potential with respect to earth

Thanks all of you for your suggestions. From the discussion what I have understood is that the potential of any point to the earth depends upon the charge of the point, distance of the point from the ground (ground capacitance!!). Thus to evaluate the potential difference (p.d.) between the floating star point and the ground we must be able to evaluate the charge of the star point. DK described that the p.d. depends on the ground capacitance, overhead cloud. This is because it affects the charge of the star point, am I right DK? But I still don=92t know how to evaluate the charge? When the star point is connected to the earth, we are simply putting a conductor between two plates of the capacitor i.e. earth and the star point which makes the p.d between them zero and all the charge at star point will be drained to the earth. Pls give ur suggestions.

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-------------------------------- Not all potentials to earth are due to static or quasi-static charge sources. For such cases, you are right in considering a capacitive coupling. Such potentials will not be related to the generated voltage. No- you can't evaluate the charge because you don't know the conditions.

Also mentioned were more mundane things such as unknown and unbalanced leakage paths, unbalanced loads to ground, and the worst is a fault to ground somewhere in one of the windings. so that the star point to ground voltage could lie between 0 and phase voltage. In the latter case, connecting the star point to ground might do serious damage.

Since you have given no information, and apparently have done no tests, it is difficult to give you a meaningful answer for your situation.

For example, is the voltage there when the generator is not running? If not- forget the cloud charges. Is it there when the generator is unloaded? Can you measure resistance between the star point and ground (generator not running)? There are things that you can do to narrow it down if it is a real problem.

So, my suggestion is that you should get a local professional consultant to examine the situation rather than hope to get free long distance advice on-line.

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Don Kelly

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