earthing

i am a student of electrical engineering. i am very confused about 'earthing' and i seek ur help and support to make my understanding
about this topic clear.plz reply some of my simple questions which r complex for me.is potential of earth always zero (even when a live conductor touch the earth)?why neutral of delta/star distribution transformer 11kv/400v is earthed where there is no earth fault relays or other protection scheme except MCCB?
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If a live wire touches earth the potential of the earth at that point is raised. There will be current through the earth at that point so that there is a voltage drop between the contact and what is called "true ground". A resistance is assumed, (measured or calculated) between the point of contact and true ground (Where d(voltage)/d(distance) is 0).
There are several reasons for grounding the neutral of a star delta transformer.
One is that triplen harmonic (3rd, 9th , 15th, etc) currents can flow in the delta (and hence in the star legs to ground as they are zero sequence) so that they don't introduce triplen voltage harmonics.
A second is that the line to earth potential on the star side is fixed in the case of unbalance or, in some cases, it prevents an unearthed system from floating up and down whenever a charged cloud passes by. Earth fault protection may be at the other end of the 11KV feeder.
If the LV side is star then the option that exists is the use of both 400V 3 phase and 230V single phase with neutral return- the latter is, for safety reasons, tied to earth. This also means that in the case of unbalanced loads, the voltage to neutral doesn't float about (getting 300 or 400V where you expect 230 V is not desirable).
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Don Kelly snipped-for-privacy@shawcross.ca
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Don Kelly wrote:

but why does line tester doesn't glow when tested on a phase which is earthed at some point due to cable or insulator leakage.but the tester glow on neutral ?
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Don Kelly wrote:

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