Page 379 - Handbook of Electrical Engineering

EARTHING AND SCREENING 367

moulded case circuit breaker, miniature circuit breaker. The international standards such as IEC 60364
and Reference 10 give tables for the limiting values of the earth loop impedance for common ratings
of fuses and circuit breakers. Once the limiting value for the circuit is determined from these tables
it is a simple calculation procedure to find the maximum length of the cable that can be allowed, as
demonstrated in 9.4.3.6.

13.3.5 Earthing Rods and Grids

An essential aspect in the design of earthing systems for land-based plants in particular is the min-
imisation of the risk of electric shock due to the creation of potential along the surface of the
ground and between the ground and metallic structures such as switchgear, overhead line poles and
fences. The creation of potential along the surface of the ground gives rise to what is defined as the
‘step potential or voltage’, and between the ground and metallic structures, the ‘touch potential or
voltage’.

13.3.5.1 Touch and step voltages
Situations arise where the soil resistivity (ρ) is very high, for example in desert locations. In these
situations the concepts of ‘touch’ and ‘step’ voltages are important, see the international standard
IEEE80, section 5. A person may be standing on a conductive surface and touching electrical equip-
ment with one or both hands. At the same time a fault occurs and its current passes through the
equipment casing to the ground, thereby creating a potential difference across the person. This is the
touch potential difference or touch voltage. In a second type of fault situation a person is standing
on conductive ground with his feet spread one metre apart. The fault current, or part of it, passes
horizontally at or near the surface of the ground. The local resistance of the ground in the path of the
current creates a potential difference across the feet of the person. This is the step potential difference
or voltage.
The magnitude and duration of these voltages, together with the resistance of the person
between his points of contact, will determine whether the person receives a minor or even a fatal
shock. If the surface layer of the ground can be reduced in conductivity by a significant amount then
the current along the surface will be small, and most of the fault current will be forced down to a lower
level in the ground. A small level of surface current and an inherently high source resistance will
tend to restrict the amount of the surface current that can be shunted into the person, thereby reducing
the risk of shock. The surface layer may be the addition of dry crushed rocks or stones, and it should
be kept reasonably shallow, e.g. 100 to 150 mm or rubber mats as used in switchrooms. Chapter
six in Reference 3 gives an excellent coverage of the subject of earthing, mathematic derivations of
complex formulae and the topics of step and touch voltages. The equations presented are well suited
to hand calculations or simple computer programming.
IEEE80 sub-divides the touch and step voltages into two categories, one for heavier persons
of typical weight 70 kg and one for lighter persons at 50 kg. The reference illustrates the fact that
the heavier the body the higher the threshold of fibrillation of the heart. For calculation purposes it is
conservative to use the 50 kg equations. The results will be about 25% lower, which will eventually
require a little more conductive material in the ground for a given situation. The reference also
introduces an additional term to the standard body resistance of 1000 ohms, which takes account of
the ‘crushed rock layer’ and the resistivities of the crushed rock (ρ s ) and the main mass of earth