Handbook of Electrical Installation Practice - BeKnowledge

INTRODUCTION
CHAPTER 7
Ground Earthing
T.E. Charlton, BEng, CEng, MIEE, MBA
(Director, Strategy and Solutions Ltd)
J.R. Wales, DFH, CEng, MIEE
(Consultant, Stemet Earthing Co. Ltd)
Earthing of electrical installations is primarily concerned with ensuring safety. In
power networks the earthing system helps to maintain the voltage of any part of the
network within a predetermined range with respect to earth under steady state and
fault conditions. If designed correctly, it should allow enough current to flow under
fault conditions to operate the protective devices installed.The rise in potential experienced
during the fault combined with the speed of fault clearance, should be such
as to minimise both the risk of electrocution to individuals near the site of fault and
damage to equipment. The widespread use of electrical appliances, both in the
factory and the home, also introduces many situations where efficient earthing is of
paramount importance, especially to prevent electric shock under fault conditions.
The two main components of an earthing system are equipotential bonding and
formal earth electrodes. Equipotential bonds seek to minimise the potential difference
experienced across exposed metallic conductive parts by connecting them
together. The formal earth electrodes normally consist of metallic components in
direct contact with the soil.They are required to disperse any fault current to ground
in a safe, controlled and effective manner.
The installation of an earth electrode is an important factor in achieving a satisfactory
earthing system. It involves burying conductive material which is in direct
contact with soil or the general mass of the earth. Soil conditions can vary enormously
from site to site and directly affect the resistance value of a given electrode.
It is thus necessary to consider the soil and other factors which affect the actual
resistance of the earth electrode at the design stage.
The earthing system consists of conductive material above ground, and metal electrodes
within the soil and the surrounding soil itself. Each of these will contribute
towards the overall resistance value. There are contact resistances, for example at
joints and at material interfaces. The contact resistance of joints must be kept to a
minimum by using appropriate materials and installation practice. In a new installation,
the most significant contact resistance is likely to be at the interface between
electrodes and soil. This arises mainly because the soil has not yet consolidated.
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