Earthing systems are governed by standard IEC 60364-3. There are
three types of systems: IT, TT and TN.
The first letter defines the neutral point in relation to
earth:
- T stands for directly earthed neutral.
- I stand for unearthed or high impedance-earthed neutral (e.g. 2200 Ω).
The second letter defines the exposed conductive parts of
the electrical Installation in relation
to earth:
- T stands for directly earthed exposed conductive parts.
- N stands for exposed conductive parts directly connected to the neutral conductor.
IT system unearthed or
impedance-earthed neutral (see Figure-1)
Letter I depicts that
The neutral is unearthed or connected to earth by a high
impedance (an Impedance of 1600 Ω is often
used).
Letter T depicts that
The exposed conductive parts of the loads are interconnected and
earthed. A group of loads can be individually earthed if it is situated
far away from the Other loads.
TT
system directly earthed neutral
First letter T depicts that
The neutral is directly earthed.
Second letter T depicts that
The exposed conductive parts of the loads are interconnected
either altogether or by a group of loads. Each interconnected group is earthed. One
exposed conductive part can be individually earthed if it is far away from the
others.
Figure 2-3: directly
earthed neutral (TT system) in low voltage
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