25-01-2013, 04:45 PM
Earthing system
Earthing system.ppt (Size: 299 KB / Downloads: 264)
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.
IEC nomenclature
The first letter indicates the connection between earth 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
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 three-phase system, or that carries the return current in a single-phase system is called neutral N
. Three variants of TN systems are distinguished:
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 to real earth at many locations to reduce the risk of broken neutrals.)
Safety
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