An interesting survey of the costs of various types of lightning damage was on a handout distributed in class.  This came from the first 3 or 4 pages in Chapter 2 of Uman's The Art and Science of Lightning Protection.  Here are a few of the figures on the handout:

The 1977 power blackout in New York City cost about $350 million.  30% of all power failures are due to lightning with an annual cost close to $1 billion.

Estimates of the costs of lightning damage to insurance companies range from 1/3 to $1 billion.  In total about 5% of insurance claims involve lightning (50% in Florida in summer)

30,000 house fires caused every year by lightning.  About half of the 20,000 wildfires every year are caused by lightning.  The cost of fighting wildfires was about $1.5 billion in 2006 (a record year).

Lightning damage to commercial airlines costs ~$2 billion per year.  Damage to military aircraft is comparable.

Lightning causes damage to perhaps 100,000 computers/year.

The house has a length of 20 m, a width of 12 meters and the top of the pitched roof is about 5 m high.  To determine the effective area we must extend the actual dimensions of the house by 2 times the height (2 x h) on all four sides of the house.

You would need to question whether it is worth the expense of installing some lightning protection (especially since I don't actually own the house).

You can do the same kind of calculation for a normal power distribution line.  We'll assume the arms on the power pole that hold the power lines are 5 m wide and are about 10 meters above the ground.  The effective area of 100 km of line then would be

 

In this case some lightning protection is probably advisable.  Often a ground wire will be run above the current carrying wires.  Lightning then will hopefully strike the ground wire (and be safely carried to ground) instead of the current carrying wires.

Protection System Components
Lightning protection for a house or building usually consists of 3 components: an air terminal (lightning rod or Franklin rod), a down conductor, and grounding.

Dimension A in both figures is 20 or 25 feet and is the maximum spacing recommended by the NFPA between lightning rods.  Dimension B in the lower figure is 20 feet and is the maximum spacing between rods located along the edge of the roof.  Two down conductors at opposite corners of the roof are shown in the top figure.  The bottom figure has a down conductor at each of the buildings corners.

For structures less than 50 feet tall

a lightning rod provides a 45^o or 60^o cone of protection.  The 45^o cone from a single lightning rod is shown at top in the figure below.

A larger structure can be protected with multiple rods with overlapping zones of protection.

A different approach is used for larger structures.  We need to recall that the striking distance, d,  is the distance between the leader tip and the moment upper connecting discharges are initiated at the ground.  The leader can potentially strike anything withing a d of the leader tip.

           
In the rolling sphere method, you imagine rolling a sphere of radius d (d is the striking distance) over the building being evaluated for lightning vulnerability.  Parts of the building touched by the sphere are at risk.  Portions that aren't touched are safe.

The height of the building at left is less than or equal to d.  The sides of the building aren't touched by the sphere, the top is.  Lightning protection would need to be installed on the roof.  The building at right is quite a bit higher than d.  Much of the sides as well as the roof can be struck by lightning.  Protection would need to be installed on the top sides of the building and the roof.  Fragments of building materials knocked off the sides of buildings by lightning are apparently a serious hazard to people on the street below.

The designers could make metal window shades and window frames part of the lightning protection system by connecting them to the down conductor.

Note the spacing of the air terminals must be such that the rolling sphere doesn't touch the roof in between adjacent lightning rods.

What value of d should be used?  Researchers have tried to relate the striking distance to the amplitude of the peak return stroke that follows.  Generally an expression of the form

is used. A class handout showed curves drawn using the most commonly assumed values for A and b.  I've one of the curves on that graph below

Protection of a roof

A roof can be protected by multiple lightning rods.  An old question whether pointed rods or blunt rods work best.  Some recent research (two articles by Moore et al, 2000 in the class articles folder) suggest that the blunt rods might be more effective.  The NFPA allows either type to be used.  Rods should be at least 10 inches tall.

So called "unconventional" rods that use radioactivity or high voltage to try to initiate an "earlier" upward connecting discharge are generally not thought to be any more effective than conventional "Franklin" rods.

A wire mesh covering the roof of a building or vulnerable installation on the ground can be used instead of lightning rods.  A wire mesh covered the administration trailer at the rocket triggered lightning site in Florida for example.

The following table gives recommendations for the spacing of the wires in a wire mesh (based on Table 4.2 in Uman's "The Art and Science of Lightning Protection")

Air terminals should be connected to as many down conductors as possible.  Lightning rods on a house should be connected to at least two down conductors (at opposite corners of the house for example).

 
There are several reasons for this:

(i) reduce impedance (connecting inductors or resistors in parallel reduces the combined impedance)

(ii) reduce B fields inside the structure (the B fields from two down conductors positioned on opposite sides of a building will point in opposite directions in the interior of the building)

The recommended minimum crossectional area is about 50 mm² for copper  (that correcponds to a radius of 4 mm, a diameter of 8 mm or roughly 1/3 inch)

Almost closed loops on the down conductor should be avoided.  This adds impedance and sparks are likely to jump across adjacent points on the down conductor.

Rather than going around a protrusion on a building wall, it would be better to go through the protrusion and keep the down conductor as straight as possible.

The down conductor should be "bonded" to (electrically connected to) metallic objects within about 5 m.  This is to reduce the chance of a spark jumping to the metallic object.

The down conductor has been connected to a nearby water pipe at (1), to a vent pipe at (2), and to an evaporative cooler at (3).

Some numbers that I might not have mentioned in class:
3 x 106 V/m is need to breakdown air at sea level
500 kV/m is the average field needed for propagation of negative polarity leaders
300 kV/m is the average field needed for positive leader propagation.

A couple of sections of copper clad grounding rods were shown in class.  Grounding rods should be at least 8 feet long and the bottom end should be at least 10 feet deep.

Grounding can be improved by connecting grounding rods to a counterpoise, a loop of grounding wire that encircles the structure.  This reduces potential differences inside the loop.

The grounding can be also be connected to a buried mesh under the structure or to interconnected rebar in the concrete base of the structure.  This is well illustrated in an article by V.A. Rakov "Lightning Protection of Structures and Personal Safety."

A handout had a table of resistivities of various materials (Table 5.1 in Uman's "The Art and Science of Lightning Protection").

material	resistiviy in ohm-meters
ocean water	0.1 - 0.5
ground, well & spring water	10 - 150
lake & river water	100 - 400
rain water	800 - 1300
commercial distilled water	1000 - 4000
chemically clean water	250000
clay	25 - 70
sandy clay	40 - 300
peat, marsh & cultivated soil	50 - 250
sand	1000 -3000
moraine	1000 - 10000
ose (calcereous remains)	3000 - 30000

 

 We'll begin class on Thursday with a calculation of resistance for a simple grounding geometry.