Page 226 - Electrical Safety of Low Voltage Systems

Safety Against Overvoltages
209
latter case, the lightning impulsive current is drained to earth, directly
or via the structures that are struck. The consequent release of a large
amount of energy (i.e., many hundreds of megajoules) is harmful to
persons as well as destructive for equipment.
The cloud-to-earth lighting is initiated by the presence of charges,
usually negative, in the lower part of the cloud. The buildup of charges
maycausetheresultingelectricfieldtoexceedthedielectricstrengthof
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the air. The breakdown of the air and an initial discharge then occur.
The discharge creates a highly conductive channel the charges can
use to descend toward ground as if it were a conductor. The channel
stops at the point where the dielectric strength of the air equals the
electric field caused by the charges. Further charges, though, traveling
from the cloud reinforce the field, perturbing the equilibrium and
allowing new discharges. Thus, new conductive channels occur in a
“zig-zag” fashion toward the earth. Such “stepped” discharge is due
to the nonuniformity of the air, caused by the punctual variation of its
parameters, such as density, humidity, etc.
The charge from the cloud progressing toward the earth induces
an equal amount of charge, but of opposite sign. The electric field
increases and an upward-directed discharge from the soil, or a struc-
ture, takes place causing an “attachment” between the upward- and
downward-directed channels. This “return stroke” causes the circu-
lation of the high-intensity (i.e., hundreds of thousands of amperes)
impulsive lightning current to ground. Such a current is characterized
by a rapid rise to the peak (i.e., within a few microseconds), a rela-
tively slow decay as well as a high-frequency content (i.e., order of
hundreds of kilohertzs).
12.3.1 Characterization of Earthing Systems Under
Impulse Conditions
The grounding system used to safely drain to earth the fault currents
at the power frequency is also employed to dissipate to ground the
lightning current. Earthing electrodes, in fact, may be connected, by
means of down-conductors, to lightning protection systems (LPS),
such as masts, installed on the roof of buildings being protected.
Grounding systems through which high-frequency currents cir-
culate, though, do not behave in the same fashion as examined in the
previous chapters for fault currents at the network frequency. Earth
electrodes, in fact, cannot be assumed to be purely resistive in the
presence of pulse currents, as the inductance of their metal parts can-
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not be neglected. Thus, the earth electrode must be modeled as an
ohmic-inductive pi circuit, and we will use the term earth impedance
instead of earth resistance.
To clarify the concept, let us consider, as an earth electrode, a
buried horizontal wire (Fig. 12.10).