Page 744 - Industrial Power Engineering and Applications Handbook

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22.5 Jointing of grounding 22.8 Tolerable potential difference at
conductors a location

As discussed in Section 29.2.5, jointing of two different We discussed in Section 2 1.1.1 the maximum tolerable
metals (copper being one) causes electrolysis at the joints, currents through a human body and their duration. The
leading to corrosion and failure of the joint. To avoid potential difference in a ground conductor at any point
this, it is recommended that the same procedure be adopted where a human body may come into contact with it during
as discussed in Section 29.2, and where the electrode the course of a ground fault should be such that the
and the connecting ground strip are of the same metal, resultant current through the human body will remain
that the joints are riveted or welded with the same metal within these tolerable limits.
after making the surface. Soldering is not recommended.
22.9 Voltage gradients
22.6 Maintenance of grounding
stations The likely positions, in which a human body may come
into contact with a phase or a ground conductor, and the
To ensure that a grounding station has not deteriorated corresponding potential differences, that he may be
and its ground resistance has not increased due to soil exposed to, are illustrated in Figure 22.9.
depletion it is mandatory to carry out a few checks
periodically to ascertain the resistance of the grounding 22.9.1 Tolerable step voltage, E,
station. If the ground resistance is found more than it
was designed for, it is possible that by proper moistening This is the difference in the surface potential to which a
of the soil or by adding more salts or chemicals to the human body may be subject when bridging a distance of
grounding pit, the desired level of ground resistance is 1 metre on the conducting ground through the feet without
achieved once more. If not, then additional grounding being in contact with any other conducting grounded
stations may have to be installed to obtain the original surface (position 1, Figure 22.9).
level of the ground resistance. The safe step voltage, E,, should not be more than the
total resistance to ground through the body, R2f,b (Section
22.10.1) x safe body current, Ib, as a function of time,
where
SECTION II
R,fsb = 6 . c, . ps + 1000 R (from equation (22.10),
discussed later)
22.7 Grounding practices in a power
generating station and I, = - l6 for a SO kg body, as in equation (21.2)
fi
This is a vast subject, on which extensive research has -- 0.157 for a70 kg body, as in equation (21.3)
been done by many authors over the years. The grounding - Jt
stations in such areas are normally spread over the entire
station, and sometimes may even extend beyond its
(6
boundary to achieve the desired results. Here we discuss .*. Es(5") I C, ' ps + 1000) x ~ O. lii for a 50 kg body
'
briefly, the basic criteria behind the requirement of a
grounding system in a power station and its design (22.5)
considerations.
The magnitude of ground voltage in such areas in the and (6 . C, . pE + 1000) x ~ 0'1s7 for a 70 kg body
event of a ground fault is very high, due to high system lir
voltage. On a ground fault, the ground path resistance (22.6)
may become a source of a high potential gradient across
the grounding conductors at a particular location. It may 22.9.2 Tolerable touch voltage (E,)
become high enough to prove fatal to a human operator This is the potential difference between the ground
coming into contact with it. To limit this potential potential rise (GPR) and the surface potential at a point
difference at all locations within a tolerable value and
achieve an equipotential distribution of a ground conductor where the person is standing on the conducting ground
with one hand in contact with a conducting grounded
over the station is the basic criterion on which is based surface (position 2, Figure 22.9). The safe touch voltage,
the design of a grounding system for a power generating Et, should not be more than the total resistance to ground
station. Our discussion is also applicable to outdoor
switchyards and large sub-stations. For detailed working, through the body, R21ps (Section 22.10.1) x safe body
current, Ib, as a function of time, where
it is advisable to refer to IEEE-80.
The following basic data are important for the design RZfps = 1.5 C, . p\ + 1000 R (from equation (22.11)
of such a grounding system. discussed later)