[In [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] systems, an [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] system defines the [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] of the conductors relative to that of the Earth's conductive surface. The choice of earthing system has implications for the [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] and [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] of the power supply. Note that regulations for earthing (grounding) systems vary considerably between different countries.[/font]
[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 ensures that in the case of an insulation fault (a "short cicuit"), a very high current flows, which will trigger an overcurrent protection device ([ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط], [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط]) that disconnects the power supply.[/font]
[A functional earth connection serves a purpose other than providing protection against [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط]. 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.[/]
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] distinguishes three families of earthing arrangements, using the two-letter codes TN, TT, and IT.
The first letter indicates the connection between [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] and the power-supply equipment (generator or transformer):
T : direct connection of a point with earth (French: terre);I : no point is connected with earth (isolation), except perhaps via a high impedance.The second letter indicates the connection between earth and the electrical device being supplied:
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 earthing system, one of the points in the [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] or [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] 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.
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط]
The conductor that connects the exposed metallic parts of the consumer is called protective earth (PE). The conductor that connects to the star point in a [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] system, or that carries the return current in a [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] system, is called neutral (N). Three variants of TN systems are distinguished:
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 earthing (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 designated as multiple earthed neutral (MEN).[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط]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-C: combined PE and N conductor all the way from the transformer to the consuming device.TN-C-S earthing system: combined PEN conductor from transformer to building distribution point, but separate PE and N conductors in fixed indoor wiring and flexible power cords.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
In a TT earthing system, the protective earth connection of the consumer is provided by a local connection to earth, independent of any earth connection at the generator.
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط]
IT network
In an IT network, the distribution system has no connection to earth at all, or it has only a high [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] connection. In such systems, an [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] is used to monitor the impedance.
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط]
[
- TN networks save the cost of a low-impedance earth connection at the site of each consumer. Such a connection (a buried metal structure) is required to provide protective earth in IT and TT systems.
- TN-C networks save the cost of an additional conductor needed for separate N and PE connections. However, to mitigate the risk of broken neutrals, special cable types and lots of connections to earth are needed.
- TT networks require [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] protection, and often an expensive time-delay type is needed to provide discrimination with an RCD downstream.
- In TN, an insulation fault is very likely to lead to a high short-circuit current that will trigger an overcurrent circuit-breaker or fuse and disconnect the L conductors. In the majority of TT systems, the earth fault loop impedance will be too high to do this, and so an RCD must be employed.
- In TN-S and TT systems (and in TN-C-S beyond the point of the split), a residual-current device can be used as an additional protection. In the absence of any insulation fault in the consumer device, the equation IL1+IL2+IL3+IN = 0 holds, and an RCD can disconnect the supply as soon as this sum reaches a threshold (typically 10-500 mA). An insulation fault between either L or N and PE will trigger an RCD with high probability.
- In IT and TN-C networks, residual current devices are far less likely to detect an insulation fault. In a TN-C system, they would also be very vulnerable to unwanted triggering from contact between earth conductors of circuits on different RCDs or with real ground, thus making their use impracticable. Also, RCDs usually isolate the neutral core, and it is dangerous to do this in a TN-C system.
- In single-ended single-phase systems where the Earth and neutral are combined (TN-C, and the part of TN-C-S systems which uses a combined neutral and earth core), if there is a contact problem in the PEN conductor, then all parts of the earthing system beyond the break will rise to the potential of the L conductor. In an unbalanced multi-phase system, the potential of the earthing system will move towards that of the most loaded live conductor. Therefore, TN-C connections must not go across plug/socket connections or flexible cables, where there is a higher probability of contact problems than with fixed wiring. There is also a risk if a cable is damaged, which can be mitigated by the use of [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] construction and/or multiple earth electrodes. Due to the (small) risks of the lost neutral, use of TN-C-S supplies is banned for caravans and boats in the UK, and it is often recommended to make outdoor wiring TT with a separate earth electrode.
- In IT systems, a single insulation fault is unlikely to cause dangerous currents to flow through a human body in contact with earth, because no low-impedance circuit exists for such a current to flow. However, a first insulation fault can effectively turn an IT system into a TN system, and then a second insulation fault can lead to dangerous body currents. Worse, in a multi-phase system, if one of the live conductors made contact with earth, it would cause the other phase cores to rise to the phase-phase voltage relative to earth rather than the phase-neutral voltage. IT systems also experience larger transient overvoltages than other systems.
- In TN-C and TN-C-S systems, any connection between the combined neutral-and-earth core and the body of the earth could end up carrying significant current under normal conditions, and could carry even more under a broken neutral situation. Therefore, main equipotential bonding conductors must be sized with this in mind; use of TN-C-S is inadvisable in situations such as petrol stations, where there is a combination of lots of buried metalwork and explosive gases.
- In TN-C and TN-C-S systems, any break in the combined neutral-and-earth core which didn't also affect the live conductor could theoretically result in exposed metalwork rising to near "live" potential!
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