Lightning One of Nature’S Most Fascinating Phenomena • Lightning Is One of the Most Beautiful and Spectacular Displays in Nature
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Lightning One of Nature’s Most Fascinating Phenomena • Lightning is one of the most beautiful and spectacular displays in nature. • It is also one of the most deadly natural phenomena known to man. • With bolt temperatures hotter than the surface of the sun and shockwaves radiating out in all directions, lightning is a lesson in physical science and humility. Lightning What is lightning? • Lightning is a massive electrostatic discharge caused by unbalanced electric charge in the atmosphere, either inside clouds, cloud to cloud or cloud to ground, accompanied by the loud sound of thunder. – Electrostatic discharge? • Think of those annoying finger zaps you get when you drag your feet across the carpet…only many more magnitudes more powerful and awesome. Lightning Where does it occur? Lightning How often does it occur? • Less than 0.1 flash/km2 per year (<0.26 flash/mi2 per year) in Western Washington. • Eastern Washington might see as high as 0.5 flash/km2 per year (1.3 flash/mi2 per year). • Whereas Florida might see 14 flash/km2 per year (37 flash/mi2 per year). – That’s 10-100 times more often than what we’d see in the Pacific Northwest. Lightning Did you know? • Lightning strikes 40–50 times a second worldwide, possibly more. – That’s at least 1.5 billion flashes per year. • The strike (arc) can be over 5 miles (8 km) long. • It can raise the immediate air temperature to over 50,000 oF. • It can be over 100 million Volts. – Typical residential and commercial electrical systems are 240/120 Volts and 480/277 Volts or some combination of the two. • It is also believed to have played an vital role in life on Earth. Lightning On a personal note…Did you know? • The odds of becoming a lightning victim in the U.S. in any one year is 1 in 700,000. • The odds of being struck by lightning in your lifetime is 1 in 3,000. • If you can hear thunder, you are within 10 miles (16 kilometers) of a storm and can be struck by lightning. – Sound travels about 1 mile every 4.6-4.7 seconds, hence the “count the seconds and divide by 5 rule” for estimating how far away the lightning strike was. Lightning How does it all work? • Lightning is not confined to thunderstorms. – It's been seen in volcanic eruptions, extremely intense forest fires, surface nuclear detonations, heavy snowstorms, and in large hurricanes. • Typically, as a thundercloud moves over the surface of the Earth, an electric charge equal to but opposite the charge of the base of the thundercloud is induced in the Earth below the cloud. – The induced ground charge follows the movement of the cloud, remaining underneath it. Lightning How does it all work? • The more common “negative discharge” occurs when negative charge accumulates at the base of thundercloud, inducing a positive charge on the ground. • Leaders form at the base of the thundercloud and travel towards the ground. – The negatively charged leader proceeds downward in a number of quick jumps (steps). Lightning How does it all work? • This initial phase involves a relatively small electric current (tens/hundreds of amperes). • The leader is almost invisible when compared with the subsequent lightning channel. • The progression of stepped leaders takes a comparatively long time (hundreds of milliseconds) to approach the ground. • When a stepped leader approaches the ground, the presence of opposite charges on the ground enhances the strength of the electric field. – If the electric field is strong enough, a positive streamer can develop . Lightning How does it all work? • As the field increases, the positive streamer may evolve into a hotter, higher current leader which eventually connects to the descending stepped leader from the cloud. • It is also possible for many streamers to develop from many different objects simultaneously, with only one connecting with the leader and forming the main discharge path. Lightning How does it all work? • Once a channel of ionized air is established between the cloud and ground this becomes a path of least resistance and allows for a much greater current to propagate from the Earth back up the leader into the cloud. • This is the return stroke and it is the most luminous and noticeable part of the lightning discharge. Lightning How does it all work? • When the electric field becomes strong enough, an electrical discharge (the bolt of lightning) occurs within clouds or between clouds and the ground. • During the strike, successive portions of air become a conductive discharge channel as the electrons and positive ions of air molecules are pulled away from each other and forced to flow in opposite directions. • The electrical discharge (averaging 30 kA for negative or 300 kA for positive lightning, and travelling at around 1×108 m/s) rapidly superheats the discharge channel, causing the air to expand rapidly and produce a shock wave heard as thunder. – Typically, a single lightning strike is made up of multiple individual strokes. – The rolling and gradually dissipating rumble of thunder is caused by the time delay of sound coming from different portions of a long stroke. Lightning What does lightning damage look like? Lightning What does lightning damage look like? Lightning What does lightning damage look like? Lightning What does lightning damage look like? Lightning Really? • Not all lightning forms in the negatively charged area low in the thunderstorm cloud. • Some lightning originates in the top of the thunderstorm, the area carrying a large positive charge. – Lightning from this area is called positive lightning and is even more powerful than “normal” or negative lightning. Lightning Really? • Positive lightning is particularly dangerous, because it frequently strikes away from the rain core, either ahead or behind the thunderstorm. • It can strike as far as 5 or 10 miles (8 or 16 kilometers) from the storm, in areas that most people do not consider to be a lightning-risk area. – This is known as the “…the bolt came from out of the blue…” phenomena. Lightning The Danger to Life • Lightning strikes injure humans (and animals) in several different ways: – Direct strike, which is usually fatal. – Contact injury, when the person was touching an object that was struck. – Side splash, when current jumps from a nearby object to the victim. – Ground strike, current passing from a strike through the ground into a nearby victim. • A strike can cause a difference of potential in the ground (due to resistance to current in the Earth), amounting to several thousand volts per foot. – Blast injuries, including hearing damage or blunt trauma by being thrown to the ground. Lightning Example of Contact Injury Lightning Example of Ground Potential Rise from Ground Strike Lightning The Danger to Life • According to the National Oceanic and Atmospheric Administration (NOAA), over the last 20 years, the United States averaged 51 annual lightning strike fatalities. • Lightning strikes can produce severe injuries, and have a mortality rate of between 10% and 30%, with up to 80% of survivors sustaining long-term injuries. – These severe injuries are not usually caused by thermal burns, since the current is too brief to substantially heat up tissues, instead nerves and muscles may be directly damaged by the high voltage producing holes directly in their cell membranes. Lightning The Danger to Life • A person injured by lightning does not carry an electrical charge, and can be safely handled. • Administer first aid before emergency services arrive. – Lightning can affect the brainstem, which controls breathing. • If a victim appears lifeless, it is important to begin artificial resuscitation immediately to prevent death. Lightning Personal Safety • If you can hear thunder, you are within 10 miles (16 kilometers) of a storm and can be struck by lightning. • If outside, follow the 30-30 rule: – Take appropriate shelter when you can count 30 seconds or less between lightning and thunder. – Remain protected, out of the weather or sheltered for 30 minutes after the last sound of thunder. Lightning Electrical and Structural Damage • Telephones, modems, computers and other electronic devices (TVs, VCRs, DVDs etc.) can be damaged by lightning. – Harmful overcurrent can reach them through the phone jack, Ethernet cable, or electrical outlet (electrical system). • In addition to electrical wiring damage, the other types of possible damage to consider include structural, fire, and property damage. Lightning Electrical and Structural Damage • Surge protectors do not provide protection against a lightning transient. – The energy from a lightning strike is too great to be absorbed by typical transient voltage surge suppressors (TVSS) that are part of the protection scheme within surge protectors. Lightning Lightning Protection • The NEC requires certain grounding, bonding and protection features which are intended to protect against lightning. – These safeguards greatly reduce the risk of shock or electrocution to a person in the house, and the risk of fires caused by lightning. – However, they are totally inadequate to prevent damage to electrical or electronic equipment. Lightning Lightning Protection • Lightning effects can be direct and/or indirect. – Direct effects are from resistive heating, arcing and burning. – Indirect effects are more probable and include capacitive, inductive and magnetic behavior. • Lightning protection, in the absolute sense, is impossible. Lightning Lightning Protection • Lightning rods – In Benjamin Franklin’s day, lightning rods conducted current away from buildings to Earth. – Now known as air terminals, they are now believed to send streamers upwards at varying distances and times according to their shape, height and other factors. • Air terminal design may alter streamer behavior – Blunt pointed rod vs. sharp pointed rod. – Air terminal design and performance is a controversial and unresolved issue. Lightning Lightning Protection • Down conductors, Bonding and Shielding – Down conductors should be installed in a safe manner through a known route on the outside of a building.