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alternative path for the fault current to flow so that it will not
endanger the user, ensure that all exposed conductive parts do not
reach a dangerous potential, maintain the voltage at any part of an
electrical system at a known value and prevent over current or
excessive voltage on the appliances or equipment.
Different earthing systems are capable to carry different
amounts of over current. Since the amount of over current
produced in different types of installation differs from each other,
required type of earthing will also differ according to the type of
installation. So in order to ensure that the installation goes with the
existing earthing system or to do any modification accordingly, we
need to have a proper idea of the present earthing system. It would
enhance the safety as well as the reliability.
As per IEC 60364-3 (International Electrotechnical
Commission) there are two types of systems:
(1) UNEARTHED SYSTEM:
• IT System.
(2) EARTHED SYSTEM:
• TT
• TN (TN-S, TN-C, TN-C-S).
The first letter defines the neutral point in relation to earth:
1. T = directly earthed neutral (from the French word Terre)
2. I = unearthed or high impedance-earthed neutral (e.g. 2,000 Ω)
The second letter defines the exposed conductive parts of the
electrical installation in relation to earth:
1. T = directly earthed exposed conductive parts
2. N = exposed conductive parts directly connected to the neutral
conductor

Unearthed System
1. IT system unearthed (high impedance earthed neutral)
First letter I = the neutral is unearthed at transformer or generator
side.
Second letter T = frame parts of the loads are interconnected and
earthed at load side

System characteristics

1. It is compulsory to install an over voltage limiter between the
MV/LV (middle voltage / low voltage) transformer neutral point

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