Lightening

LIGHTENING
Presented by
Md Asif Hasan

Introduction
Lightening is the occurrence of a
natural electrical discharge of very
short duration and high voltage
between a cloud and the ground
or within a cloud, accompanied by
a bright flash. More simply, it is the
meeting of negative charges with
positive charges

Some Facts about Lightening
 Duration of one stroke of lightning, 1/5th of second.
 Lightning can travel at speeds of 60,000 m/sec.
 Lightning containing an average charge of 30 to 50 lakhs volts
 During lightening heat exceeds 50,000 degrees F (3 times hotter than
the surface of the sun)

Mechanism of Lightning
The lightning occurs in phases. The scientists discovered that the lightning occurs
in three main phases-
 The first phase: A negative charge descends from the cloud toward the ground in
part of a second.
 The second phase: The positive charge on the ground meets with that negative
charge.
 The third phase: As a result, a strong electric spark generates and goes toward
the cloud. The returned electric charge and that is the lightning beam, all of
these four phases happens in 25 Millisecond.

Occurrence
 Tall objects such as trees and
skyscrapers are commonly struck by
lightning.
 Mountains also make good targets. The
reasons for this is their tops are closer
to the base of the storm cloud.
However, this does not always mean tall
objects will be struck.it all depends on
where the charges accumulate. Lightning
can strike the ground in open field even if
the tree line is close by.

Damage due to lightning
Lightning striking a structure can result in damage to the structure itself and to
its occupants and contents, including failure of equipment and especially of
electrical and electronic systems.
It injuries or sometimes even kills animals and people instantly.
It can cause fire and shatter buildings resulting in lot of destruction and
damage.
It can burn trees and also causes forest fire

Safety Measures
 Take shelter in a building or car
 Stay about 15 feet away from each other
 Do not lie flat on ground
 Stay away from all types of water bodies because water is an excellent
conductor of electricity
 It is safer to use mobile phones and cordless phones during lightning
 Do not take baths and showers during storms, as water is an excellent
conductor of electricity.

Lightening in Bangladesh
Fig: Statistics Of Lightning-related
Fatalities in Bangladesh 1990-2017
Fig: Gender Variation Of Lightning-
related Fatalities in Bangladesh

Fig: Socioeconomic Conditions Affecting Activities Of
Lightning Fatalities

Lightning Protection System
Lightening Conductor: A lightning
conductor is fixed on the top of tall
buildings to protect them from
damage due to the lightning

 Overhead Ground Wires: The ground wires
are placed above the line conductors at such
positions that practically all lightning strokes
are intercepted by them (i.e. ground wires).
The ground wires are grounded at each tower
or pole through as low resistance as possible.
 Lightning arrester or Surge diverter: A
lightning arrester or a surge diverter is a
protective device which conducts the
high voltage surges on the power system
to the ground which provide protection
for indirect strokes.

The Earthing Screen: It consists of a network
of copper conductors (generally called shield
or screen) mounted all over the electrical
equipment in the sub-station or power
station. The shield is properly connected to
earth on at least two points through a low
impedance. On the occurrence of direct stroke
on the station, screen provides a low
resistance path by which lightning surges are
conducted to ground. The limitation of this
method is that it does not provide protection
against the travelling waves which may reach
the equipment in the station

Lightening

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