Lightning Safety

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Created on Wednesday, 26 August 2009 12:52

Last Updated on Saturday, 10 August 2013 21:38

Written by Khalid Shoaib AP2MKS

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Lightning Safety

from Larry Kendall, K5END on June 14, 2009
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Lightning-human encounters cause burns, trauma and electrical interference with physiological processes, often with fatal results. This article addresses personal safety concerns with lightning. Antenna and grounding practices are covered quite well in recent QST articles as well as ARRL and other literature and are therefore not iterated here. Lightning protection for backpacking and blue-water sailing are not covered comprehensively in this article.

The following three points are important.

Big Point 1: The first strike of lightning in a thunderstorm is just as deadly as the last. For some reason, people tend to dismiss the danger of lightning for at least the first few strikes. Electrical storms don't have to "warm up" to be deadly. If you can hear thunder, you are vulnerable immediately. Seek shelter.

Big Point 2: A tree is a poor choice for shelter. Lightning striking the tree will flashover in a deadly penumbral shape around the tree as it seeks paths to ground. Secondly, the tree sap will boil and turn to steam rapidly, exploding the trunk of the tree. This is a bad place to be when it happens.

Big Point 3: Lightning is second only to floods in terms of "natural" fatalities. Fear lightning more than earthquakes, tornadoes, wildfires or volcanoes. In the United States, Florida has the highest frequency of lightning events; lightning in Alaska is rare.

LIGHTNING BASICS

Lightning is the plasma path that allows charge equalization between the atmosphere and the ground or between regions of the atmosphere. This process is nearly continuous worldwide. The process of lightning path generation as a plasma wave forges its way through the air is both fascinating and esoteric. Air does break down under electric fields and lightning occurs. That is what matters.

A lightning strike is a "current source" and is effectively independent of load impedance. Call it an electrical Tsunami. You can neither stop it nor out run it. Protect yourself as you would from a Tsunami: stay out of its path!

A lightning strike is composed of several strokes as various charged regions are equalized. Stroke "leaders" may originate and extend from cloud to ground, ground to cloud or cloud to cloud. You can observe the individual strokes in the duration of a single strike especially well if at a distance when wind across the ground is significant where the strike occurs. The paths shift slightly with the wind during the series, giving the appearance (to human visual perception) of "ribbon lightning." Strokes often fork into diverse paths as the charge finds the most efficient routes toward equalization.

Yes, Virginia, lightning can strike the ground where the sky overhead is clear and blue; thus the expression, "out of the blue." Lightning can travel horizontally for miles before striking the ground. If you can hear thunder, you are within range of a strike.

Cloud-to-ground voltages range from 100 million to 1 billion Volts.
Typical peak current is 30,000 Amperes. This means that only 0.0000033 of the electrical current in a routine lightning strike is lethal.
Typical length of a lightning path is 5 kilometers.
The short time domain of a lightning stroke means that a large bandwidth is conducted and radiated as energy. This is why lightning static noise is present everywhere across the radio bands. It is less noticeable on FM modes, but it is still there on the bands (lightning is noticeable on FM broadcast mostly as "dead air" when their transmitter/antenna takes a strike!) What does this mean to us? It means that the protective grounding configuration must account for and have minimum impedance for all frequencies from DC to UHF.

Power of a single lightning strike:

Visible light 1-3%

Sound >10%

Heat <50%

Radio waves <50%

Duration ranges from 0.001 to 0.5 seconds.

The region around a ground strike will cause a voltage gradient in the top two feet or so of the soil until the current is dissipated. Persons standing with feet spread or laying on the ground are at increased risk. (Anecdotally, it is claimed that cattle die when the poor heifer's axis is aligned with this gradient. Who knows?)

Shrapnel from objects disintegrating near a ground strike is another common source of injury, especially in rocky areas.

LIGHTNING PROTECTION

The best defense to lightning is to seek safe shelter within an ENCLOSED structure, which means you need to be aware of the weather and have a plan beforehand. For example, any boater who finds himself in peril on a lake when a thunderstorm emerges needs to work on his situational awareness, if not consider another form of recreation. Think ahead and plan. The human species supposedly became more intelligent as a result of the last ice age. Those who were able to plan ahead were the ones who survived!

Buildings which are NOT SAFE are those with exposed openings. These include beach shacks, picnic shelters, pavilions, carports, and baseball dugouts. Porches are especially dangerous.

Once inside a suitable building, stay away from electrical appliances and plumbing fixtures. Lightning can travel great distances through power lines, especially in rural areas. This means a distant storm poses a risk inside your home, especially in the absence of proper grounding. Electrical appliances pose a risk to the user, ESPECIALLY corded telephones. Computers are also dangerous as they usually are connected to an ISP facility and the house AC. It should be obvious this is not the time for a shower, bath or a hot tub party.

An enclosed metal vehicle makes a good shelter. When lightning strikes a metal car, it is conducted through the car's metal structure and then arcs to the ground, at the easiest point and usually from the axle to ground. Aftermarket (ham) antennas may compromise the safety within a vehicle. Roll the windows up (glass is a pretty good inhibitor to plasma) and avoid contact with any conducting paths leading to the outside of the vehicle. Keep in mind that tornadoes often coincide with thunderstorms, and a car is not adequate shelter from a tornado.

Don't fall prey to the urban legend: the myth of tires. The tires have nothing to do with lightning safety. A car is safer because you are surrounded by a conductor, and Gauss's law, #1 in Maxwell's 4 famous collective equations is a good source to find an explanation of why this matters. In short, the voltage drop from the top of the car where the lightning hits to the floorboard where your behind is planted is enough protection to keep the air inside the vehicle from ionizing and providing a path for the lightning. BUT, I wouldn't go raising my hands to the headliner to test the protection factor! The "myth of tires" is pointed out here because of the following unsafe vehicles. Convertible (or fiberglass) vehicles offer little safety from lightning, even if the top is "up". Other vehicles, which are NOT SAFE during lightning storms, are those, which have open cabs, such as golf carts, tractors, and construction equipment. Motorcycles do not offer adequate protection from lightning.

Water or material on the surface of windows may heat with rapid steam generation or flashover conduction and cause the glass to shatter. Be aware of this possibility. This author is not aware of documented examples of this, but includes it as a possibility and recommends facing the interior of the car or building during shelter from an intense electrical storm.

Use the "30/30 rule." If the interval between the light flash and the thunder is less than 30 seconds, seek shelter and remain sheltered for 30 minutes after the last strike of the storm. "Counting seconds" is not an accurate method to estimate lightning distance! The speed of sound is dependent on air density and other factors, and there is no accounting for triangulation. Note that thunder may be a rumble rather than a sharp report. This is because the observer would be hearing various regions of the lightning path, all of which travel varied distances at the speed of sound, which also varies. A sharp thunder report implies the site of the ground strike is nearby, nothing more.

For outdoor activities:
The first outdoor procedure is to get indoors. Recreational pilots have a saying about weather. "It's better to be on the ground wishing you were flying than to be flying and later wish you were on the ground." The same logic applies to lightning. Know the weather forecast before you venture outside.

HAVE A PLAN and make sure the responsible party members understand the plan. Designate individuals to monitor the weather.

Know safety shelters in advance.
Designate individuals to organize and lead evacuation.

This practice will save softball, little league and soccer players. Better to forfeit the game or skip the practice than to die for no reason at all. If the coach is not compliant, don't let him reduce your life expectancy or that of your child. Write him off to Darwin. Few will remember the outcome of the game for very long. The death will be remembered for much longer.

Golfers, listen up! Think about this. You're standing in wide open spaces, you are the tallest objects in the vicinity, you installed metal electrodes on your shoes (damp leather against your feet) pressed into the damp earth, and you're swinging a pretty fair "lightning rod" (carbon graphite can conduct too!) several feet above your head, if not holding an umbrella constructed with "pointy-ended" (perfect for coronal discharge initiation of a ground leader!) metal ribs extending a couple of feet in all directions. Call it a day, go inside, have a "beverage" and tell war stories (which is probably the important part of the outing anyway. Am I right?)

Backpacking and hiking require special instruction beyond the scope of this article. But in short, get off the ridges, spread the group out and stay away from the trees. Minimize your footprint on the ground; it would be better to squat with feet together, rather than lie flat. The group members should spread out considerably so that a strike would affect fewer members and the other members would still be able to apply first aid and begin medical evacuation.

Bicyclists, head for the nearest shelter or take cover. Stay aware of the weather and have a plan. If thunderstorms are predicted, consider whether you should ride another day.

Remember:

1. Lightning is an everyday killer across most of the United States.

2. The best protection from lightning is to stay out its path by seeking shelter.

3. Become informed and have a plan.

courtesy : eham.net