Earthing systems..

[ishannivanka]

hi all
*What do you think about the important to know the type of earthing system in doing an installation design?

* what are the advantages and disadvantages of TT,IT,TN,TN-s,TN-c,TN -C-S earthing systems?

 

[Ridma1986]

1)Main objectives of an earthing system are

Provide an alternative path for the fault current to flow so that it will not endanger the user

Ensure that all exposed conductive parts do not reach a dangerous potential

Maintain the voltage at any part of an electrical system at a known value and prevent over current or excessive voltage on the appliances or equipment

Different earthing systems are capable of carrying different amounts of over current. since the amount of over current produced in different types of installation differs from each other, required type of earthing will also differ according to the type of installation. so in order to ensure that the installation goes with the existing earthing system or else to do any modification accordingly, we need to have a proper idea of the present earthing system.It would enhance the safety as well as the reliability.

And by knowing about the earthing system we can also check whether it matches with the present regulations and standards.

2)Comparison of Earthing systems


Cost comparison

TT

requires proper residual current device to ensure protection which in turn would increase the cost

TN

saves the cost of a low impedance earth connection at the site of each consumer

IT

it also needs an additional protective device which increase the cost



Safety comparison

TT

A residual-current device can be used as an additional protection to enhance safety.

TN

An insulation fault may lead to a high short circuit current which may harm the consumer

IT

A single insulation fault is unlikely to cause dangerous currents but a second fault may turn the system to a TN system and cause more damage



hpe dis wl help u....

 

[ericmark]

TT (Means Ground to Ground from Latin) Does not rely on the suppliers earth and where it is important for the ground and all metal to be at same voltage as with petrol stations, and boats is often only method that can be used. But current through the earth rod is normally limited so often also requires a RCD as well.
IT (Means isolated ground) Not really used only time I have found this is on a tunnel boring machine in Hong Kong and I think mainly as it had a cheap step down transformer from the 660 volt supply. A real pain to fault find and really a non starter.
TN (This means Ground and Neutral) Is part of below.
TN-s Neutral and ground are separate except at origin. Very safe and very few restrictions
TN-c Neutral and ground use same cable many legal restrictions only the supplier is allowed to use this system.
TN-C-S Is a combination of TN-s and TN-c where the supply authority use TN-c but it is split to TN-s at customers premises. Also called PME and is very common. But can’t be used for supply to petrol stations, Boats, and Caravans and because the combined bit is in suppliers part sometimes had to work out difference between TN-s and TN-C-S in theory it should have multi-earthing points but in practice often no where near enough earth points.

Problem is the supplier makes the choose between systems not the user as where a number of premises are supplied from one transformer there is a danger of metal of different voltage connected to different premises unless they are all the same.

So except where law requires a TN-s or TT system as user you have very little say. The supply authorities do try to provide an earth but they do not have to and you may get two houses with different supply types next to each other where work is in progress and one house has been converted to TN-C-S and other is on waiting list.

This refers to UK. In other countries other conditions and rules exist and often this is because ground conditions do not always allow a good connection to be made for the whole of the year. Moisture is required and this is not always there.