05-05-2014, 12:06 PM
Earthing system
Earthing system .ppt (Size: 299 KB / Downloads: 255)
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.
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
Properties
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 RCD protection and often an expensive time delay type is needed to provide discrimination with an RCD downstream
Application examples
Most modern homes in Europe have a TN-C-S earthing system. The combined neutral and earth occurs between the nearest transformer substation and the service cut out (the fuse before the meter). After this separate earth and neutral cores are used in all the internal wiring.
Older urban and suburban homes in the UK tend to have TN-S supplies with the earth delivered through the lead sheath of the underground lead and paper cable.
Some older homes, especially those built before the invention of residual-current circuit breakers and wired home area networks, use an in-house TN-C arrangement. This is no longer recommended practice