community does not endorse them. There is no scientific evidence at present for their better performance claimed by their manufacturers respect to the conventional Franklin rod system, which is being claimed by their manufacturers.
There are a number of references in which the protection efficacy of the LPS is demonstrated by high voltage impulse breakdown studies conducted on geometrically scaled down models in laboratory. Both conventional and non-conventional systems have been subjected to test. The limitation of such an approach has been elaborated in the literature. Serious limitation originates from the fact that only the geometry is scaled down and not the other physical parameters and, as a result, the phenomena in natural lightning cannot be emulated in the laboratory.
As mentioned in an earlier section, at present it is not possible to make a reliable quantification of the threat due to a direct hit, as well as nearby strokes. Significant progress in several directions is necessary for taking up such a task. An integrated lightning current and field measurement system is essential in the launch pad area, whose data can be utilized for further work.
Further Reading
1. Berger K. The Earth Flash. In: Golde RH, ed. Lightning. London: Academic Press; 1977.
2. Vernon Cooray. The mechanism of the lightning flash. In: The Lightning Flash. United Kingdom: IET: Vernon Cooray, IEE Power and Energy Series 34; 2003.
3. Godfrey R, Mathews ER, McDivitt JA. Analysis of Apollo 12 lightning incident. Nasa Report MSC-01540 1970.
4. Golde RH. Lightning Currents and Related Parameters. In: Golde RH, ed. Lightning. London: Academic Press; 1977.
5. IEC 62305. International Standard on Protection Against Lightning, 62305–1: General principles; 62305-2: Risk assessment; 62305-3: Physical damage to structures and life hazard; 62305-4: Electrical and electronic systems inside structures. Geneva: International Electrotechnical Commission; 2006.
6. Rohan Jayaratne. Thunderstorm electrification mechanism. In: The Lightning Flash. United Kingdom: IET: Vernon Cooray, IEE Power and Energy Series 34; 2003.
7. Krider EP, Christian HJ, Dye JE, et al. Natural and Triggered Lightning Launch Commit Criteria. Atlanta, GA, USA: The 86th American Meteorological Society Annual Meeting, 12th Conference on Aviation Range and Aerospace Meteorology; 2006; January 2006.
8. Kumar U, Nagabhushana GR. Analysis of Lightning Protection System for India Satellite Launch Pad. Orlando, Florida: National Interagency Coordination Group Lightning Conference; 2000.
9. Kumar U. Experimental Investigation with the Scaled-Down Models for the Post-stroke Potential Differences & Currents in UT/MST and LPS, Final report on the project sponsored by ISRO-IISc Space Technology Cell. 2002.
10. Kumar U, Nelson TJ. Analysis of the Air Termination System of the Lightning Protection Scheme for the Indian Satellite Launch Pad. Proc of IEE on Science & Measurement Technology. 2003;150(1):3–10.
11. Kumar U, Hedge V, Darji P. Investigations on the Voltages and Currents in the Lightning Protection System of the Indian Satellite Launch Pad-I During a Stroke Interception. Proc of IET Science Measurement and Technology. 2007;1(5):225–231.
12. Kumar U. Lightning Protection of Satellite Launch Pads. In: Lightning Protection. United Kingdom: IET: V Cooray, IEE Power and Energy Series 58; 2010.
13. Lightning and Space Program, August 1998, NASA Facts AC 321/867-2468.
14. John F Kennedy Space Center, FS-1998-08-16-KSC, August 1998.
15. LIS/OTD 0.5 Degree High Resolution Full Climatology (HRFC), Goddard Space Flight Center. Available from http://gcmd.nasa.gov/records/GCMD_lohrfc.html; accessed 16.02.12.
16. Rakov VA, Uman MA. Lightning Physics and Effects. Cambridge: Cambridge University Press; 2005.
17. NASA: Design Considerations for Lightning Strike Survivability, Preferred Reliability Practices, Practice no. PD-ED-1231.
18. Roeder WP, McNamara TM. A Survey of the Lightning Launch Commit Criteria. Atlanta, GA, USA: The 86th American Meteorological Society Annual Meeting, Second Conference on Meteorological Applications of Lightning Data; 2006.
19. Schaffar A, Lemeur P, Gobin V, Bertuol S. ARIANE 5 Lightning Verification Plan. Toulouse, France: International Conference on Lightning and Static Electricity; 1999; Paper No. 1999-01-2334.
20. Uman MA. Lightning. New York: Mc-Graw Hill; 1969.
21. Uman MA, Rakov VA. The interaction of lightning with airborne vehicles. Progress in Aerospace Sciences. 2003;39:61–81.
22. Wikipedia (2012). Available from: http://en.wikipedia.org/wiki/Cape_Canaveral_Air_Force_Station_Space_Launch_Complex_41, accessed 16.02.2012.
23. Williams E. Charge structure and geographical variations of thunderclouds. In: Lightning Protection. United Kingdom: IET: V Cooray, IEE Power and Energy Series 58; 2003.
24. Xichang Satellite Launch Centre. Available from www.sinodefence.com/space/facility/xichang.asp; accessed 16.02.2012.