The main objectives of the grounding are to:
(1) Provide an alternative path for the fault current to flow so that it will not endanger the user
(2) Ensure that all exposed conductive parts do not reach a dangerous potential
(3) Maintain the voltage at any part of an electrical system at a known value so as to prevent over current or excessive voltage on the appliances or equipment.
The qualities of a good grounding system are:
(1) Must be of low electrical resistance
(2) Must be of good corrosion resistance
(3) Must be able to dissipate high fault current repeatedly
System Grounding:
In electricity supply systems, an grounding system defines the electrical potential of the conductors relative to that of the Earth's conductive surface. The choice of grounding system has implications for the safety and electromagnetic compatibility of the power supply. Note that regulations for grounding (grounding) systems vary considerably between different countries. A protective earth (PE) connection ensures that all exposed conductive surfaces are at the same electrical potential as the surface of the Earth, to avoid the risk of electrical shock if a person touches a device in which an insulation fault has occurred. It also ensures that in the case of an insulation fault, a high fault current flows, which will trigger an overcurrent protection device (fuse, MCB) that disconnects the power supply. A functional earth connection serves a purpose other than providing protection against electrical shock. In contrast to a protective earth connection, a functional earth connection may carry a current during the normal operation of a device. Functional earth connections may be required by devices such as surge suppression and electromagnetic-compatibility filters, some types of antennas and various measurement instruments. Generally the protective earth is also used as a functional earth, though this requires care in some situations.
IEC nomenclature:
International standard IEC 60364 distinguishes three families of grounding 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):
Table: IEC nomenclature
T
|
direct
connection of a point with earth (French: terre);
|
I
|
No point is
connected with earth (isolation), except perhaps via high impedance.
|
The second letter indicates the connection between
earth and the electrical device being supplied:
Table: IEC nomenclature
T
|
direct
connection with earth, independent of any other earth connection in the
supply system;
|
N
|
Connection to
earth via the supply network.
|
TN network:
In a TN grounding 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.
TN system
Table: Three variants of TN systems
TN‑S
|
PE and N are separate
conductors that are connected
Together only near the power
source.
|
TN‑C
|
A combined PEN conductor
fulfils the functions of
both a PE and an N conductor
|
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, whereas within the building separate PE and N conductors are used. In the UK, this
system is also known as protective
multiple grounding (PME), because of
the practice of connecting the
combined neutral-and-earth conductor to real earth at many locations,
to reduce the risk of broken neutrals - with a similar system in Australia
being signed as multiple earthed neutral (MEN).
|
TN-S:
Separate protective earth (PE) and neutral (N) conductors from transformer to consuming device, which are not connected together at any point after the building distribution point.
TN-S systemTN-C:
Combined PE and N conductor all the way from the transformer to the consuming device.
TN-C system
TN-C-S grounding system:
Combined PEN conductor from transformer to building distribution point, but separate PE and N conductors in fixed indoor wiring and Flexible power cords.
TN-C-S system
It is possible to have both TN-S and TN-C-S
supplies from the same transformer. For example, the sheaths on some
underground cables corrode and stop providing good earth connections, and so
homes where "bad earths" are found get converted to TN-C-S.
TT network:
TT system
IT network:
In an IT network, the distribution system has no connection to earth at all, or it has only a high impedance connection. In such systems, an insulation monitoring device used to monitor the impedance.
IT system
Example of The Various Earthing Systems Included In The Same Installation :
Various
Earthing systems
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