Minute Lectures
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.
Minute Lectures
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.
Minute Lectures
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.
Minute Lectures
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)
Minute Lectures
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.
Minute Lectures
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