5.4.1 - Earthing conductors
The earthing conductor is commonly called
the earthing lead. It joins the installation earthing terminal
to the earth electrode or to the earth terminal provided
by the Electricity Supply Company. It is a vital link in
the protective system, so care must be taken to see that
its integrity will be preserved at all times. Aluminium
conductors and cables may now be used for earthing and bonding,
but great care must be taken when doing so to ensure that
there will be no problems with corrosion or with electrolytic
action where they come into contact with other metals.
Where the final connection to the earth
electrode or earthing terminal is made there must be a clear
and permanent label Safety Electrical Connection - Do
not remove (see
{Fig 5.17}). Where a buried earthing conductor is not
protected against mechanical damage but is protected against
corrosion by a sheath, its minimum size must be 16 mm² whether
made of copper or coated steel. If it has no corrosion protection,
minimum sizes for mechanically unprotected earthing conductors
are 25 mm² for copper and 50 mm² for coated steel.
If not protected against corrosion the
latter sizes again apply, whether protected from mechanical
damage or not.
Earthing conductors, as well as protective
and bonding conductors, must be protected against corrosion.
Probably the most common type of corrosion is electrolytic,
which is an electro-chemical effect between two different
metals when a current passes between them whilst they are
in contact with each other and with a weak acid. The acid
is likely to be any moisture which has become contaminated
with chemicals carried in the air or in the ground. The
effect is small on ac supplies because any metal removed
whilst current flows in one direction is replaced as it
reverses in the next half cycle. For dc Systems, however,
it will be necessary to ensure that the system remains perfectly
dry (a very difficult task) or to use the 'sacrificial anode'
principle.
A main earth terminal or bar must be provided
for each installation to collect and connect together all
protective and bonding conductors. It must be possible to
disconnect the earthing conductor from this terminal for
test purposes, but only by the use of a tool. This requirement
is intended to prevent unauthorised or unknowing removal
of protection.
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