Volume 4 | 2016
______________________________________________________________________
Alex Z. Kattamis, Ph.D., P.E., CFEI; Matthew Pooley, Ph.D.;
Patrick F. Murphy, Ph.D., P.E., CFEI
Introduction
According to the insurance industry, lightning is responsible for more than $5 billion in total insurance losses annually.
i
Properly designed and installed lighting protection systems may help to mitigate losses
resulting from lightning strikes to structures by intercepting and routing lightning energy to the earth.
Benjamin Franklin is often credited with first describing the idea of capturing lightning energy via a metallic rod ii
:
“I say, if these things are so, may not the knowledge of this power of points be of use to mankind; in preserving houses, churches, ships, etc.
from the stroke of lightning; by directing us to fix on the highest parts of those edifices upright rods of iron, made sharp as a needle and gilt to prevent rusting, and from the foot of those rods a wire down the outside of the building into the ground; or down round one of the shrouds of a ship and down her side, till it reach’d the water?
Would not these pointed rods probably draw the electrical fire silently out of a cloud before it came nigh enough to strike, and thereby secure us from that most sudden and terrible mischief!” iii
Several recognized authorities issue standards for the design and implementation of such systems
(including the National Fire Protection Association [NFPA] and Underwriters Laboratory [UL]).
Perhaps the most relied on is NFPA 780, “Standard for the Installation of Lightning Protection Systems,” which was adopted as an American National Standard.
Lightning protection systems not designed according to
such standards may not offer the desired protection to the structure from lightning strikes.
Theory of Rolling Spheres
An understanding of the theory of lightning propagation can be helpful in appreciating the design and function of lightning protection systems (LPSs).
A majority of cloud ‐ to ‐ ground strikes occur due to downward negative leaders, which are narrow, negatively charged pathways that reach down from the lower regions of thunderclouds toward the Earth.
Once a negative leader has formed, the narrow
geometry of the ionized column that protrudes down from the thundercloud concentrates the electric field at its leading tip and induces further ionization of the adjacent air.
This cycle continues until the negative leader reaches ground, at which point positive ions from the Earth flow up the negatively charged leader, in a process known as a return stroke.
The huge current in the return stroke, and the subsequent ionization of the surrounding air, is what gives rise to the bright flash and loud thunderclap of a lightning bolt.
Due to the cyclic nature of the ionization, a negative leader typically advances in
discrete steps of around 50 m, with pauses on the order of 50 µs between advancement events.
Lightning protection system designs are based in part on a methodology intended to ensure ensuring that the advancing negative leader does not encounter an unprotected section of a structure during the final advancement step in which it connects to the Earth.
iv
Specifically, during the 1970s, it was observed that when the negative leader ends an advancement step within around 50 m of the Earth, the next step is likely to leap to the Earth, and can do so via a structure.
v
This led to the development of the rolling sphere method for estimating the zone of protection afforded by a tall lightning rod.
The rolling sphere method is implemented by imagining a sphere of radius 150 ft (~50 m) positioned adjacent to the lightning rod, such that it rests against the rod at a single point.
The region between this sphere’s edge and the rod is the zone of protection in which structures can be placed such that they are protected against lightning strikes.
Within the zone of protection, an advancing negative leader cannot be within its 50 ‐ m step range of the structure without also being within 50 m of the rod.
Thus, a negative leader that advances toward the Earth in the vicinity of the rod is unlikely to strike the ground or structures within the zone of protection, because the rod offers a preferential conductive path within range of the final negative leader advancement step.
The rolling sphere method can be expanded to protect structures that are too large to fall within a practically sized single lightning rod.
Lightning strike protection terminals are positioned such that an imaginary 150 ‐ ft
(~50 ‐ m) sphere can be rolled over the building without contacting the structure, but instead only contacting the lightning ‐ strike terminals, as shown in the figure below, where the blue ‐ shaded region indicates the zone of protection offered by the lightning rods.
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Design Basics
A lightning protection system designed and implemented according to NFPA 780 includes the following elements: a network of strike termination devices (lightning rods); a network of conductors to move lightning energy from the strike termination devices toward earth; a network for ground terminations
(ground rods); equipotential bonding; and surge protection devices.
Each is discussed further below.
Strike Termination Device Network
Strike termination devices, also referred to as air terminals or lightning rods, are placed on the upper portions of the protected structure.
They are used to intercept the lightning strike.
The placement of the strike termination devices, as set forth by
NFPA 780, is derived according the method of rolling spheres described above.
For the designer and installer,
NFPA 780 sets forth the spacing of terminals based on the structure to be protected.
A basic structure shown below, assumed to be 30 ft high, would allow for a maximum spacing of
20 ft between 10 ‐ inch ‐ tall air terminals, per NFPA 780.
Conductor Network
The conductor network includes the conductors that interconnect the strike termination devices and down conductors for routing lightning currents to ground termination devices.
Conductors are intended to provide a low ‐ resistance path preventing the lightning currents from traveling through high ‐ resistance building materials (wood, sheetrock, brick, etc.).
Lightning currents through such building
materials are expected to produce high heat, which can lead to fire and/or explosion.
According to NFPA 780, there should be a two ‐ way path via conductors from each air terminal, with the exception of dead ends.
The conductor length extending to a dead end is restricted to a maximum length of 16 ft.
Both copper (minimum cross ‐ sectional area 29 mm
2
) and aluminum (minimum cross ‐ sectional area 50 mm
2
) are allowed for use as conductors, with the important caveat that the two should never be connected to one another without a proper connector, to avoid galvanic corrosion.
Bimetallic connectors that exclude moisture can be used for a connection between copper and
aluminum wires.
A series of other restrictions are set forth by NFPA 780 with regard to the layout of the conductor network.
In particular, the location of conductors in the network relative to other conductive material incorporated in the structure must be considered in order to prevent potentially damaging discharge w w w . e x p o n e n t . c o m
arcs between the conductor network and the structure.
For example, NFPA 780 recommends that a conductor should not form an angle of less than 90° or a bend radius of less than 8 inches.
A radius of curvature tends to cause the electric field adjacent to the conductor to increase, thus increasing the chance of flashover between the conductor and nearby grounded metallic bodies such as duct work,
piping, and other systems.
According to NFPA 780, at least two down conductors should be used per structure.
If the perimeter of the structure exceeds 250 ft, NFPA 780 recommends one additional down conductor for every 100 ft (30
m) of perimeter or fraction thereof.
Ground Termination Network
Ground terminations are used to connect the lighting protection system to earth.
NFPA 780 recommends that each down conductor be connected to a ground rod, independent from the electrical service ground system, which may be governed, depending on jurisdiction, by the National Electrical
Code (NEC) NFPA 70.
NFPA 780 recommends that ground rods be not less than 1/2 inch (12.7
mm) in diameter and 8 ft (2.4
m) long, and they must be buried to at least 10 ft (i.e.
the top of the rod is buried to 2 ft.
NFPA 780 recommends that rods be copper ‐ clad steel, solid copper, hot ‐ dipped galvanized steel, or stainless steel.
NFPA 780 is silent regarding a maximum allowable resistance for such a ground, whereas NFPA 70 250.53(A)(2) indicates a requirement of 25 ohms or less.
Equipotential Bonding
An important consideration in LPS design is the use of equipotential bonding.
Equipotential bonding is the creation of conducting pathways between other grounded metal systems that also provide a path to the ground, and is important in order to prevent potentially damaging discharge arc events between
such pathways.
NFPA 780 recommends that all such grounded metal systems be bonded to the lighting protection system via a conductor sized as the roof conductor network.
NFPA 780 defines a bonding distance for structures of 40 ft or less in height via the following:
6
where:
D = calculated bonding distance
h = height of the building n = 1 where there is only one down conductor within the zone (100 ft from the bond location); n = 1.5
where there are only two down conductors within the
zone; n = 2.25
where there are three or more down conductors within the zone.
K m
= 1 for air, or 0.50
for building materials such as masonry or brick.
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For example, in the situation pictured to the right, a down conductor passes by a condenser fan for an air conditioning unit.
This unit’s exterior is a metal body grounded to the electrical service.
Applying the following to the equation: h = 30 ft (building height), n = 1, and
K m
= 1 (air), a bonding distance of 5 ft is calculated.
Therefore if the spacing between edge of the fan and the down conductor is less than 5 ft, the condenser fan housing must be bonded to the conductor.
Bonding of non ‐ grounded metal bodies, that offer differences in electric resistivity along alternate paths to ground, must also be considered, per NFPA 780.
A common example set forth in NFPA 780 is that of a metallic window frame.
In this case, a bond between the window frame and the down conductor is required per NFPA 780 only if a + b is
greater than the bonding distance D .
Studies have shown that bonding in general is often overlooked by LPS designers and installers.
vi
When grounded metal systems that can provide a path to ground are not properly bonded, arcing can occur, leaving a source for fire and explosion due to lightning strike.
vii
Surge Protection
According to NFPA 780, surge protection devices (SPDs) are to be installed at all incoming power and telecom lines.
Per NFPA 780, the SPD must comply with UL 1449 Standard for Transient Voltage Surge
Suppressors and should be installed in accordance with NFPA 70.
The role of such devices is to prevent
unwanted voltage surges, which could damage household electronic equipment.
Conclusion
Although LPSs are not mandated by a uniform nationwide code, there are several standards for LPS installations.
These standards generally recommend that an LPS include the following elements: a network of strike termination devices (lightning rods), a network of conductors to move lightning energy from the strike termination devices toward earth, a network for ground terminations (ground rods),
equipotential bonding, and surge protection devices.
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For Further Information, Contact:
Alex Z.
Kattamis, Ph.
D., PE, CFEI
Senior Managing Engineer
(212) 895 ‐ 8127 | akattamis@exponent.com
Matthew Pooley, Ph.D.
Scientist
(212) 895 ‐ 8146 | mpooley@exponent.com
Patrick F.
Murphy, Ph.
D., PE, CFEI
Senior Managing Engineer
(212) 895 ‐ 8115 | pmurphy@exponent.com
References i NLSI. 2008. Lightning costs and losses from attributed sources. Compiled by the National Lightning
Safety Institute. ii
Krider, E.P. 2006. Benjamin Franklin and lightning rods. Physics Today 59(1). iii Labaree, L.W., W.B. Wilcox, C.A. Lopez, B.B. Oberg, E.R. Cohn, et al. (Eds.), 2006. The Papers of
Benjamin Franklin Yale University Press, New Haven, CT, Vol. 4, 19. iv
Uman, M.A. Lightning. Dover Publications, Inc, §1.2. v
Zipse, D.W. Lightning protection systems: Advantages and disadvantages. 1994. IEEE Transactions on Industry Applications 30(5) Sept./Oct. vi Tobias, J.M. et al., 2001. The basis of conventional lightning protection technology: A review of the scientific development of conventional lightning protection technologies and standards. Report of the
Federal Interagency Lightning Protection User Group, June. vii
Loehr, K. 2007. Safety standards for lightning protection: Without the proper system in place, lightning will produce heat, fires and even explosions. PM Engineer. w w w . e x p o n e n t . c o m