~ National Lightning Safety Institute ~
Good Lightning Shelters For Outdoor Workers
By Richard Kithil, President & CEO, NLSI
Sudden thunderstorms may bring an urgency for outdoor workers to cease work and quickly relocate to refuge. Several safety measures should be considered by management and by individual workers alike, including:
NLSI's paper entitled "An Overview of Lightning Detection Equipment" provides background information on threat detection and notification. Ordinary wood buildings provided for worker comfort, lunch breaks, or safety from rain or sun are not safe from lightning. What constitutes a safe location and why? This paper discusses characteristics of lightning, some behavioral aspects of it, “safe” and “not safe” structures, and their placement on typical properties. Some examples of suitable shelters are pictured.
2. Characteristics of lightning
Lightning strikes are arbitrary and random. Lightning has been recorded traveling from cloud sources 40 miles distant to cause injuries and deaths. Average temperatures are in the 50,000 degrees F. range. Median current levels are in the 25kA range (10mA can stop the human heart). Some 40% of lightning is forked with two or more ground attachment points. Thunder always accompanies lightning: “sound and light.” Hearing thunder indicates that that lightning was within hearing range … 6 to 8 miles normally.
3. Behavior of lightning on struck objects
High-frequency current flowing on a metal conductor generates an electromagnetic field. One effect of this is to confine amperages towards the outside of it. This is called “skin effect.” The thickness of the layer of restricted penetration is called “skin depth.” The higher the lightning frequency, the smaller the depth. By example, a copper wire conductor at 50 Hz has a skin depth of about 10 mm. However, lightning events induced on similar cables have much higher frequencies, on the order of many tens of Hz, even MHz, so in this case the skin depth is less than 1 mm. Skin depth is proportional to the square root of the inverse of the frequency.
People react much differently to lightning than do metal objects. The human body, being some 65% salt water, is a good conductor. Direct lightning strikes can follow either internal or external pathways or both. Indirect lightning damage mechanisms include: 1) flashover from an intended conductor (for example, a gazebo or tree) to an unintended conductor (such as a person seeking refuge from rain or hail); 2) step and touch voltages where a person’s hands or feet intercept electrical differentials, which then seek to equalize via the body; and 3) interruption of normal electrical heart beats leading to arrhythmia.
4. Lightning-safe structures
Knowing the above described behavior of lightning upon, say, an automobile, it is apparent that a fully enclosed metal vehicle is a lightning-safe shelter. Other all-metal mobile equipment — such as airplanes, buses, vans, and construction equipment with enclosed mostly-metal cabs — also are safe. A cautionary note, however, will emphasize that the “outer metal shield” should not be compromised. This means:
Unsafe vehicles include those made of fiberglass and other plastics, plus small riding machinery or vehicles without enclosed canopies, such as motorcycles, farm tractors, golf cars, and ATVs.
Metal buildings are lightning-safe places. So, too, are large permanent structures made of masonry and wood. Once again, the caveat is not to become a part of the pathway conducting lightning. This means avoiding all electrical circuits, switches, powered equipment, metal doors and windows, hand rails, and so on. Small post-supported structures, such as bus stops or picnic shelters, are not safe and cannot be made safe for people.
Metal shipping containers (also known as Conex containers or MilVans) can be easily modified to become cheap, effective, portable, and rapidly deployable shelters. Used ones are OK. Double walls are better than single walls. Cut out openings for ventilation. OSHA requires two separate doors. Install metal screening (2 x 2 inches) at all openings, along with simple awnings to help keep out rain. Install battery-powered lights; never install any AC-powered equipment. Maybe place some wooden benches along the walls for comfort. Inspect interiors periodically for critters, such as bees, bugs, snakes, and so on. Containers do not need to be grounded.
5. Shelter placement on properties
How much time, in minutes, is required to get to a lightning-safe shelter from different locations on a property? We suggest that 3-4 minutes, even under rapid evacuation, is adequate for reaching safety. Each site location is unique and different. Here are some examples of decisions to be made when seeking shelter:
To attain 100% lightning safety is not possible. But lightning safety training for outdoor workers and pre-planned defenses can assure a best attempt to achieve high levels of safety. Hear thunder? When to stop activities? Hear more thunder? Get ready to evacuate to safe shelter. What’s a safe place? How long to stay inside the shelter? These and other questions must be answered well in advance of the thunderstorm evacuation emergency.