Lightning is a visible discharge of static electricity that can occur within or between clouds or between the earth and a cloud. It is caused by high electrostatic stresses that ionize the air, allowing discharge between charged clouds and tall structures on the ground. A lightning protection system aims to safely intercept lightning strikes using air terminals, down conductors to direct current to ground terminals buried in the earth. The risk of lightning strikes on a structure is assessed using flash density maps and calculations of the equivalent collective area to determine if protection is required.
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LIGHTINING PROTECTION.pptx
1.
2. Lightning is the visible
discharge of static
electricity within a
cloud, between clouds,
or between the earth
and a cloud.
3. Current levels sometimes in
excess of 4000 KA,
Temperatures to 50,000
degrees F (water freezes at 32
degrees Fahrenheit under
normal conditions), and
Speed approaching one third
the speed of light (NLSI).
4. 1. Charged clouds induce
opposite charges on the
apex of tall structures, e.g.
towers, chimneys, trees,
mountains etc.
5. High electrostatic stresses at the
ends of these objects then ionizes
the air, resulting in lowering the
resistance of the path between the
cloud and the object, thus allowing
a discharge to occur between them.
Hence strikes the highest or most
pointed object in the area.
6. 2. Occurs when a sudden
potential difference
between a cloud and the
earth is established
instantly. This is induced
soon after a strike of the
first kind.
7. e.g. if there is a discharge
between cloud 1 and 2, cloud 3
can be suddenly left with
greater potential difference
gradient immediately adjacent
to it than the air can withstand
and a sudden strike to the
earth will occur.
8. Lightning risk assessment
methodology is provided
to assist the building
owner or architect /
engineer in determining
the risk of damage due to
lightning
9. If Nd > Nc lightning protection
should be installed, Where
Nd = Yearly lightning strike
frequency;
Nc = Tolerable lightning
frequency.
10. Nd = (Ng)(Ag)(C1)
Where:
Ng = the yearly average flash
density in the region where the
structure is located.
Ag = the equivalent collective area
of the structure in km2.
C1 = the environmental coefficient.
11. Yearly Flash density is
measured in km2/year, i.e.
flashes/km2/yr.
The data is obtained on flash
density maps.
These are obtained from the
meteorological offices,
e.g. A 10-year flash density
map
12. Refers to the ground area
having the same yearly
direct lightning flash
probability as the structure.
The effects of height and
location are also considered
in the area computations.
15. This accounts for the
topography of the site of
the structure and any
other objects within a
distance of 3H
Given as follows:
16.
17. This is a measure of the
damage risk to the
structure including factors
affecting risks to the
structure, environment
and the monetary loss.
It is calculated as follows:
18.
19.
20.
21.
22.
23. If Nd > Nc lightning protection
should be installed, Where
Nd = Yearly lightning strike
frequency
Nc = Tolerable lightning
frequency
24. EXAMPLE:
Lightning flash density (Ng) = 4
Relative structural location (C1) = 2
Rectangular structure L = 80m
W = 50m
H = 30m
Lightning strike frequency (Nd) = 3.93
C2=3, C3=3, C4=1, and C5=5
Qs: Should the structure be protected
or not?
34. The lightning protection
system has a zone of
protection.
A single vertical conductor
protects an area in circular
form of the building having
its center at the conductor
and reaching equal to twice
the height of the conductor.
35. It is however better to
provide a protection with a
zone consisting of a cone
with its apex at the top of
the vertical conductor and
a base of radius equal to
the height of the conductor.
36. There are basically three types
of lightning protection system
which are:
1. The simple lightning rod
2. The lightning rod with taut
wires
3. The lightning conductor with
meshed cage
37. 1. The simple lightning rod
The lightning rod is a metallic
capture tip placed at the top
of a building, and is earthed
to the ground by one or more
down-conductors and earth
plates or crow’s feet.
38.
39.
40. 2. Rod with taut wires
The tin plated copper
wires are stretched above
the structure to be
protected.
For special structures e.g.
rocket launching areas,
etc.
41.
42. 3. Meshed cage
This involves fixing numerous
down conductors symmetrically
all round the building.
This type is highly
recommended for highly
exposed buildings with very
sensitive installations like
computer rooms.