Background for the AMS Statement on Lightning Protection Systems
by C.B. Moore
New Mexico Institute of Mining and Technology
Lightning is the primary natural cause of fires and,
throughout recorded history, has damaged structures built by humans. That it
preferentially strikes tall objects has long been recognized; Artabanis, an advisor
to Xerxes, is quoted as saying 2300 years ago: "(God's) bolts fall ever
on the highest houses and tallest trees". In his poem, De Rerum Natura
published in 55 BC, Titus Carus Lucretius asked about thunderbolts "Why
has he (Jupiter) a special fondness for high places so that we see most
traces of his fire on mountain tops?" and "Why does he demolish the
holy shrines of the gods and his own splendid abodes (which were located on
high places) with a devastating bolt?". Although this preference of
lightning for striking elevated objects has long been known, no successful
efforts using this knowledge for lightning protection were recorded until
Benjamin Franklin announced in 1752 that the strikes to a tall metal rod
could be conducted to Earth by a wire, without damage to the structure on
which the rod and wire were installed (Franklin, 1753).
THE ORIGINS OF
STANDARDS FOR THE INSTALLATION OF LIGHTNING PROTECTION SYSTEMS.
The use of lightning protection on land-based buildings increased greatly
during the early 19th century but the information on how best to install
lightning protection was not widely known. There were no texts or standards
In 1904, following the British code, W. S. Lemmon, B.
H. Loomis and R. P. Barbour (Lemon et al., 1904) prepared Specifications for Protection of
Buildings Against Lightning which was adopted for American use by the
National Fire Protection Association
PROVIDED BY LIGHTNING RODS EQUIPPED WITH DOWN CONDUCTORS. The
protection provided by lightning rods equipped with down conductors. In the
years since the specifications for protection of buildings against lightning
were adopted and suitable protection systems were installed on buildings,
significant protection against the damage caused by lightning has been
demonstrated. During a seven-year period beginning in 1907, the reports of
fire losses to protected farm buildings in
During World War II, the decision as to whether lightning protection would be provided for certain manufacturing facilities operated by private contractors for Army Ordnance was left up to the contractor. Records show that from
1943 to 1946 the unprotected Indiana Ordnance Works lost approximately 500,000 pounds of explosives, an explosives magazine, a shipping house and operating building, and had other damage exceeding $100,000 as the result of six lightning strikes. In contrast, the immediately adjacent protected Hoosier Ordnance Plant reported no losses from lightning. A survey by Army Ordnance of the lightning experience for the period 1944 through 1948 shows that protected structures were struck 330 times with negligible damage whereas the unprotected structures were struck 52 times and the damage exceeded $130,000. The efficacy of lightning rods in preventing damage from strikes has been established so well over the years that lightning protection is now routinely required on large buildings open to the public.
THE INITIATION OF LIGHTNING STRIKES. Lightning strikes usually begin in thunderclouds from which a "leader" carrying negative charges descends toward the Earth. Schonland and Collens (1934) discovered that the negative leader descends in a step-wise fashion from a thundercloud, provoking upward-going positive streamers from exposed objects on the ground as a result of the local intensification of the atmospheric electric field beneath the negative stepped leader. One or more of the rising positive streamers may intensify into a positive leader which connects to the approaching negative leader and allows negative charge to drain to the Earth as a "return stroke" when Earth potential moves up the negative channel. Less frequently, lightning strikes lower positive charges to earth. When these strikes occur, they are often more damaging than the negative ones; positive strikes usually are more energetic and have sustained "continuing currents" that can ignite fires.
Of special interest for lightning protection is the determination of which object on the Earth is most likely to furnish the successful upward-going streamer that provides the connection to the descending leader. From studies of the physics of lightning and of long sparks, it is now known that the development and propagation of plasma streamers and of the more conductive leaders occur only when the local electric fields are very strong. It is also known that the tips of curved electrodes concentrate and intensify any ambient electric fields to which they are exposed. This provides an explanation for why elevated objects are the preferred recipients of lightning strikes; the strong fields around their tips caused by the charges carried by approaching leaders provide the conditions necessary for the generation of upwardly propagating leaders that rise to connect with downward-moving leader, thus completing the strike discharge path to earth and determining the strike point.
PROTECTION AGAINST DIRECT LIGHTNING STRIKES. The fundamental principle in the protection of life and property against lightning is to provide a means by which a lightning discharge can enter or leave the earth without resulting damage or loss. In practice, this is accomplished by providing preferred receptors for nearby strikes then conveying these discharges to Earth and distributing the charges brought down by lightning into the ground so that they do not cause local hazards.
out in the 1882 Report of the Lightning Rod Conference, it is important that
the tips of the designated strike receptors "be high enough to be the
most salient features of the building no matter from what direction the storm
cloud may come". While
that the local electric field be intensified more rapidly than it is weakened
by the formation and motion of ions around its tip and that the rod respond
successfully before competing streamers form above some other objects in its
vicinity. The early successes with
Another aspect of lightning physics that has been studied recently is that of the efficacy of ground rods in transferring lightning discharges to the earth. It is well known that large potential gradients develop at ground level when lightning strikes an isolated tree; the resulting voltages appearing on the ground surface between the legs of animals standing nearby may exceed hundreds of kilovolts, causing fatal currents to flow through their legs. Arcing occurs radially outward from ground rods when the current from lightning discharges conducted to the rod exceeds 20 kA. The use of radial ground conductors buried below ground level is now recommended in the 2000 edition of NFPA 780 as a means of distributing lightning charges to their ultimate destination, the surface of the earth.
THE NEED FOR A STANDARD TO GUIDE THE INSTALLATION OF LIGHTNING PROTECTION SYSTEMS. It is now well established that properly installed and maintained lightning-rod based protection systems significantly decrease lightning damage. Since the installation of an adequate lightning protection system is not simple, a standard is needed to ensure that proper procedures are followed by the installer. To be effective in providing protection for a structure that is struck, the following requirements must be met:
1. A sufficient number of rods must extend above the upper portions of the structure to be protected and their tips must be so exposed that one of them becomes the locally-preferred strike receptor upon the close approach of an initiating leader, descending from a thundercloud,
2. The connection between the strike receptor and the Earth, the "down conductor," must be able to carry the lightning current without significant heating,
3. The impedance to the flow of current in the down conductor must be sufficiently low that "side flashes" to objects in the vicinity do not occur as a result of high voltages developed by the passage of the current,
4. The connection from the down conductor to the Earth must allow the lightning current to flow into the ground without the development of large electrical potential differences on the earth's surface and without creating hazards to personnel or structures nearby,
5. All large metal components of the structure should be connected electrically to its down conductor system to minimize capacitance effects and to allow the transfer to Earth of the "displacement currents" that flow when the external electric fields are changed abruptly by the lightning discharge.
6. Surge protection should be provided for the electrical service and for all electronic equipment within the structure.
complexities, standards that specify the requirements which must be met in
order to ensure the adequacy of each lightning protection installation are
essential and are required by most industrial countries. As given above, the standard that is widely
used in the
Anderson, R., 1879: Lightning Conductors – Their History,
Nature and Mode of Application, E. & F. N. Spohn, 16
Franklin, B., 1753: How to secure Houses &c. from
lightning. Poor Richard’s Almanac, reproduced in Experiments &
Observations of Electricity (1774), I.B. Cohen, ed., 1941: Harvard University
Lemon, W.S., B.H. Loomis and R.P. Barbour, 1904:
Specifications for Protection of Buildings Against Lightning. National Fire
NFPA, 2001: The Standard for the Installation of Lightning Protections Systems, NFPA 780, National Fire Protection Association,, Quincy, MA (http://www.nfpa.org/Codes/index.asp).
Schonland, B.F.J. and H. Collens, 1934: Progressive lightning. Proc. Roy. Soc. London, 114, 654-674.
Symons, G.J., Editor, 1882: Report on the Lighting Rod
Conference, E. & F. N. Spohn, 16