Main Types of Earthing Systems

Introduction

The process of connecting the metallic frame i.e. non-current part of electrical equipment or some electrical part of the system such as the neutral point of the secondary side of the distribution transformers or the star point in a star connected system to the earth is called grounding or earthing.

The potential of the earth is to be considered zero for all practical purposes. Earthing is used to connect any electrical equipment to earth with a very low resistance making it to attain the earth’s potential. This ensures the safe discharge of electrical energy due to failure of the insulation line coming in contact with the casing, and so forth. As a result, earthing brings the potential of the body of the equipment to zero, that is, the earth potential, hence protecting the operating personnel against electrical shock.

The contact with metalwork made live by a fault is termed to as indirect contact. One widely used technique of providing some measure of protection against such contact is by earthed equipotential bonding and automatic disconnection of supply. This entails the bonding together and connection to earth of:

• All metalwork associated with electrical equipment and systems, referred to as exposed conductive parts such as conduit, trunking and the metal cases of equipment.
• All metalwork likely to introduce a potential including earth potential, termed to as extraneous conductive parts such as gas, oil and water pipes, structural steel work, radiators, sinks and baths.

The effect of all this bonding is to create a zone in which all metalwork of different services and systems will, even under fault conditions, be at a largely equal potential. Furthermore, there is a low-resistance earth return path, ensuring the fast protection operation to prevent danger. The resistance of such an earth return path will depend upon the system. These systems have been designated in the IEEE regulations using the letters T, N, C, and S. These letters stand for:

T – Terre (French for earth) and meaning a direct connection to general mass of earth.

N – Neutral (indicates that there is also the connection of a conductor to the star or neutral point of the supply transformer, which is continuous throughout the distribution system and terminates at the consumer’s intake position.

C – Combined

S – Separate

When these letters are grouped they form the classification of a type of system. The first letter in such a classification denotes how the supply source is earthed. The second indicates how the metalwork of an installation is earthed. The third and fourth indicate the functions of neutral and protective conductors. In the following sections we shall look at the main earthing systems classified per aforementioned description.

Types of Earthing Systems

TT System

A TT system has the star or neutral point of supply transformer directly connected to earth by means of an earth electrode and the earthing of the consumer’s installation is also directly connected to earth via an earth electrode. This system is common feature of an overhead line supply in a rural area and is illustrated in the figure below:

TN-S System

A TN-S system has the star point of the supply transformer connected to earth. Additionally the installation metalwork is connected to the earthed neutral/star point of the supply source via the the outer metallic sheaths of the distribution cable and, ultimately, the service cable. Thus, there are separate (S) metallic earth and neutral conductors throughout the system.

TN-C-S System

TN-C-S system is actually the TN-S system as discussed above however, with the supply cable sheath being used also as the neutral, that is, it forms a combined earth/neutral conductor known as Protective Earthed Neutral (PEN) conductor. The installation earth and neutral are separate conductors. As demonstrated in the diagram below, the system inside the consumer’s premises continues to have separate (S) earth and neutral conductors.

Figure 1.2: TN-C-S system (Single-phase) – Combined earth and neutral conductor for supply. Separate earth and neutral in consumer’s installation.

This system is also referred to as Protective Multiple Earthing (PME).

Bottom Line

To prevent the risk of electric shock, it is paramount to provide a path for earth leakage currents to operate the circuit protection and to make every effort to maintain all the metalwork at the same potential. This is accomplished by bonding together the metalwork of electrical and non-electrical systems to earth. The path for leakage currents would then be via the earth itself in TT systems or by a metallic return path in TN-S or TN-C-S systems.

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