In the
regulations for electrical installations, the types of earthing systems are identified
as follows, depending on the relationship of the source (supply authority
network) and of the exposed conductive parts of the installation, to earth.
These are
(1) TN -
earthing of the installation is done to that supplied by the supply authority
(2) TT -
supply authority earth and the installation earth is independent
(3) IT -
supply authority has effectively an isolated neutral and the installation has
an independent earth
In these, the
first letter denotes the earthing arrangement at the supply authority side and
the second letter denotes the relationship of the exposed conductive parts of
the installation to earth. With the First letter, T (short for terra or earth)
refers to a direct connection of one or more points of the source to earth, and
I (short for isolated) indicated that all live parts are isolated from earth or
one point connected to earth through a high impedance. With the Second letter,
T denotes a direct electrical connection of the exposed conductive parts of the
consumers installation to earth, independently of the earthing of any point of
the supply authority side, while N denotes a direct electrical connection of
the exposed conductive parts to the earthed point of the supply authority side,
which for ac is usually the neutral point.
[There are
further sub-divisions of the TN system, but are considered to be beyond the
contents of this course. Even the details of the earthing systems other than
the one used in Sri Lanka will not be dealt with in this]
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