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?
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
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).
Don Kelly email@example.com
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