Types of Earthing Systems – What does TT, IT & TN Earthing mean?
Standards Used for Earthing Systems Definitions
Over the past century, Electrical Safety Standards have evolved into highly developed systems that cover all the major aspects for a safe installation, including Earthing Systems. In Low-voltage (LV) Electrical Installations, the reference standard IEC 60364 is used for the measures to be implemented to guarantee the protection of personnel and property.
The IEC 60364 standard has defined three types of Earthing Systems, namely TT, IT, and TN systems. Since IEC publishes International Standards for all electrical, electronic, and related technologies and is the leading international organization in its field, IEC 60364 is the apex level document that informs the standards for LV Electrical Installations around the world. Therefore, the three types of earthing systems defined in IEC 60364 are also recognised in many national standards. BS 7671: 2008, also known as the IEE Wiring Regulations 17th Edition is the British Standard published on January 2008 used by the UK and other countries. Similarly, Indian Standard IS 732:1989 (R2015) is used in India for electrical installations.
Types of Earthing Systems
As mentioned above, the three major types of Earthing Systems used by IEC 60364 are
- 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.
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.
For the first letter, “T denotes that the start point of the source is solidly connected to earth, which is usually at a location very near to the winding.
I denote that the start point and the winding are isolated from earth. The start point is usually connected to an inductive impedance or resistance. Capacitive impedance is never used.”
And for the second letter, “T denotes that the consumer has solidly earthed independently of the source earthing method.
N denotes that a low impedance conductor is taken from the earth connection at the source and routed directly to the consumer for the specific purpose of earthing the consuming equipment.
S denotes that the neutral conductor routed from the source is separate from the protective earthing conductor, which is also routed from the source. This implies that five conductors need to be routed for a three-phase consumer.
C denotes that the neutral conductor and the protective earthing conductor are one and the same conductor. This means that four conductors need to be routed for a three-phase consumer.”
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 type of Earthing System, connection to the supply source is directly connected to earth & load end or installation metalwork 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. Such a fault 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. The IT Earthing System is used for power distribution systems such as substations or generators.
TN-S Earthing System
In this system, the Ground Conductor & Neutral 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 & the neutral terminal and these terminals are linked together.
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This article is part of our series of articles on Lightning Protection, Surge Protection & Earthing, you can read more with the following links:
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