An earthing system is process of connecting a conductive object to the earth for by-pass in the event of fault current, leakage current, or an overvoltage condition.
Electrical distribution systems must have an earthing systems arrangement for ensure a safe and effective operation of electrical equipment.
There are mainly 3 types of arrangement in earthing system; TT, TN and IT
For TT and IT System
The 1st letter, determine the condition for distribution or source point and “T” stands for Terra or Earthed and “I” stands for isolated.
The 2nd letter determines connection of exposed conductive parts or installation point to earth, “T” stands for independently Earthed and “N” for via neutral.
TT System: The letters stand for “T” is French word, stands for earth. The first “T” tells us that the supply transformer has a connection to earth and the second “T” tells us that there is also a connection to earth.
Within the installation this connection to earth is completed by installing an earth rod or earth electrode. The earthing conductor connects directly onto the earth electrode. The earth electrode is often a long metallic rod that gets hammered directly into the ground.
IT System: In IT system, at supply source or distribution point, the neutral point is not connected to earth or it is only earthed via a high impedance connection earthing system used in IT systems, the neutral point is not connected to earth. The purpose of this is to provide power in an environment where the electrical equipment is sensitive to earth leakage current.
TN Earthing systems have 3 variety of forms; TN-S, TN-C, and TN-C-S. The 3rd and 4th letters define the combination of neutral and earth. Here, “S” stands for Separate; “C” stands for Combined and “C-S” for Combined in source of supply and separate in installation side.
TN-S: The TN-S earthing system is a method of connecting the supply transformer to the earth. The “T” in TNS stands for earth, “N” stands for “Neutral” while the “S” stands for “Separate.” This means that in the TN-S earthing arrangement, the neutral, earth conductor, and CPCs are all kept separate both inside and outside of the installation.
To connect to the supplier’s earth transformer, the installation’s earthing conductor is connected directly to the metallic sheathing of the supply cable. This connection is typically made by the distribution network operator.
In a TN-S earthing system, the earth conductor (green and yellow cable) connects directly to the sheathing of the supply cable.
TN-C: TN-C system is a widespread system, where in the transformer substation, the earth and neutral connections are combined into a single connection, which reduces the number of connections required.
However, this type of earthing system does not provide as much protection against lightning strikes as TN and TN-S systems. Only two wires are connected to the sockets, and in case of emergency contact between phase conductor and electrical appliances, a short circuit occurs.
The disadvantages of this system are that the neutral conductor can burn out or break off, and appliances often lose their protection grounding. An RCD (residual current protective device) can be installed to protect.
TN-C-S: In TN-C-S, earthing system, the letters “T”, “N”, “C”, and “S” represent different components of the system. The “T” stands for Terra, which represents the supply transformer’s connection to earth.
The “N” stands for Neutral, which represents the neutral conductor. The “C” shows the combination of the Neutral and earthing conductors in a pen conductor, and the “S” represents the separation of the earthing conductor and Circuit Protective Conductors from the Neutral conductor.
This type of earthing system is also known as Protective Multiple Earthing (PME) as the supply cable is connected to earth multiple times.