If lightning were to strike a metal- enclosed building, the current would be constrained to the exterior of the metal enclosure, and it would not even be necessary ...
86th AMS Annual Meeting 2nd Conference on Meteorological Applications of Lightning Atlanta, Georgia, January 29 – February 2, 2006
Lightning Protection: History and Modern Approaches Vladimir A. Rakov Department of Electrical and Computer Engineering University of Florida, Gainesville
Lightning Protection: History and Modern Approaches
1. Franklin rod system 2. “Faraday cage” approach
3. Placement of air terminals 4. Behavior of grounding systems under direct lightning strike conditions 5. Bonding Requirements 6. Non-conventional approaches to lightning protection
2
2
1. Franklin rod system (first described in 1753) Air terminal
Down conductor
Ground terminal
Lightning protection system for houses proposed (most likely by G. Ch. Lichtenberg) in 1778. Adapted from Wiesinger and Zischank (1995). 3
1. Franklin rod system Air terminal locations (UL 96A, Fig. 6.2, 1998)
Air terminal
Down conductor
Ground terminal
A= 20 feet (6 m) maximum spacing for 10 inch (254 mm) air terminal height or 25 feet (7.6 m) maximum spacing for 24 inch (610 mm) air terminal height. B= 2 feet (610 mm) maximum spacing from corner, roof edge or ridge end.
Metallic roofs whose thickness is 4.8 mm (3/16 in.) or greater do not require air terminals (NFPA 780). 4
2. “Faraday cage” approach In 1876, James Clerk Maxwell proposed that a gunpowder building be completely enclosed with metal of sufficient thickness, forming what is now referred to as a Faraday cage.
Zone 1
Zone 2
If lightning were to strike a metalenclosed building, the current would be constrained to the exterior of the metal enclosure, and it would not even be necessary to ground this enclosure. In the later case the lightning would merely produce an arc from the enclosure to earth. The Faraday cage effect is provided by all-metal cars and airplanes. Modern steel-frame buildings with reinforcing metal bars in the concrete foundation connected to the building steel provide a good approximation to a Faraday cage. 5
Zone 3
0 1 1 2
Zone 0
2
3
Zone 1 SPD = Surge Protective Device
The general principles of topological shielding. Adapted from Vance (1980)
2. “Faraday cage” approach
Lightning strike to a car with a live rabbit inside. Courtesy of S. Sumi. 6
2. “Faraday cage” approach
Video frame of a lightning strike to an aircraft on takeoff from the Kamatsu Air Force Base, Japan, during winter. Courtesy of Z. I. Kawasaki.
7
2. “Faraday cage” approach
Cape Canaveral Air Force Station, Launch Pads 40/41. Courtesy of R. Kithil. 8
2. “Faraday cage” approach
Cape Canaveral Air Force Station, Launch Pad 41. Courtesy of R. Kithil. 9
3. Placement of air terminals Electrogeometrical model (EGM) Ng=const
Capture surfaces
rs rs
rs
Illustration of capture surfaces of two towers and earth’s surface in the electrogeometrical model (EGM). rs is the striking distance defined as the distance from the tip of the descending leader to the object to be struck at the instant when an upward connecting leader is initiated from this object. Vertical arrows represent descending leaders, assumed to be uniformly distributed (Ng=const) above the capture surfaces. Adapted from Bazelyan and Raizer (2000). 10
3. Placement of air terminals
4
3
1 2
{
rs = 10 I0.65, m where I is in kA
I, kA
rs , m
10
45
30
91
170
282
Striking distance, rs, versus return-stroke peak current, I [curve 1, Golde (1945); curve 2, Wagner (1963); curve 3, Love (1973); curve 4, Ruhling (1972); x, theory of Davis (1962); , estimates from twodimensional photographs by Eriksson (1978); , estimates from three-dimensional photography by Eriksson (1978). Adapted from Golde (1977) and Eriksson (1978). 11
3. Placement of air terminals 102
Finding rs = f(I)
•
101
For Q = 5 C I = 33 kA
Q
Assume leader geometry, total leader charge Q, and distribution of this charge along the channel.
I = 10.6 Q0.7
•
Assume critical average electric field between the leader tip and the strike object at the time of initiation of upward connecting leader from the object (200-600 kV/m)
•
100
I peak/ Q impulse neg. first strokes n=89
Finding rs = f(Q)
•
Use an empirical relation between Q and I to find rs = f(I)
12
10-1 100
101
I
102
Scatter plot of impulse charge, Q, versus return-stroke peak current, I. Note that both vertical and horizontal scales are logarithmic. The best fit to data, I = 10.6 Q0.7, where Q is in coulombs and I is in kiloamperes, was used in deriving rs = 10 I0.65 Adapted from Berger (1972).
3. Placement of air terminals Rolling-Sphere Method
rs = 46 m (150 ft) (NFPA 780, 2004), corresponds to I = 10.1 kA (95% of currents exceed this value)
rs rs rs
Illustration of the rolling-sphere method (RSM). The shaded area is that area into which, it is postulated, lightning cannot enter. Adapted from Szczerbinski (2000). 13
4. Behavior of grounding systems under direct lightning strike conditions
Photograph of surface arcing associated with the second stroke (current peak I=30 kA) of flash 9312 triggered at Fort McClellan, Alabama (Rg=260 Ω). The lightning channel is outside the field of view. One of the surface arcs approached the right edge of the photograph, a distance of 10 m from the rocket launcher. Adapted from Fisher et al. (1994). V= I x Rg=7.8 MV. 14
5. Bonding Requirements Dsoil = I Zg/ Eb If I = 60 kA, Zg = 25 Ω and Eb= 300 kV/m, Dsoil = 5 m
LPS air terminal
Dair = 0.12Zg+0.1L Dair LPS down conductor
I = the lightning peak
Wooden pole L
LPS grounding system
Protected Object
Dsoil
current Zg = the grounding impedance Eb = the breakdown electric field in the soil
Buried Services
Bond whenever you cannot adequately isolate Illustration of safety distances in the soil (Dsoil) and in air (Dair). Adapted from Kuzhekin et al. (2003). 15
5. Bonding Requirements
PI =
1 1 + (I /31)2.6
(1)
Dsoil = IZg/Eb, m
(2)
The IEEE peak current distribution given by equation (1) and corresponding values of Dsoil given by equation (2).
Peak current, I, kA
5
10
20
40
60
80
100
200
Percentage exceeding tabulated value, ΡI · 100%
99
95
76
34
15
7.8
4.5
0.8
0.42
0.83
1.7
3.3
5.0
6.7
8.3
17
Dsoil, m (Zg = 25 Ω, Eb = 300 kV/m)
16
5. Bonding Requirements
structure
clamp
gap
arrester
information line
pipe
power line
Bonding of external services near ground level. Taken from Wiesinger and Zischank (1995). 17
6. Non-conventional approaches to lightning protection Early Streamer Emission (ESE) systems
Lightning struck point B (without air terminal), which was within the claimed protection radius of 30 m of the ESE air terminal at point A. The distance between A and B is 18 m. After the strike the manufacturer installed an additional ESE Terminal at point B. Taken from Chrzan and Hartono (2003). 18
6. Non-conventional approaches to lightning protection Lightning elimination systems Evolution of lightning elimination (dissipation) system claims: •
Corona current can discharge the thundercloud.
• Corona charge can neutralize an approaching lightning leader.
•
Taken from the European Power News, February 2005
Corona charge can suppress or delay the formation of an upward connecting leader from the protected object.
However, Multipoint corona systems (dissipation arrays) provide only local lightning protection. They reduce the number of lightning strikes to their own surface and the object components directly covered by them. The question of extending the protection area of such systems still remains open (Aleksandrov et al., 2005). 19
Thank You
Lightning strike to the Washington Monument (169 m high) on July 1, 2005 20
1. Franklin rod system
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Surface arcing
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