Page 62 - Electrical Safety of Low Voltage Systems

CHAPTER4

The Earth

Is it a fact, or have I dreamt it, that by means of
electricity, the world of matter has become a great
nerve, vibrating thousands of miles in a
breathless point of time?
nathaniel hawthorne (1804–1864)

4.1 Introduction
Not all electric faults cause circulation of current through the earth.
However, regardless of the grounding system adopted, if a fault en-
ergizes an exposed-conductive-part, a potential difference between its
enclosure and the ground will appear. As a result, current will circu-
late through the body of the person touching the faulty item, as the
actual earth becomes an available return path to the supply source. 1
This is because the ground is a conductor, with a definite resistivity,
which varies according to its make up.
It is, then, important for the electrical safety of persons to under-
stand the electrical behavior of the earth as a return means for fault
currents.

4.2 The Earth Resistance
Currents can be impressed through the earth by means of ground
electrodes.
Ground electrodes are conductive parts, which may be embedded
in a specific conductive medium, in intimate electric contact with the
earth. Their purpose is to guarantee safety by providing an effective
access to the earth for fault currents.
When a potential difference is applied between two electrodes,
a current field will be inevitably impressed into the soil. We now
consider a homogeneous and isotropic 2 soil, and hemispherical

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