Generally, there are 3 types of Earthing Systems namely TN
System, TT System and IT System.
TN System: A
system having one or more points of the source of energy directly earthed, the
exposed conductive parts of the installation being connected to that point by
protective conductors. TN systems however are sub-divided into 3 main categories
which are;
TN-C System: A
system in which neutral and protective functions are combined is a single
conductor throughout the system.
All exposed conductive parts of an installation are connected
to PEN conductor.
TN-S System: A
system that having separate neutral and protective conductors throughout the
system.
The protective conductor (PE) is the metallic covering of
the cable supplying the installations or a separate conductor.
All exposed conductive parts of an installation are
connected to this protective conductor via the main earth terminal of the installation.
TN-C-S System: A system in which neutral and
protective functions are combined in a single conductor in part of the system.
In TN-C-S system, the supply is TN-C and the arrangement in
the installations is TN-S. this is also known as Protective Multiple Earthing
and the PEN conductor is referred to as the combined neutral and earth (CNE)
conductor.
The supply system PEN conductor is earthed at several points and an earth
electrode may be necessary at or near the consumer’s installation.
All exposed conductive parts of an installation are
connected to the PEN conductor via the main earthing terminal and the neutral
terminals being linked together.
TT System: A
system having one point of the source of energy directly earthed, the exposed
conductive parts of the installation being connected to earth electrodes
electrically independent of the earth electrodes of the source.
All exposed conductive parts of an installation are
connected to an earth electrode which is electrically independent of the source
earth.
IT System: A
system having no direct connection between live parts and Earth, the exposed
conductive parts of the electrical installation being earthed.
All exposed conductive parts of an installation are
connected to an earth electrode.
The source is either connected to Earth through deliberately
introduced earthing impedance or is isolated from Earth.
The IT system is commonly used in hospital’s operation
theatre.
