Introduction
Costs of various types of lightning damage copy pps 25-28 in Uman’s book
1977 blackout New York City cost about $350 million
30% of power failures due to lightning, annual cost close to $1 billion.
insurance cost estimates range from 1/3 to $1 billion
5% of insurance claims involve lightning (50% in Florida in summer)
30,000 house fires caused by lightning
about half of the 20,000 wildfires every year are caused by lightning
fire fighting cost about $1.5 billion in 2006 (a record year)
commercial airline costs ~$2 billion
(roughly the same amount for military aircraft)
lightning causes damage to perhaps 100,000 computers/year
quick calculation of odds that my house might be struck
12 m x 20 m x 5 m (pitched roof)
(10 + 12 + 10) x (10 + 20 + 10) = (32) x (40) = (0.032 km) x (0.04 km) = 0.00128 km2
0.00128 km2 x 6.6 strikes/(km2 yr) = 0.00845 or about 0.01. About 1 strike every 100 yrs
power line
(50 m) x (100 km) x Ng = (0.05 km) x (100 km) x 6.6 strikes/(km2 yr) = ~ 30 strikes
Schedule
today: protecting structure
Thursday: protecting electronics & human life
Goal
not to be equipped with enough information to be able to design and install a lightning protection system (online links?)
NFPA 780:2004
National Fire Protection Association “Standard for the Installation of Lightning Protection Systems” Quincy Mass.
International Electrotechnical Commission,
IEC62305-n:2006 (n can be 1,2,3,4 or 5)
Geneva, Switzerland
rather to want to and to be able to look at existing systems with new understanding.
3 components: air terminal, down conductor, grounding
Figure 3.3: air terminals on buildings with pitched roof, gently sloping roof, and flat roof
(note more than one down conductor, symmetrically situated)
Figure 3.4 and Figure 3.5
House and large building with all the various connectors etc.
Zone of protection
structures less than 50 feet tall
45o or 60o cone angle, zone of protection
rolling sphere (radius equal to striking distance)
striking distance try to relate this to peak first return stroke current
d = A(Ipeak)b Fig. 3.6
short building (h<d) versus taller building (h>d)
Fragments off tops of tall buildings struck by lightning is apparently a serious hazard
anything touched by the rolling sphere is vulnerable. Want it to touch lightning rods but not the roof in between
Problem is the small return strokes, don’t really know how many there are
(NLDN threshold is 5 or 6 kA)
Table 4.1 p 74
protection level radius minimum peak I % rs>Imin
I 20 m 3 kA 99%
II 30 5 97
III 45 10 91
IV 60 16 84
NFPA 780:2004 recommends 30 m or 46 m
Protection of a roof
1. multiple connected rods
2. wire mesh (5 to 20 m spacing)
3. metal roof (NFPA >= 3/16” thick)
Lightning rods (“Franklin rods”)
blunt or pointed
some recent experimental evidence that blunt rods might work better
Moore et al
GRL 27 1487-1490 2000
J. Appl Meteorol 39 593-609
NFPA 780:2004 either type may be used, must be at least 10” tall, within 2 feet of ends of roof ridges.
non conventional rods (radioactive sources that ionize air or high voltage that initiate a discharge) generally thought to be ineffective
Kennedy Space Center
overhead cables supported on insulators
sketch
Table 4.2 p. 75
protection level radius Mesh size
I 20 m 5 m x 5 m
II 30 10 x 10
III 45 15 x 15
IV 60 20 x 20
Down Conductors
connect to max. number possible, small structure should have at least 2
(i) reduce impedance (inductances in parallel)
(ii) reduce B fields inside the structure
minimum crossectional area ~ 50 mm2 for copper (r = 4 mm)
avoid almost closed loops
sketch
bond to metallic bodies within about 5 m
sketch
3 x 106 V/m needed for air breakdown
500 kV/m average field needed for neg. leaders
300 kV/m for pos. leaders
Grounding
grounding rod at least 8 feet long
bottom end at least 10 feet deep
counterpoise – buried wire that surrounds the structure
reduces potential differences inside the loop
buried mesh (underneath structure, perhaps rebar in concrete base) connected to counterpoise is best.
table of resistivities
sometimes add chemicals to soil to lower resistivities
example calculation of grounding resistance
voltage potential sometimes exceeds soil breakdown (100 to 500 kV/m)
and surface arcs form. This effectively enlarges the grounding electrode.