Protective earthing has to be carried out when risk exists of parts of struc-
tures not intended for current-carrying purposes becoming charged to dangerous
voltages either through accidental contact with H.T. circuits or through current
creeping across from the H.T. circuit. It operates by providing a good conducting
path to earth for any such leakage currents. Consequently, when a human
body comes into contact with the structure, only an infinitesimal current will
flow to earth through it whilst the greater part passes through the metallic con-
nection to earth. The relative values of these two currents is dependent on the
existing condition of the earth connection, and its resistance should therefore be
kept permanently at the lowest possible jfigure. The important role of this
protective earthing, viz., the prevention of danger to persons, makes careful
workmanship and avoidance of all risk of mechanical or chemical damage
imperative.
The cross-section of the earthing wire and the surface of the earthing plate
must be chosen to suit the expected volume of current flowing to earth. In
overhead line systems it is advisable to make the cross-section of copper earthing
wires at least -04 square inch and of iron earthing wires at least -16 square inch.
The copper should be tinned and the iron galvanised. Earth wires should be
as open to inspection as possible and, where laid in the earth, should be of solid
wire not of stranded cable. Good metallic contact at the connection points
is essential. When connecting the earth wire to iron intermediate lead washers



OVERHEAD TRANSMISSION LINES



167



should be inserted ; and the connection should be covered with a waterproof
covering or paint.
