Explanation on
Step Potentials due to a direct lightning strike
UE
± 150 m Radius Voltage Cone
10
© copyright Surge Technology (Pty) Ltd
In the above example the tree gets struck and the man on the right hand side is the same distance from the tree
as the cow. Due to the distance between the cows feet the cow is exposed to a much higher voltage (Red) than
the man (Blue). The second man on the left is closer to the tree and exposed to an even higher voltage
(Orange) and would most possibly be killed. If the two men were holding hands then they would both be
exposed to the total voltage between them (bottom of Orange to top of Blue)
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Directors: P. Narismulu, SG. Kuni, L. Xate
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UE
Step Potentials
U1
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U2
© copyright Surge Technology (Pty) Ltd
This picture shows the same effect but as voltage rings (gradients) radiating out from the point of strike. If each
ring indicated 5000 Volts, then the man would have 5000 Volts between his legs and the cow would have
10 000 Volts between its legs.
Based on the above you can see why it is dangerous to stand next to a tall object during a storm.
The following picture shows a strike to a flag pole on a golf course.
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From these pictures you can see how the voltage is developed between you feet when in close
proximity to a strike. It is the difference in voltage between your feet that causes the current to flow
through your body which can easily cause death. Thus to be safe keep your feet as close together as
possible. The voltages and currents indicated are for explanation only.
Step voltage -it all depends on where your feet are
0 volts
50,000
volts
100 metres
STRIKE POINT
30,000 Amps
0.5 m between feet =
high voltage between
your feet
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The following picture shows cattle killed due to a lightning strike in close proximity. It was the voltage between
their feet that caused them to die.
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The same principle applies to electrical and electronic equipment. Any cable between equipment will also have
a difference in potential between the 2 x devices and then lightning surge currents will rather flow in the low
resistance cables that in the ground, damaging equipment at both ends.
Lightning up to 1 kilometre away can cause damage to sensitive electrical and electronic equipment. A lightning
strike 100 meters away can induce as much as 2000 Volts per meter of cable. A lightning strike 1 kilometre
away can induce as much as 200 Volts per meter of cable.
Danger due to Lightning Strokes
ABC Company
approx. 20,000,000 lightning strokes in South Africa per year
MCR
110 kV
data
telephone
km
2
400/230 V
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mobile phone
TV
Step and Touch Potentials 210109.doc