lightning protection systemWhy Lightning Protection

Lightning is a capricious, random and unpredictable event. Its' physical characteristics include current levels sometimes in excess of 400 kA, temperatures to 50,000 degrees F., and speeds approaching one third the speed of light. Globally, some 2000 on-going thunderstorms cause about 100 lightning strikes to earth each second. USA insurance company information shows one homeowner's damage claim for every 57 lightning strikes. Data about commercial, government, and industrial lightning-caused losses is not available. Annually in the USA lightning causes more than 26,000 fires with damage to property (NLSI estimates) in excess of $5-6 billion.

The phenomenology of lightning strikes to earth, as presently understood, follows an approximate behavior:

  1. The downward Leaders from a thundercloud pulse towards earth seeking out active electrical ground targets.
     
  2. Ground-based objects (fences, trees, blades of grass, corners of buildings, people, lightning rods, etc., etc.) emit varying degrees of electric activity during this event. Upward Streamers are launched from some of these objects. A few tens of meters off the ground, a "collection zone" is established according to the intensified local electrical field.
     
  3. Some Leader(s) likely will connect with some Streamer(s). Then, the "switch" is closed and the current flows. We see lightning.

Lightning effects can be direct and/or indirect. Direct effects are from resistive (ohmic) heating, arcing and burning. Indirect effects are more probable. They include capacitive, inductive and magnetic behavior. Lightning "prevention" or "protection" (in an absolute sense) is impossible. A diminution of its consequences, together with incremental safety improvements, can be obtained by the use of a holistic or systematic hazard mitigation approach, described below in generic terms.

Lightning Rods

In Franklin's day, lightning rods conducted current away from buildings to earth. Lightning rods, now known as air terminals, are believed to send Streamers upward at varying distances and times according to shape, height and other factors. Different designs of air terminals may be employed according to different protection requirements. For example, the utility industry prefers overhead shielding wires for electrical substations. In some cases, no use whatsoever of air terminals is appropriate (example: munitions bunkers). Air terminals do not provide for safety to modern electronics within structures.

Air terminal design may alter Streamer behavior. In equivalent e-fields, a blunt pointed rod is seen to behave differently than a sharp pointed rod. Faraday Cage and overhead shield designs produce yet other effects. Air terminal design and performance is a controversial and unresolved issue. Commercial claims of the "elimination" of lightning deserve a skeptical reception. Further research and testing is on-going in order to understand more fully the behavior of various air terminals.

Down Conductors, Bonding and Shielding

Down conductors should be installed in a safe manner through a known route, outside of the structure. They should not be painted, since this will increase impedance. Gradual bends (min. eight inch radius) should be adopted to avoid flashover problems. Building steel may be used in place of down conductors where practical as a beneficial part of the earth electrode subsystem.

Bonding assures that all metal masses are at the same electrical potential. All metallic conductors entering structures (AC power, gas and water pipes, signal lines, HVAC ducting, conduits, railroad tracks, overhead bridge cranes, etc.) should be integrated electrically to the earth electrode subsystem. Connector bonding should be thermal, not mechanical. Mechanical bonds are subject to corrosion and physical damage. Frequent inspection and ohmic resistance measuring of compression and mechanical connectors is recommended.

Shielding is an additional line of defense against induced effects. It prevents the higher frequency electromagnetic noise from interfering with the desired signal. It is accomplished by isolation of the signal wires from the source of noise.

Grounding

The grounding system must address low earth impedance as well as low resistance. A spectral study of lightning's typical impulse reveals both a high and a low frequency content. The high frequency is associated with an extremely fast rising "front" on the order of 10 microseconds to peak current. The lower frequency component resides in the long, high energy "tail" or follow-on current in the impulse. The grounding system appears to the lightning impulse as a transmission line where wave propagation theory applies.

A single point grounding system is achieved when all equipment within the structure(s) are connected to a master bus bar which in turn is bonded to the external grounding system at one point only. Earth loops and differential rise times must be avoided. The grounding system should be designed to reduce ac impedance and dc resistance. The shape and dimension of the earth termination system is more important a specific value of the earth electrode. The use of counterpoise or "crow's foot" radial techniques can lower impedance as they allow lightning energy to diverge as each buried conductor shares voltage gradients. Ground rings around structures are useful. They should be connected to the facility ground. Exothermic (welded) connectors are recommended in all circumstances.

Cathodic reactance should be considered during the site analysis phase. Man-made earth additives and backfills are useful in difficult soils circumstances: they should be considered on a case-by-case basis where lowering grounding impedances are difficult an/or expensive by traditional means. Regular physical inspections and testing should be a part of an established preventive maintenance program.

Transients and Surges

Ordinary fuses and circuit breakers are not capable of dealing with lightning-induced transients. Lightning protection equipment may shunt current, block energy from traveling down the wire, filter certain frequencies, clamp voltage levels, or perform a combination of these tasks. Voltage clamping devices capable of handling extremely high amperages of the surge, as well as reducing the extremely fast rising edge (dv/dt and di/dt) of the transient are recommended. Adopting a fortress defense against surges is prudent: protect the main panel (AC power) entry; protect all relevant secondary distribution panels; protect all valuable plug-in devices such as process control instrumentation, computers, printers, fire alarms, data recording & SCADA equipment, etc. Further, protect incoming and outgoing data and signal lines. Protect electric devices which serve the primary asset such as well heads, remote security alarms, CCTV cameras, high mast lighting, etc. HVAC vents which penetrate one structure from another should not be ignored as possible troublesome electrical pathways.

Surge suppressors should be installed with minimum lead lengths to their respective panels. Under fast rise time conditions, cable inductance becomes important and high transient voltages can be developed across long leads.

In all instances, use high quality, high speed, self-diagnosing protective components. Transient limiting devices may use a combination of arc gap diverters-metal oxide varistor-silicon avalanche diode technologies. Hybrid devices, using a combination of these technologies, are preferred. Know your clamping voltage requirements. Confirm that your vendor's products have been tested to rigid ANSI/IEEE/ISO9000 test standards. Avoid low-priced, bargain products which proliferate the market (caveat emptor).

Detection

Lightning detectors, available at differing costs and technologies, sometimes are useful to provide early warning. An interesting application is when they are used to disconnect from AC line power and to engage standby power, before the arrival of lightning. Users should beware of over-confidence in such equipment which is not perfect and does not always acquire all lightning data.

Education

Lightning safety should be practiced by all people during thunderstorms. Preparedness includes: get indoors or in a car; avoid water and all metal objects; get off the high ground; avoid solitary trees; stay off the telephone. If caught outdoors during nearby lightning, adopt the Lightning Safety Position (LSP). LSP means staying away from other people, taking off all metal objects, crouching with feet together, head bowed, and placing hands on ears to reduce acoustic shock.

Measuring lightning's distance is easy. Use the "Flash/Bang" (F/B) technique. For every count of five from the time of seeing the lightning stroke to hearing the associated thunder, lightning is one mile away. A F/B of 10 = 2 miles; a F/B of 20 = 4 miles, etc. Since the distance from Strike A to Strike B to Strike C can be as much as 5-8 miles. Be conservative and suspend activities when you first hear thunder, if possible. Do not resume outdoor activities until 20 minutes has passed from the last observable thunder or lightning.

Organizations should adopt a Lightning Safety Policy and integrate it into their overall safety plan.

Testing

Modern diagnostic testing is available to mimic the performance of lightning conducting devices as well as to indicate the general route of lightning through structures. This testing typically is low power, 50 watt or less. It is traceable, but will not trip MOVs, gas tube arrestors, or other transient protection devices. Knowing the behavior of an event prior to occurrence is every businessman's earnest hope. With such techniques, lightning paths can be forecast reliably.

Codes & Standards

The marketplace abounds with exaggerated claims of product perfection. Frequently referenced codes and installation standards are incomplete, out dated and promulgated by commercial interests. On the other hand IEC, IEEE, MIL-STD, FAA, NASA and similar documents are supported by background engineering, the peer-review process, and are technical in nature.

Summary

It is important that all of the above subjects be considered in a lightning safety analysis. There is no Utopia in lightning protection. Lightning may ignore every defense man can conceive. A systematic hazard mitigation approach to lightning safety is a prudent course of action.

References

  1. API 2003, Protection Against Ignitions Arising out of Static, Lightning, and Stray Currents, American Petroleum Institute, Washington DC, December 1991.
  2. Golde, G.H., Lightning, Academic Press, NY, 1977.
  3. Hasse, P., Overvoltage Protection of Low Voltage Systems, Peter Peregrinus Press, London, 1992.
  4. Hovath, Tibor, Computation of Lightning Protection, John Wiley, NY, 1991.
  5. IEEE Std 1100, Powering and Grounding of Sensitive Electronic Equipment, IEEE, NY, NY. 1992.
  6. KSC-STD-E-0012B, Standard for Bonding and Grounding, Engineering Development Directorate, John F. Kennedy Space Center, NASA, 1991.
  7. Morris, M.E., et.al., Rocket-Triggered Lightning Studies for the Protection of Critical Assets, IEEE Transactions on Industry Applications, Vol. 30, No. 3, May/June 1994.
  8. Sunde, E.D. Earth Conduction Effects in Transmission Systems, D. Van Nostrand Co., NY, 1949.
  9. Towne, D., Wave Phenomena, Dover Publications, NY.
  10. Uman, Martin, Lightning, Dover Publications, NY, 1984.
  11. Viemeister, Peter, The Lightning Book, MIT Press, Cambridge MA, 1972.


What is Lightning

Lightning is a giant spark of electricity in the atmosphere between clouds, the air, or the ground. In the early stages of development, air acts as an insulator between the positive and negative charges in the cloud and between the cloud and the ground. When the opposite charges builds up enough, this insulating capacity of the air breaks down and there is a rapid discharge of electricity that we know as lightning. The flash of lightning temporarily equalizes the charged regions in the atmosphere until the opposite charges build up again.

Lightning can occur between opposite charges within the thunderstorm cloud (intra-cloud lightning) or between opposite charges in the cloud and on the ground (cloud-to-ground lightning).

Lightning is one of the oldest observed natural phenomena on earth. It can be seen in volcanic eruptions, extremely intense forest fires, surface nuclear detonations, heavy snowstorms, in large hurricanes, and obviously, thunderstorms.

What are cloud flashes?

A cloud flash is lightning that occurs inside the cloud, travels from one part of a cloud to another, or from the cloud to the air.

What is a "stepped leader?"

A stepped leader is a stream of weakly charged particles that flows from the cloud – it moves towards the ground, starting and stopping, and sometimes branching, trying to find the path of least resistance.

Is it possible to have thunder without lightning?

No, it is not possible to have thunder without lightning. Thunder is a direct result of lightning. However, it is possible that you might see lightning and not hear the thunder because it was too far away. Sometimes this is called “heat lightning” because it occurs most often in the summer.

Is lightning always produced by a thunderstorm?

Thunderstorms always have lightning (thunder is caused by lightning, and you can't have a thunderstorm without thunder), but you can have lightning without a thunderstorm. Lightning can also be seen in volcanic eruptions, extremely intense forest fires, surface nuclear detonations, and in heavy snowstorms.

What causes thunder?

Thunder is caused by lightning. The bright light of the lightning flash caused by the return stroke mentioned above represents a great deal of energy. This energy heats the air in the channel to above 50,000 degrees F in only a few millionths of a second! The air that is now heated to such a high temperature had no time to expand, so it is now at a very high pressure. The high pressure air then expands outward into the surrounding air compressing it and causing a disturbance that propagates in all directions away from the stroke. The disturbance is a shock wave for the first 10 yards, after which it becomes an ordinary sound wave, or thunder. Thunder can seem like it goes on and on because each point along the channel produces a shock wave and sound wave.

What is dry lightning?

Dry lightning is lightning that occurs without rain nearby. The NOAA Storm Prediction Center routinely forecasts dry lightning because this kind is more likely to cause forest fires.

What is a “bolt from the blue”?

 A “Bolt from the Blue” is a cloud-to-ground flash which typically comes out of the back side of the thunderstorm cloud, travels a relatively large distance in clear air away from the storm cloud, and then angles down and strikes the ground. These lightning flashes have been documented to travel more than 25 miles away from the thunderstorm cloud. They can be especially dangerous because they appear to come from clear blue sky.

A helmeted bicyclist experienced a lightning strike to the head under fair weather conditions with a cloudless sky. It was determined that the bolt probably originated in a thunderstorm that was about 16km away and obscured by mountains.

Lightning strikes the ground approximately 25 million times each year in the U.S. According to the NWS, the chance of an individual in the U.S. being killed or injured during a given year is one in 240,000. Assuming an average life-span of 80 years, a person's odds over their lifetime becomes one in 3000. Assuming the average person has ten family members and others with whom they are close, then the chances are one in 300 that a lightning strike will closely affect a person during their lifetime.

Does lightning always strike the tallest object?

Never say always! Lightning usually strikes the tallest object. It makes sense that the tallest object is most attractive, because it is the easiest path for the lightning to take.

What type of electricity is lightning?

Lightning is an electrostatic discharge accompanied by the emission of visible light and other forms of electromagnetic radiation.

How many volts and watts are in lightning?

Lightning can have 100 million to 1 billion volts, and contains billions of watts.

Why are positive lightning bolts deemed more dangerous than the more common negatively charged bolts?

Positive lightning is often considered more dangerous because its electrical field is stronger (forming at the top of the storm), the flash duration is typically longer, and its peak charge can be much greater than a negative strike. Plus, positively charged lightning can occur near the edge of a cloud or strike more than 10 miles away – when people aren't aware of the danger.

Does lightning strike from the sky down, or the ground up?

The answer is both. Cloud-to-ground lightning comes from the sky down, but the part you see comes from the ground up. A typical cloud-to-ground flash lowers a path of negative electricity (that we cannot see) towards the ground in a series of spurts. Objects on the ground generally have a positive charge. Since opposites attract, an upward streamer is sent out from the object about to be struck. When these two paths meet, a return stroke zips back up to the sky. It is the return stroke that produces the visible flash, but it all happens so fast - in about one-millionth of a second - so the human eye doesn't see the actual formation of the stroke.

How hot can lightning make the air?

Energy from lightning heats the air anywhere from 18,000 degrees Fahrenheit to up to 60,000 degrees Fahrenheit.

What causes lightning to be colored rather than the usual white or blue?

Lightning can appear to be many different colors depending on what the light travels through to get to your eyes. In snowstorms, where it is somewhat rare, pink and green are often described as colors of lightning. Haze, dust, moisture, raindrops and any other particles in the atmosphere will affect the color by absorbing or diffracting a portion of the white light of lightning.

How does the Earth benefit from lightning?

The earth benefits from lightning in several ways. First, lightning helps the Earth maintain electrical balance. The Earth is recharged by thunderstorms. The Earth's surface and the atmosphere conduct electricity easily—the Earth is charged negatively and the atmosphere, positively. There is always a steady current of electrons flowing upwards from the entire surface of the Earth. Thunderstorms help transfer the negative charges back to Earth (lightning is generally negatively charged). Without thunderstorms and lightning, the earth-atmosphere electrical balance would disappear in 5 minutes. Lightning also makes ozone-producing chemicals.

What happens to the ground when lightning strikes it?

What tends to happen when lightning strikes ground is that it fuses dirt and clays in to silicas. The result is often a glassy rock (called a fulgurite) in the shape of a convoluted tube. Fulgurite has been found all over the world, but is relatively rare. The color depends on the minerals in the sand that was struck. The shape in the ground is the shape of the path the lightning current followed in the ground. There is often damage to grasses along this path too.

Lightning traveling down a tree trunk turns water to steam. If it gets under the bark into the surface moisture of the wood, the rapidly expanding steam can blast pieces of bark from the tree, and the wood along the path is often killed.

Can lightning strike the same place twice?

Lightning does hit the same spot (or almost the same spot) more than once, contrary to folk wisdom. It could be simply a statistical fluke (i.e., with all the lightning that occurs, eventually lightning will strike somewhere near a previous lightning strike within a short period of time). It could also be that something about the site makes it somewhat more likely to be struck. Typically, when lightning strikes something on the ground, the object that is struck sends a faint channel upward that joins the downward developing flash and creates the connection to the ground. Taller objects are more likely than shorter objects to produce the upward channel. But it is also possible that something that locally affects the ability of the ground to conduct electricity (such as the salt or moisture content of the ground at the time, the presence or absence of rock, standing water, pipes or other metal objects in the ground), the terrain shape, the shape of leaves or twigs, or something else might make a particular location more likely than another nearby location to be struck.

When and where does lightning most frequently strike?

Lightning comes from a parent cumulonimbus cloud. These thunderstorm clouds are formed wherever there is enough upward motion, instability in the vertical, and moisture to produce a deep cloud that reaches up to levels somewhat colder than freezing.

These conditions are most often met in summer. In general, the US mainland has a decreasing amount of lightning toward the northwest. Over the entire year, the highest frequency of cloud-to-ground lightning is in Florida between Tampa and Orlando. This is due to the presence, on many days during the year, of a large moisture content in the atmosphere at low levels (below 5,000 feet), as well as high surface temperatures that produce strong sea breezes along the Florida coasts. The western mountains of the US also produce strong upward motions and contribute to frequent cloud-to-ground lightning. There are also high frequencies along the Gulf of Mexico coast, the Atlantic coast in the southeast US, and inland from the Gulf. Regions along the Pacific west coast have the least cloud-to-ground lightning.

Does lightning happen during the winter?

Lightning occurs less frequently in the winter because there is not as much instability and moisture in the atmosphere as there is in the summer. These two ingredients work together to make convective storms that can produce lightning. Without instability and moisture, strong thunderstorms are unlikely.

During the winter, the land surface is cooler because there is not as much heating by the sun to warm it up. Without warm surface temperatures, the near-surface air wouldn't rise in the atmosphere very far. Thus, the kinds of deep (8-15 km deep) thunderstorms that develop in the summertime wouldn't develop.

Warm air holds more water vapor. And, when water vapor condenses into liquid water cloud drops, latent heat is released which fuels the thunderstorm. So, warm, moist air near the surface (and the proper conditions aloft to give you lots of instability) can result in deep convection, which may produce lightning discharges.

Clouds become electrified when strong updrafts (fueled by the instability and moisture) bring supercooled liquid water drops and ice crystals at temperatures less than freezing (0 oC) together. In this environment, interactions between the ice crystals and supercooled water droplets produce electric charges. The exact mechanisms by which this charging happens remain unknown. The electrical charges build up until they are strong enough to overcome the resistance of the surrounding air. The breakdown of the electric fields produced by these charges is the lightning bolt.

What is thundersnow?

Although thunderstorms are less common in the winter, sometimes lightning can occur within snowstorms. Called thundersnow, relatively strong instability and abundant moisture may be found above the surface, such as above a warm front, rather than at the surface where it may be below freezing. Thundersnow is sometimes observed downstream of the Great Salt Lake and the Great Lakes during lake-effect snowstorms, too.

How many flashes a year are there?

Over the contiguous 48 states, an average of 20,000,000 cloud-to-ground flashes have been detected every year since the lightning detection network covered all of the continental US in 1989. In addition, about half of all flashes have more than one ground strike point, so at least 30 million points on the ground are struck on the average each year in the US. Besides cloud-to-ground flashes, there are roughly 5 to 10 times as many cloud flashes as there are ground flashes.

Cloud-to-ground lightning can kill or injure people by direct or indirect means. The lightning current can branch off to a person from a tree, fence, pole, or other tall object. It is not known if all people are killed who are directly struck by the flash itself. In addition, flashes may conduct their current through the ground to a person after the flash strikes a nearby tree, antenna, or other tall object. The current also may travel through power or telephone lines, or plumbing pipes to a person who is in contact with an electric appliance, telephone, or plumbing fixture.

Similarly, objects can be directly struck and this impact may result in an explosion, burn, or total destruction. Or, the damage may be indirect when the current passes through or near it. Sometimes, current may enter a building and transfer through wires or plumbing and damage everything in its path. Similarly, in urban areas, it may strike a pole or tree and the current then travels to several nearby houses and other structures and enter them through wiring or plumbing.

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