i am working as a electrical engineer and i feel that my concept about
grounding is not very clear.is the voltage of grounded object always
zero?.what happens if a distribution transformer with delta on primary
and star secondary,neutral being grounded, one phase of secondary is
grouded at some point say 100m from transformer?plz guide me
Well, if you have the neutral grounded on the wye (star) side of the
transformer and then you ground one phase "at some point" you get a FAULT,
along with great quantities of current and perhaps some interesting
| i am working as a electrical engineer and i feel that my concept about
| grounding is not very clear.is the voltage of grounded object always
| zero?.what happens if a distribution transformer with delta on primary
| and star secondary,neutral being grounded, one phase of secondary is
| grouded at some point say 100m from transformer?plz guide me
That would apply your line-to-neutral voltage across the two ground points.
Assuming separate grounding electrodes, you would be applying full voltage
through the ground and would see current as a result, and wasting a lot of
Assuming 25 ohms between electrodes, and 230 volts line to neutral, that
would be an earth current of 9.2 amps wasting 2116 watts.
I'll speculate on why you might be asking this:
If you are referring to a fault grounding, and wondering if that would be
enough current to trip overcurrent protection breakers, the answer is not
very likely. This is what ground fault protection is for.
If the grounding at the transformer is impedance grounding, then the fault
current will be even less, based on what the grounding impedance is. This
is a hazard situation as it requires immediate attention to find and remove
the fault before another phase also faults. Such imepdance grounded systems
must have a ground fault alarm to get your attention to the issue fast.
This approach to wiring is rare, but sometimes needed in very critical uses
to ensure continued power flow under single fault conditions.
Whatever conductor is chosen to be grounded (and it usually is the star
point) that needs to always be the one to be grounded everywhere in that
separately derived system. If the star point is grounded at the transformer
that supplies the system, but is not carried out as a conductor and you
need to ground the system somewhere else for some reason, or need a new
neutral, then you have to add on a zig-zag transformer (or just derive a
whole new system with another delta-star transformer).
On 11/11/06 4:49 AM, in article
Working as an electrical engineer does not make you one. Are you licensed?
If you are not, you are opening yourself and your employer to liability if
anyone gets hurt. If you indeed are a registered PE, you are obligated by
law not to practice beyond your knowledge and capability.
-- Fermez le Bush