grounding of transformers

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
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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 fireworks.
Charles
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On 11/11/06 4:49 AM, in article snipped-for-privacy@i42g2000cwa.googlegroups.com, "sanju"

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.
Bill -- Fermez le Bush
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| 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).
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| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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