ERICO Practical Guide to Electrical Grounding - Manual (Page 31)

LIGHTNING - AN OVERVIEW
Lightning is an electrical discharge within clouds, from
cloud to cloud, or from cloud to the earth. Lightning
protection systems are required to safeguard against
damage or injury caused by lightning or by currents
induced in the earth from lightning.
Clouds can be charged with ten to hundreds of millions of
volts in relation to earth. The charge can be either negative
or positive, although negative charged clouds account for
98% of lightning strikes to earth. The earth beneath a
charged cloud becomes charged to the opposite polarity. As
a negatively charged cloud passes, the excess of electrons in
the cloud repels the negative electrons in the earth, causing
the earth’s surface below the cloud to become positively
charged. Conversely, a positively charged cloud causes the
earth below to be negatively charged. While only about 2%
of the lightning strikes to earth originate from positively
charged clouds, these strikes usually have higher currents
than those from negatively charged clouds. Lightning
protection systems must be designed to handle maximum
currents.
The air between cloud and earth is the dielectric, or
insulating medium, that prevents flash over. When the
voltage withstand capability of the air is exceeded, the air
becomes ionized. Conduction of the discharge takes place
in a series of discrete steps. First, a low current leader of
about 100 amperes extends down from the cloud, jumping
in a series of zigzag steps, about 100 to 150 feet (30 to 45
m) each, toward the earth. As the leader or leaders (there
may be more than one) near the earth, a streamer of
opposite polarity rises from the earth or from some object
on the earth. When the two meet, a return stroke of very
high current follows the ionized path to the cloud, resulting
in the bright flash called lightning. One or more return
strokes make up the flash. Lightning current, ranging from
thousands to hundreds of thousands of amperes, heats the
air which expands with explosive force, and creates
pressures that can exceed 10 atmospheres. This expansion
causes thunder, and can be powerful enough to damage
buildings.
The National Weather Services of the National
Atmospheric Administration (NAA) keeps records of
thunderstorm activity. This data is plotted on maps showing
lines of equal numbers of thunderstorm days (days in which
there was at least one occurrence of thunder is heard). Such
isokeraunic charts show a wide geographic variation of
thunderstorm activity, from more than 90 days per year in
central Florida to less than 5 on the West Coast. (Fig. 2-1)
Such charts cannot predict the lightning activity at any
location, but make it possible to judge the extent
of exposure and the potential benefits of a lightning
protection system. However, the overriding concerns in
protection must be the protection of people and the
reliability of equipment.
23 Chapter 2: Building Lightning Protection
1
6
5
5
30
40
10
5
10
10
50
50
50
50 50
50
50
50
50
50
40
40
40
60
60
60
70
70
40
5
20
30
40 40
50
30
49
9
5
10
20
20
30
30
40 30
30
30
30
30
30
20
20
20
30
40
40
50
60
70
80
80
70
60
60 70
80
90
90
80
70 60
30
40
50
20
Isokeraunic Map
Fig. 2-1
This isokeraunic map shows mean annual number of days with thunderstorms in the United States.
The highest frequency is encountered in south central Florida.
Grounding Book 4/14/99 10/5/99 6:01 PM Page 23 (Black plate)