Lightning strikes!
By E Trengove and KJ Nixon, University of the Witwatersrand
In view of the recent ‘deaths by lightning’ in and around the Johannesburg area, it is appropriate to revisit the basic physics of lightning, the
mechanisms of lightning injury and offer some lightning safety guidelines.
N
ormally an article on lightning protection would focus on the
protection of electrical equipment and systems. However, it
is good to remind ourselves that lightning is a force of nature
that also causes harm to people, animals and property. Furthermore,
rural people who work outdoors tending the land or herding animals
are particularly vulnerable to lightning strikes. Many rural houses are
structurally too unsound to protect inhabitants adequately against
lightning. In addition to those killed by lightning, there are no records
of how many people are injured by lightning strikes, but there could
potentially be a large number of rural people who are affected [1].
South Africa has a relatively high number of lightning fatalities.
Reports in the lay press indicate that there were 84 lightning fatalities
in South Africa between January 2009 and November 2010 [1], but
it should be borne in mind that newspapers are not a reliable source
of information since:
• Not all lightning deaths are reported in the newspapers
• Accounts are usually anecdotal and are not based on any kind of
forensic investigation
Mechanisms of lightning injury
There are currently five generally accepted mechanisms of lightning
death and injury, namely, a direct strike, a touch voltage, a side
flash, a step potential and upward streamers [2], [3], [4], [5], [6]. Each
mechanism will be explained in some detail.
Direct strike
Death or injury due to a direct strike, graphically represented in Figure 2, occurs when a lightning stroke connects directly with a person.
Due to the potential difference between the point where lightning
strikes the person and his/her feet, current flows through the body
into the earth. Although one might imagine that this would be the
greatest cause, it only accounts for 3 - 5% of lightning injuries [2].
The basic physics of lightning
A very basic explanation of the physics of lightning is as follows:
• A thundercloud forms when positively charged icy particles are
separated from negatively charged water droplets within a cloud;
• The negative charge in the cloud attracts positive charges on earth;
• They attract each other and streams of charge start moving towards each other, the negative downward leader moving down
from the cloud and the positive upward leader moving upwards
from the earth;
• When the downward and upward leaders meet, an electric current
rushes from the cloud to the earth and we see a lightning flash.
Figure 2: Direct strike.
Figure 3: Touch voltage. Figure 4: Side flash.
Touch voltage
A touch voltage, as shown in Figure 3, occurs when a person is
touching an object when it is struck by lightning, also called a touch
potential or contact potential. If lightning strikes something like a telephone wire or an electrical conductor, a person can be injured even
if the lightning strikes quite far away but s(he) is touching something
connected to the point of strike, for example a landline telephone or
an electrical appliance.
Side flash
Figure 1: The basic physics of lightning.
A side flash can harm somebody standing close to an object that
is struck by lightning. Part of the lightning flash will keep travelling
down to the ground along that object, but part of it ‘jumps’ to the
nearby person and travels to ground through the person, as shown
in Figure 4.
E+C SPOT ON • April 2013
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Figure 5: Step potential.
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Figure 6: Upward leaders.
Step potential
When lightning strikes an object, for example a tree, the lightning
current goes into the earth (see Figure 5). As the current spreads
out through the earth, from a higher to a lower concentration of
negative charge, it could happen that a person has one foot is
standing in an area of higher charge than the other foot. If the
person’s body constitutes a path with lower resistance than the
ground, the lightning will travel through that body. This also happens to animals, for example cattle and sheep – with lightning
current travelling through their bodies between their forelegs
and hind legs.
Upward leaders
During thunderstorm conditions, upward leaders start to form
from points on the ground, attracted by the opposing charge of
the cloud. A lightning flash occurs when the downward leader and
the upward leader connect and form a channel for the lightning
discharge. An upward leader could form from a person’s body,
typically from the top of the head of somebody standing upright.
Even if the upward and downward leaders never connect to complete a full lightning strike, the upward leader can be so strong
that a person could still be hurt (see Figure 6). This happened
during a football match in 1998 between Moroka Swallows and
Jomo Cosmos in Johannesburg [4] - lightning struck nearby and
several players were injured by upward leaders.
Conclusion
In South Africa, many people have limited literacy and it is a
society with many different languages, so the information in any
lightning campaign should be expressed as simply as possible.
In this context, the following content is suggested for lightning
awareness and education purposes:
• If you are close to a solid, permanent structure, then… ‘when
thunder roars, go indoors’. You are safer indoors than outdoors.
Remember that lightning can occur at the edges of clouds, so do
not wait until it starts raining before you take safety precautions.
If you are outdoors or if the only shelters are flimsy or temporary,
squat down or sit on the ground to keep your height as reduced
as possible. Make your contact with the ground as small as possible, so do not lie down on the ground.
Do not touch metal objects like electric wires, fences, or pluggedin electrical appliances like a washing machine.
Do not take a bath or shower, wash dishes or wash your hands.
Do not lie down on the ground.
Do not talk on a landline telephone, like a Telkom phone.
If you use a fire for cooking, put it out.
Do not stand under or near a tree.
You are safe in a car, taxi or bus as long as you keep the windows
closed.
References
[1] Trengove E and Jandrell IR. 2011. Strategies for Understanding
Lightning Myths and Beliefs. International Journal of Research
& Reviews in Applied Sciences, vol. 7, no 3.
[2] Cooper MA and Ab Kadir MZA. 2010. Lightning Injury Continues
to be a Public Health Threat Internationally Proceedings of the
21st International Lightning Detection Conference, Orlando.
[3] Cooray V. 2003. The Lightning Flash. IEE Power Series, London.
The Institution of Electrical Engineers.
[4] Anderson RB, Jandrell IR and Nematswerani HE. 2002. The Upward Streamer Mechanism Versus Step Potentials as a Cause
of Injuries from Close Lightning Discharges. Transactions of the
SAIEE, vol. 93, no 1.
[5] Carte AE, Anderson RE and Cooper MA. 2002. A Large Group of
Children Struck by Lightning. Annals of Emergency Medicine.
[6] Dlamini WM. 2009. Lightning Fatalities in Swaziland: 2000 – 2007.
Natural Hazards, vol. 50.
Estelle Trengove is a registered Professional Engineer (Pr Eng)
and a lecturer at Wits University's School of Electrical and
Information Engineering. She recently obtained a PhD for a
thesis on lightning myths and beliefs in southern Africa and
the role that they play in personal safety.
Enquiries: Email [email protected].
Dr Ken Nixon is an Associate Professor in the School of
Electrical and Information Engineering at the University of
the Witwatersrand. He is a member of the IEEE and a Fellow
of the SAIEE. His interests include software engineering, the
smart grid, high voltage engineering, lightning protection and earthing.
Enquiries: Tel. 011 717 7203 or email [email protected].
E+C SPOT ON • April 2013
About the authors