Earthing Clasifications and Advantages

Most domestic installations in the UK for instance have TN-S system at present, but the TN-C-S also known as PME-Protective Multiple Earthing, is increasingly becoming more popular in new domestic installations because of its lower cost and superior earth contact resistance. For example the maximum Ze specified by a distributor is 0.35ohms for TN-C-S supplies compared to 0.8ohms for TN-S supplies.

However, under certain fault conditions (PEN conductors of the supplies becoming open circuit external to the installation) a potential can develop between the conductive parts connected to the PME earth terminal and the general mass of earth. However since there are multiple earthing points on the supply network and bonding is provided within the building complying with BS7671, the risk ia considered to be small.

For special locations such as metalwork in a caravan or boat, PME should not be used. Additional precautions have to be taken when installing PME in a petrol filling stations, mines and quarries.

In TT systems, the supplier does not provide an earth connection. The consumer provides their own connection to earth ie by installing a suitable earth electrode local to the installation. The circumstances in which a distributor will not provide a means of earthing for the consumer are usually in areas or countries where the distributor cannot guarantee the earth connection back to the source, e.g. a low voltage overhead supply, where there is the likelihood of the earth conductor either becoming somehow disconnected or even stolen.

TT earthing presents significant challenges to the electrical installer, as its earth resistance is unconcomfortably high and may vary with the weather. When it is wet, the contact with earth will be better than when it is dry.

TN-C and IT systems are both very uncommon in the UK. This is because TN-C requires an exemption from the Electrical System, Quality and Continuity Regulations, and an IT system is not permitted for a low voltage public supply in the UK becuase the source is not directly earthed.

Hope this helps.

TT is a common method of distribution in rural districts, as the expense of providing a third overhead wire all the way from the distribution transformer can be avoided by providing a separate earth electrode locally to the point of use. There are also safety implications in avoiding an overhead earth return wire: in the event of the earth wire becoming broken by, say, a fallen tree branch the installation at the point of use would be at risk. Avoiding the overhead earth return means the point of use remains safe.

In the UK, most TT installations will require a 500mA RCD installed at the house inlet terminals to compensate for the possibility of the earth return path being poor. In the event of an earth fault, it is more likely that the RCD will trip rather than the 6, 16, 20 or 32A breaker provided on the individual distribution circuits, thereby affording a greater level of protection than were one not to have been installed.

http://en.wikipedia.org/wiki/Earthing_system makes an interesting spot for further reading on this subject.