~ National Lightning Safety Institute ~

Section 4.11

1. Summary

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:

• Early threat detection
• Notification of affected persons
• Evacuation to safe shelters
• Re-assessment of threat levels
• Resumption of activities

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. Safe and not safe structures

Knowing the above described behavior of lightning upon, say, an automobile, it is apparent that a fully enclosed metal vehicle is a 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:
1) Windows need to be rolled up.
2) Person must not make any interior contact with external objects, such as radio dials, metal door handles, two-way radio microphones, etc.
3) Person should avoid all other objects that penetrate from inside to outside.

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 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.

Good Shelters for Lightning Protection		Poor Shelters for Lightning Protection

5. Shelter placement on properties

How much time, in minutes, is required to get to 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:
1) Is there a pickup truck nearby that you can get to faster than an alternative shelter?
2) If you have a choice between a metal shelter and a plastic shelter at equivalent distances, choose the metal shelter.
3) If the only nearby structure is locked, seek an overhanging roof, where possible.
4) If you are caught with no shelter of any type nearby, crouch to the lowest possible position, avoiding all nearby metal objects.

6. Conclusion

To attain 100% lightning safety is not possible. But 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.

7. References

• Kithil, R. (2009). Lightning Protection For Engineers, Louisville, CO: National Lightning Safety Institute.
• Kithil, R. and Rakov V. (2001). "Small Shelters and Safety from Lightning," International Conference on Lightning and Static Electricity, Seattle, WA, September 2001.
• This NLSI website at www.lightningsafety.com