Queries Regarding Concepts of Earthing

Code requirements (in the US, anyway) mandate the neutral and ground points in distribution transformers. In figure 2 your fault path is short of the tranformer core reducing the effectiveness of your overcurrent devices.

The various types of earthing arrangements are there for the convenience of the user depending on the effects on their application.

This wiki article explains it well: http://en.wikipedia.org/wiki/Earthing_system

In a *typical plant* there will likely be several layers of distribution transformers at which point(s) the earthing scheme(s) may vary dependent on the loads serviced.

Earth/ground for a particular device is directly related the power which services it. Electricity takes the path of least resistance. Parallel paths which result from a fault can and do happen, and stray voltage can be found in seemingly non-associated equipment as a result.

Don't think of energy/power/voltage as a bucket dumping like water. If stray energy finds it way to ground it wants to find it's source. It doesn't go full strength to every available ground in the system.

All of this is relies heavily on having the equipment connected correctly from the get-go.

have you seen these . . .

http://www2.schneider-electric.com/documents/technical-publications/en/shared/electrical-engineering/dependability-availability-safety/low-voltage-minus-1kv/ect172.pdf

http://www.designers.schneider-electric.ru/Attachments/ed/ct/earthing_systems_evolution_worldwide.pdf

http://www2.schneider-electric.com/documents/designers/top-downloads/090197c6800bcbde_.pdf

Or http://www.eon-uk.com/distribution/CiCdocs/01%20Technical%20Documents/CN%20Combined/Earthing%20Manual/E2%20Earthing%20Manual%20Earth%20Guidance%20Notes.pdf

Regarding question 3. The reason the current flows through point A, is that it is trying to get back to the source of the current. Electrical current only travels in complete paths. The source of the voltage trying to push the current is T1. Current starting there can only flow through paths that connect back to T1. Tying one side of every transformer to the earth is not relevant, if the current starts in T1 it has to end in T1.

Your diagram doesn't show point B being tied to the common neutral. If it is, then an alternate path for the current would be: through the earth to point B, through the common neutral to T1 bypassing point A. However that current will still flow through CT1800/SA giving you the over current protection. This alternate path may,or not, be a lower resistance than the path through point A. With parallel paths, current will flow through all available paths. The amount of current in each path, will be determined by the relative resistances. Think of this example, if you have two lamps connected to one outlet, one of them with a 100W lamp, the other one a 10W lamp. The total current from the outlet will be the sum of the current through the two lamps, however the current through the 100W lamp will be 10 times as much as the current through the 10W lamp.

Actually point B is tied to common neutral - i forgot to draw! and i understand your alternate path theory. But what if its not connected to common neutral as shown. Would fault current take that path then?

Thanks all I'll read those pdf's now.

If there is no connection between point B and the common neutral, then there is no path for the current to flow through.