Small Shelters and Safety from Lightning
By Richard Kithil, President & CEO, NLSI
Vladimir Rakov, Ph.D., Dept. of Electrical and Computer Engineering,
University of Florida
Copyright © 2001 Society of Automotive Engineers,
Published as Paper Number 2001-01-25,
Used with permission
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.
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,
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."
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 an independent consulting, education, and research organization
providing objective information on lightning safety issues. 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. 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.