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Earthing configurations

Earthing or not?

Is that the question? It is often thought that an electrical appliance can only be earthed or not earthed. But the earthing connection comes from the socket. What about the other end of the wire?

We should be aware that several earthing configurations exist.

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Earthing configurations

Expert work

Not every configuration ensures safety and functionality for each environment, and some configurations are more cost-effective than others.

Choosing the right configuration is expert work.

It is recommended that the voltage and current distributions that would be effected in case of a fault be investigated before opting for a certain configuration.

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Earthing configurations

The local earth electrode

In most cases, exposed conductive parts of equipment (metallic frames and enclosures) are connected to a local earth electrode in the basement of the building.

This is always useful, if not for conducting an earth fault current, then for conducting a lightning strike.

But this will neither exclude every risk of electrical shock nor guarantee flawless operation of electronic devices.

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Earthing configurations

Different earthing configurations

The power for the equipment is supplied by a transformer, which is situated on the site itself or is part of the distribution network. The star point of this supply transformer may be earthed to provide extra safety (but it does not have to be).

The same goes for the casing or housing on the consumer side.

If both earthing systems are present (consumer side and supply side), they may or may not be connected.

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Earthing configurations

The abbreviations for earthing configurations

The first character describes the supply side.

The second character describes the consumer side.

The third character describes the way of connection, if any.

The letters and their meanings are: • T – terre (earthed) • • • • I – isolée (not earthed) N – neutre (neutral conductor) C – commune (combined) S – séparée (separate)

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Earthing configurations

Without an earthed star point: IT (Isolée – Terre)

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Earthing configurations

The IT configuration

Major advantage: the system can continue to operate after the first earth fault. In most cases, this is prohibited, except when the continuity of the supply is critical (e.g. surgery rooms, chemical industry) This configuration needs to be combined with a monitoring device that can detect an earth fault. An earth fault should be repaired as soon as possible A Residual Current Device (RCD) is required (a Residual Current Monitoring System, or RCM might be better to prevent nuisance tripping)

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Earthing configurations

With an earthed star point

If the discipline to repair each earth fault quickly cannot be guaranteed, it is recommended that the star point of the transformer be earthed Earthing the star point is by far the most common practice When this is done, there are still several possible configurations left: TN-C, TN-S, TN-C-S, and TT

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Earthing configurations

The TN-C configuration (Terre-Neutre-Commune)

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Earthing configurations

TN-C or TN-S?

In a TN-C configuration, the extraneous parts are connected to the neutral wire. The main disadvantage is that the currents in the neutral could cause EMC problems (= interference with signals of electronic devices).

To counter this, a separate earthing cable could be provided: that is the TN-S system. This system is superior in performance but more expensive.

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Earthing configurations

The TN-S configuration (Terre-Neutre Séparée)

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Earthing configurations

The TN-C-S configuration

A good compromise can be the TN-C-S system, in which there is a separate cable inside the building where EMC problems could occur, and only the neutral wire outside the building:

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Earthing configurations

Another option: the TT-configuration (Terre-Terre)

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Earthing configurations

The TT configuration

TT combines features of the IT and TN configurations. It has a better EMC than TN-C, it is cheaper than TN-S, and safer than IT.

The disadvantage is that it requires a Residual Current Device, and, in some cases, EMC problems might still occur.

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Earthing configurations

Round-up

Several earthing configurations exist.

Choosing the optimal one is a complex decision that should be made by experts.

In the event of a fault, the voltage and current distributions should be investigated.

Arguments that influence the choice: required security of supply, cost, assumed discipline of the users, EMC problems, RCD or RCM or fuses required.

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Earthing configurations

Links and references

• Standard IEC 60364-3, available from www.iec.ch

• A Systems Approach to Earthing • Earthing Systems – Fundamentals of Calculation & Design • Earthing Systems – Basic Constructional Aspects available from http://www.leonardo-energy.org/drupal/node/3937