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Types of Earthing

There are five types listed in BS 7671 Wiring Regulations:

  1. TN–S
  2. TN–C–S
  3. TT
  4. TN–C
  5. IT

The first letter relates to the connection between earth and the source of power supply, i.e. generator or transformer; and the second relates to the connection between earth and the electrical device the power is supplying; with each letter meaning as follows:

  • T = Earth. Direction connection with Earth (From the French word for ground: Terre).
  • N = Neutral. Connection to earth via the supply network.
  • S = Separate.
  • C = Combined.
  • I = Isolated. No connection with earth, unless through a high impedance.

Which type of earthing system should be used?

It is imperative that the right type of electrical earthing system is chosen since it will affect both safety and the electromagnetic compatibility of the power supply.

Providing a Protective Earth (PE) connection will ensure that all exposed conductive surfaces have the same electrical potential as the earth’s surface, thereby preventing anyone touching the device in the case of a short circuit from suffering an electric shock. A short circuit will also cause a PE connection to give rise to a greatly increased current flow, triggering fuses and circuit breakers and disconnecting the power supply.

A Function Earth (FE) connection is used for devices such as antennas, and may carry a current during normal operation.

The three common types of earthing systems operate as follows:

TN–S

This type of electrical earthing system connects the neutral source of energy with the earth at one point only (or as near as practically possible), and with the consumer’s earthing terminal commonly connected to the metallic sheath or armour of the distributor’s service cable into the premises. With a TN–S the PE connection and N are separate conductors that are connected together only near the power source.

TN–C–S (PME)

A TN–C–S earthing system, typically known as Protective Multiple Earthing (PME), connects the supply neutral conductor of a distribution main with earth at course and at intervals along its stretch. The neutral conductor is also used to return earth faults currents safely to source by the provision of a consumer’s earthing terminal linked to the incoming neutral conductor.

TT

The neutral of the energy source is connected as with the TN–S system, however there is no provision for the consumer’s earthing, therefore they must provide their own connection to earth.

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