I ask this question purely out of interest. When I look at wiring diagrams for houses, hotels etc., I can't help feeling that the relationship between the Earth and Neutral wiring surely means that these two must be connected together. Now, since they are certainly not connected together at the consumer premises, are they therefore connected together at the generating station or step down transformer?
In the main they are connected at the generator or transformer. Earth systems are defined as TT, TN-S, TN-C-S and IT the latter the earth and neutral are not connected but in the main the IT system is not used in UK. Only place I have ever worked on it is a tunnel boring machine in Hong Kong.
The TT system has the earth at transformer and house connected through the ground and the ohms between the two is often very high in the order of 200 ohms and is enough to trip an RCD but not blow a fuse.
The TN system has a wired connection between transformer and house. With TN-S this is separate wire and there is only an earth at the transformer. With the TN-C-S over part of the run the earth and neutral are combined and it uses multiple earth rods it is also called PME. It is hard to tell if a house has TN-S or TN-C-S some times you can see at the head which type but not always. As a result the supplier (DNO) has to tell you what earth system is in use. Because if your neighbour used TN-C-S and you used TT and you touched both metal soil pipes outside the DNO normally has to tell you what system you are to use. However if there is a big gap between houses then really not a problem. Some premises are not allowed to use TN-C-S for example supplies to caravans, boats and petrol stations. And in the main the IT system in banned. Exception is output from shaver socket for example.
Should there be a fault which connects the earth to line the current that flows should open a protective device so when the supply is installed a reading of the earth loop impedance (ELI) is made to ensure this will be the case. This includes adding an extra socket for example. And the readings are recorded on a minor works certificate or the installation certificate and then are re-checked every 10 years or with change of occupant which ever sooner when a periodical inspection report is done. There is a change in name for that can't remember the new name.
Very many thanks ericmark for your kind reply. It does indeed answer my question very comprehensively. I don't pretend to understand fully all parts of your answer, but I've grasped enough to understand why it is essential that most systems have the earth and neutral connected. Again, many thanks.[/u][/b]