In the preceding article we studied the basics of Earthing and also the basic most Earthing type where the neutral is grounded at source and optionally grounding is done even at customer side.
Besides this, International Standard – IEC 60364 formally defines different types of Earthing arrangements. Let us examine them here in detail.
IEC Standard for Earthing
IEC Standard 60364 specifies a Two Letter Codes to identify type of earthing. It also defines three families of Earthing arrangements.
- The two letter code is based on Source Side – Device Side Earthing.
- The First Letter indicates how the Earthing is done on Source side (Generator / Transformer).
- The Second Letter indicates how the Earthing is done on Device side (place where electricity is consumed at customer premises).
The Letters used are as follows:
T – (French word “Terre” meaning Earth) – It means direct connection of a point to earth
I – It means that either no point is connected to Earth or it is connected via high impedance
N – It means that there is direct connection to neutral at the source of installation which is in turn connected to the ground
Based on a combination of these three letters, there are three families of Earthing arrangements proposed by IEC as below:
- TN Network
- TT Network
- IT Network
In TN type of earthing system, one of the points of the source side (Generator or Transformer) is connected to earth. This point is usually the star point in a three phase system. The body of the connected electrical device is connected to earth via this earth point on the source side. See fig. below which depicts this:
In above diagram:
PE – Acronym for “Protective Earth” – is the conductor that connects the exposed metallic parts of the consumer’s electrical installation to the ground.
N –Also called Neutral. It is the conductor that connects Star point in a 3 phase system to the earth.
There are three sub-types of TN networks as below:
TN-S: In this, separate conductors for Protective Earth (PE) and Neutral run from Consumer’s electrical installation till the source. They are connected together only at the power source.
TN-C: In this, there is a combined conductor called PEN (Protective Earth-Neutral) which is connected to earth at the source.
TN-C-S: In this type of earthing, part of the system uses a combined PEN conductor for earthing, whereas for remaining part of the system uses separate conductor for PE and N.
Usually, the combined PEN conductor is used near the source of the system.
In TT type of earthing system, consumer employs its own local earth connection in the premises, which is independent of any earth connection at source side.
This type of earthing is preferred in telecommunication applications, because this system is free of any high or low frequency noise that comes through neutral wire connected to the equipment.
In IT type of earthing system, there is either no connection to earth at all, or it is done via a high impendence earthing connection.
Country Specific Earthing Standards
UK – Uses Protective Multiple Earthing (PME) – which is a form of TN-C-S type of earthing
Australia / New Zealand – Also uses TN-C-S type of earthing know as Multiple Earth Neutral (MEN) system
USA / Canada – Uses TN-C for the feed from Transformer but uses TN-C-S within the structure at customer premises
France / Japan / Denmark – Uses TT type of earthing and customer must make its own arrangement for its own earthing connection