Earthing configurations

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Earthing configurations
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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.

3
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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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 (or provided by the
  utility for small installations).
• 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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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.
• If both earthing systems are present (consumer
  side and supply side), they may or may not be
  connected.

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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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Without an earthed star point IT (Isolée Terre)

• IT systems tend to be used in very specific
  circumstances
• IT systems are governed by specific regulations

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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.
  operating theatres, 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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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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The TN-C configuration (Terre-Neutre-Commune)
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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 protected earth cable
  could be provided that is the TN-S system. This
  system is superior in performance but more
  expensive.

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The TN-S configuration (Terre-Neutre-Séparée)
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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
PCC
N
PEN
PE
PE
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Another option the TT-configuration (Terre-Terre)
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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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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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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
  www.leonardo-energy.org/drupal/apguide