Getting Down to Earth - Earthing Explained

Introduction

An earthing system or grounding system (US) connects specific parts of an electric power system with the ground, typically the Earth's conductive surface, for safety and functional purposes. The choice of earthing system can affect the safety and electromagnetic compatibility of the installation.

Regulations for earthing systems vary considerably among countries, though most follow the recommendations of the International Electrotechnical Commission (IEC).

Purpose of Earthing

There are three main purposes for earthing:

• System Earthing

System earthing serves a purpose of electrical safety throughout the system that is not caused by an electrical fault. Its main purpose is to prevent static build-up and to protect against power surges caused by nearby lightning strikes or switching. Static build-up, as induced by friction is dissipated to the Earth. In the event of a surge, a lightning arrester, a surge arrester or a SPD will divert the excess current to the Earth before it reaches an appliance.

• Equipment Earthing

Equipment earthing serves a purpose of electrical safety in an electrical fault. Its main purpose is to prevent equipment damage and the risk of an electric shock. When current flows from a line conductor to an earth wire, as is the case when a line conductor makes contact with an earthed surface in a Class I appliance, an automatic disconnection of supply (ADS) device such as a circuit breaker or a RCD automatically opens the circuit to clear the fault. Technically, this type of earthing is not an earth connection.

• Functional Earthing

Functional earthing serves a purpose other than electrical safety.

Example includes electromagnetic interference (EMI) filtering in an EMI filter, and the use of the Earth as a return path in a single-wire earth return distribution system.

IEC Terminology

All Earthing Systems (ES) are meant for the protection of human beings and equipment. However, each earthing system has specific advantages and disadvantages which are required to be considered at the time of the installation. In the tertiary sector and industry, needs change, and it is becoming increasingly important to choose the right earthing system in order to ensure the cohabitation of high and low currents and meet application requirements.

International standard IEC60364 distinguishes three families of earthing arrangements, using the two-letter codes TN, TT, and IT.

The first letter indicates the connection between earth and the power- supply equipment (generator or transformer):

"T" - Direct connection of a point with earth

"I" - No point is connected with earth, except perhaps via a high impedance.

The second letter indicates the connection between earth or network and the electrical device being supplied:

"T" - The Earth connection is by a local direct connection to earth, usually via a ground rod.

"N" - The Earth connection is supplied by the electricity supply network, either separately to the neutral conductor (TN-S), combined with the neutral conductor (TN-C), or both (TN-C-S). These are discussed in detail later in this article.

Today, the three earthing systems, defined by the standards IEC364 are in practice and they are:

• The TN system
• The TT system
• The IT system (isolated or impedance-earthed neutral) 

The TN System

In a TN earthing system, one of the points in the generator or transformer is connected with earth, usually the star point in a three-phase system.

The body of the electrical device is connected with earth via this earth connection at the transformer. This arrangement is a current standard for residential and industrial electric systems The conductor that connects the exposed metallic parts of the consumer's electrical installation is called protective earth (PE). The conductor that connects to the star point in a three-phase system, or that carries the return current in a single phase system, is called neutral (N).

Three variants of TN systems are distinguished

A)    TN−S

PE and N are separate conductors that are connected together only near the power source.

B)    TN−C

A combined PEN conductor fulfills the functions of both a PE and an N conductor. (on 230/400 V systems normally only used for distribution networks)

C)    TN−C−S

Part of the system uses a combined PEN conductor, which is at some point split up into separate PE and N lines. The combined PEN conductor typically occurs between the substation and the entry point into the building, and earth and neutral are separated in the service head.

It is possible to have both TN-S and TN-C-S supplies taken from the same transformer. For example, the sheaths on some underground cables corrode and stop providing good earth connections, and so homes where high resistance "bad earths" are found may be converted to TN-C-S. This is only possible on a network when the neutral is suitably robust against failure, and conversion is not always possible. The PEN must be suitably reinforced against failure, as an open circuit PEN can impress full phase voltage on any exposed metal connected to the system earth downstream of the break. The alternative is to provide a local earth and convert to TT. 

The main attraction of a TN network is the low impedance earth path allows easy automatic disconnection (ADS) on a high current circuit in the case of a line-to-PE short circuit as the same breaker or fuse will operate for either L-N or L-PE faults, and an RCD is not needed to detect earth faults

The TT System

In a TT earthing system, the protective earth connection for the consumer is provided by a local earth electrode and there is another independently installed at the generator. There is no 'earth wire' between the two. The fault loop impedance is higher, and unless the electrode impedance is very low indeed, a TT installation should always have an RCD as its first isolator.

The big advantage of the TT earthing system is the reduced conducted interference from other users' connected equipment. TT has always been preferable for special applications like telecommunication sites that benefit from the interference-free earthing. Also, TT networks do not pose any serious risks in the case of a broken neutral. In addition, in locations where power is distributed overhead, earth conductors are not at risk of becoming live should any overhead distribution conductor be fractured by, say, a fallen tree or branch.

In pre-RCD era, the TT earthing system was unattractive for general use because of the difficulty of arranging reliable automatic disconnection (ADS) in the case of a line-to-PE short circuit (in comparison with TN systems, where the same breaker or fuse will operate for either L-N or L PE faults). But as residual current devices mitigate this disadvantage, the TT earthing system has become much more attractive providing that all AC power circuits are RCD-protected.

The IT System

In an IT network, the electrical distribution system has no connection to earth at all, or it has only a high-impedance connection. Either there is no earthing at the supply, or it is done via a high impedance connection.

This type of earthing is not used for distribution networks but is frequently used in substations and for independent generator-supplied systems.

These systems are able to offer good continuity of supply during operation.

Comparison between different earthing systems and conclusion

Critical criteria of different earthing systems are tabulated below. This table facilitates in selecting right kind of earthing system depending on technical, commercial & safety requirements.

Did You Know: To avoid accidental shock, current sensing circuits are used at the source to isolate the power when leakage current exceeds a certain limit. Residual- current devices (RCCBs or RCBOs) are used for this purpose. In industrial applications, earth leakage relays are used. This protection works in the range of milli- Amps and can be set from 30 mA to 500 mA. L&T Electrical & Automation has comprehensive LV portfolio of RCCB & RCBOs along with Earth leakage Relays to provide protection against residual current.

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