TT, IT & TN earthing solutions for safe earthing

Electrical Safety Standards have evolved into highly developed systems that cover all the major aspects for a safe installation, including the earthing systems. In Low-voltage (LV) Electrical Installations, the reference standard IEC 60364 is used to implement the measures to guarantee the protection of personnel and property.

This standard has defined three types of Earthing Systems; TT, IT, and TN systems. Since IEC publishes International Standards for all electrical, electronic, and related technologies is the leading international organisation in its field, IEC 60364 is the apex level document that informs the standards for LV Electrical Installations around the world. Indian Standard IS 732:1989 (R2015) is IEE Wiring Regulations and is used in India for electrical installations.

Types of Earthing Systems
The three major types of Earthing Systems used by IEC 60364:

• TT
• IT
• TN – TN-C, TN-S, TN-C-S

The TN system is further subdivided into TN-C, TN-S and TN-C-S and thus we will refer to 5 types of Earthing Systems prevalent worldwide.

NOMENCLATURE
The First Letter of each system refers to the power source from a star-connected winding. The Second Letter refers to the consuming equipment that needs to be earthed. From the ‘Handbook of Electrical Engineering: For Practitioners In The Oil, Gas And Petrochemical Industry’ by Alan L. Sheldrake

To put it in simpler terms:
T = Direct connection to the Earth, T stands for Terra meaning earth
I = Isolated
N = Neutral
S = Separate
C = Combine
The most common systems are TT and TN. A few countries, such as Norway, use the IT system. The table below table lists examples of the Earthing Systems used for public distribution (LV Consumers) in a few countries.

TT Earthing System
In this connection to the supply source is directly connected to earth and load end or installation metal work is also directly connected to the earth. Therefore, in case of an overhead line, the mass of the earth will be the return path for the line. The neutral and earthing conductor must be separated through the installation because the power distributor only provides the supply neutral or protective conductor for the connection to the consumer.

IT Earthing System
The distributor system does not have any connections to earth or it has only a high impedance connection. The basic feature of IT earthing system is that in the event of a fault between phases and earth, the system can continue to operate without interruption. It is referred to as a “first fault”. Thus, usual earthing protection is not effective for this system and this type is not meant for consumer power supply, it is used for power distribution systems such as substations or generators.

TN-S Earthing System
The Ground Conductor and Neutr al Conductor are separate throughout the distribution system. The protective conductor is the metallic covering of the cable supplying the installation. All the exposed conductive parts of the installation are connected to this protective conductor or via the main earthing terminal of the installation.

TN-C Earthing System
The Neutral and the protective earth are combined into a single conductor throughout the system. All the exposed & conductive parts of the installation are connected to the PEN conductor. As per the 8(4) of the Electricity Safety, Quality and Continuity Regulations 2002, “Consumer shall not combine the neutral and protective functions in a single conductor in his customer’s installation”.

TN-C-S Earthing System
The Neutral and the protective earth are combined in a single conductor in a part of the system. This type of earthing is also known as multiple protective earthing. The supply system PEN conductor is earthed at two or more points and an earth electrode may be necessary at or near the consumer’s installation. All the exposed conductive parts of the installation are connected to the PEN conductor via the main earthing terminal and the neutral terminal and these terminals are linked together.

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