~ National Lightning Safety Institute ~

Section 5.5.5

1. Abstract

Small open shelters are common on athletic fields, golf courses, parks, roadside picnic areas, schoolyards, and elsewhere. Many of these shelters are built to protect against rain or sun, not lightning. What can be done to minimize risk/maximize safety for people inside them under direct and indirect lightning strike conditions? Although there is no such thing as a lightning-proof small outdoor shelter, a properly designed and installed lightning protection system may make a difference. Sometimes the difference is between life and death.

2. Lightning Hazard

There are five mechanisms of lightning related injury and death: direct strike, touch voltage, side flash (or surface arc), unconnected upward discharge, and step voltage (Lee, 1977; Cooper, 2000). About half of cloud-to-ground flashes form two or more terminations when striking the earth. Lightning is known to strike tall objects considerably below their tops. Many lightning discharges are likely to produce electrical arcs developing radially along the ground surface from the strike point. In the context of lightning safety, a conservative and prudent approach always is suggested.

3. General Information on Lightning Protection of Structures

A basic lightning protection system (LPS) against direct strikes to an ordinary structure includes (1) air terminals, (2) down conductors, and (3) ground terminals. These three elements of the LPs must form a continuous conductive path (actually at least two paths) for lightning current, with all connections between the elements typically being accomplished by bolting or welding. The function of the LPs is to intercept lightning and safely direct its current to ground. A metal roof and/or structural metal framework (including metal support posts) can be a part of the LPs if they are electrically continuous. A ground terminal may be a ground rod or a buried bare conductor wire encircling the structure (also called a loop conductor or a ring electrode). The ring electrode grounding design is beneficial in that it also serves to equalize potential on the ground surface and to intercept electric arcs developing along or under the ground surface toward the shelter from lightning strikes outside the ring.

4. Shelters Unprotected from Lightning

In the absence of the three-element lightning protection system described above, the structure should be considered unprotected from lightning. Small shelters with or without lightning protection should be avoided where possible during thunderstorms. A disclaimer statement should be posted on each unprotected shelter by the organization running the outdoor facility. Such a disclaimer should include a clear statement that the structure does not afford protection from lightning. It would also be appropriate here to include a concise lightning guideline for personal safety. These "lightning safety tips" are available from the US Golf Association, the National Weather Service, the National Lightning Safety Institute, and other groups.

5. Shelters Protected from Lightning

A small shelter equipped with a properly designed and installed LPs may provide reasonable protection from direct lightning. It is essential, however, that a person inside the shelter does not touch any element of the LPs and tries to position himself or herself at approximately the same distance from all down conductors. Electrical signs, pop machines, water faucets, irrigation controllers, etc. can increase personal hazards inside the shelter should lightning strike nearby. Shelters should be located in relatively low areas, preferably surrounded by a large number of trees of approximately the same height. They should not be located on hilltops, near metallic fences, power poles, tall trees, or near water. A small shelter, even one protected as described here, should be viewed as the last resort option. Refuges with much higher safety levels such as large buildings and fully enclosed metal vehicles should be sought instead when time permits.

A small recreation shelter with peaked roof should have an overhead shield wire system or Franklin rod system. Shield wires should be suspended on tall poles to form a "tent" over the shelter. Overhead shield wires should be connected to buried ground terminals via down conductors on the poles. Franklin rods on the roof should be connected to at least two down conductors on two diagonally opposite sides of the shelter; buried ground terminals should be connected to each of the down conductors.

To improve grounding efficiency and to equalize ground potential inside the shelter we suggest the use of buried metal mesh connected to the ground terminals. Flooring made of clean rock and placed on the mesh may provide additional safety advantages. As described in IEEE Std 80, "Guide for Safety in AC Substation Grounding", coarse gravel 4-6 inches in depth retards evaporation of topsoil moisture. IEEE Std 80 includes an abstract of a French study (Bodier, 1948) describing coarse gravel as an effective insulating barrier.

6. Case Studies

1. Massachusetts, 1996. A golf course shelter was located on a hilltop scenic vista with several tall trees located close by. Three golfers sought refuge from a violent hailstorm in the shelter. Lightning struck an adjacent tree and ground surface arcs entered the shelter. One person was killed and two were injured.

2. Illinois, 1985. A group of seven or eight people was huddled in an open wooden shelter during a thunderstorm. One person, wearing a Sony Walkman radio headset, stood just inches away from the wet asphalt shingled roof. When lightning struck, he was killed and others were injured.

3. Colorado, 1994. Two hunters were cooking on a charcoal stove under an outdoor shelter with a metal roof. One person was standing on a metal table positioned inside the shelter. His head was close to the roof. Lightning hit the roof. He was killed and his companion was injured.

7. Conclusion

This paper outlines general principles of the lightning protection of small open shelters. The most recent edition of NFPA-780 "Standard for the Installation of Lightning Protection Systems" should be consulted for further information. It contains many details with diagrams for the installation of air terminals, placement and sizing of conductor wires, use of aluminum vs. copper, suggested grounding design options, maintenance, etc.

It is generally possible to find a local company that installs lightning protection on buildings and trees. Look in the Yellow Pages for "Lightning Protection" and "Electrical Contractors." These companies should follow minimum standards recommendations of the NFPA-780 "Standard for the Installation of Lightning Protection Systems". The Underwriters Laboratories guideline is similar and is called "Installation Requirements for Lightning Protection Systems – UL96A."

8. References

1. ANSI/IEEE Std 80-1986, IEEE Guide for Safety in AC Substation Grounding, Institute of Electrical and Electronics Engineers, New York, NY.

2. Cooper, M.A. 2000. The fifth mechanism of lightning injury. In Proc. 2000 Intl. Lightning Detection Conf., Tucson, Arizona, 2 p.

3. Installation Requirements for Lightning Protection Systems – UL96A (1998), Underwriters Laboratories, Northbrook IL.

4. Lee, W.R. 1977. Lightning injuries and death. In Lightning, vol. 2, Lightning protection, ed. R.H. Golde, pp. 521-543.

5. NFPA 70 (1999), National Electrical Code, National Fire Protection Association, Quincy MA.

6. NFPA 780 (1997), Standard for Installation of Lightning Protection Systems, National Fire Protection Association, Quincy MA.

9. Contact Information

1. Richard Kithil, Founder and CEO of the National Lightning Safety Institute. NLSI is a non-profit consulting, education, and research organization providing objective information on lightning safety issues. Tel. 303-666-8817, Fax 303-666-88786, Email: info at lightningsafety dot com and website: www.lightningsafety.com

2. Vladimir A. Rakov, Professor of Electrical and Computer Engineering at the University of Florida, Gainesville, FL. He is the author or co-author of over 230 technical publications on various aspects of lightning and lightning protection. Tel. 352-392-4242, Fax 352-392-8381, Email: rakov@ece.ufl.edu and website: www.eel.ufl.edu/~lightning.

Recommended Lightning Safety Guideline:
If you can hear thunder or see lightning, you should move to reasonably safe shelters, such as large buildings or all metal vehicles.