How Do the Voltage Between Neutral and Ground Decrease When the Value is 1.5v?

what is v/g??...copper strips in earth pits?...No rods? is it mesh earthing?...5-18ohms is high value. should be less than 1 ohm.

Do a soil resistivity test and calculate the required earthing scheme. no of rods, depth etc.

Finally, why do you worry about voltage between neutral and earth?

i am worried because we have to start our plant on DCS system

and DCS system contain very senstive equipments which can not bear that v/g. due to this voltage our two no. of data cards are burned.

due to this v/g panels of MCC and DCS get charged and v/g rises to 152 V.

Then there is a grounding problem. see #6 below.

The earth conductor is there to operate the circuit protective devices in the event of a fault. It has no other purpose.

The voltage between earth and neutral indicates an imbalance current in the neutral, which is what it is there for.

So any attempt to control the voltage between neutral and earth is futile, fatuous and abstruse.

Sandeep,

That isn't bad, but if there is instrumentation involved there could be errors.

One time I was faced with a problem when there was only a few hundred millivolts involved and had to make a separate ground to get around lots of ground problems. I used a number of long driven copper rods with chemical "assist". There were five very sensitive systems all being controlled and monitoted by a large computer system.

If you are measuring the resistance in a "simple" manner you will not be getting a really meaningful number. This measurement requires the use of multiple test rods and a special bridge.

I have measured the "apparent" ground resistance of a 20 foot 5/8" copper driven ground and was surprised to find around 20 ohms. I used a variac with a low voltage isolation transformer and ammeter and voltmeter. I used the power company's ground as the reference.

I never checked the resistance the right way to see the difference.

If you make a small pit around the top of the rod and load it with alum and then add water and more alum (often and lots) you will reduce the resistance somewhat. Your ground may be better than the power company's. Mine was!

geo3

It is not possible to "control the neutral voltage"; there isn't a problem here that requires a solution!

It is normal practice to tie the -ve of the extra-low-voltage side of the instrumentation power supply to an instrument ground/earth within the panel, and run a conductor from here to the earth pit separately from the power ground/earth conductor. Many operations call this "clean instrument earth".

Make sure that under no-fault conditions, there is no current in the earth conductors!

End of.

Is the installation spread over several building each with it's own earth pits?

If it is, a neutral conductor connected between each building will show varying neutral/earth voltages at each building. Bonding between each building would be the only solution if this were the case.