Neutral to earth voltage?

Question

In one of the IT Clusters the Neutral to Earth voltage is as high as 18V in the 7th floor. But the neutral to earth voltage in all other floors are below 3 volts. The loads are balanced. Currents in all the phases are balanced. We laid new earth cable from earth pit then also the voltage didnt reduce. To confirm the problem we fed supply from 7 th floor to 5th floor there also the voltage was 18 volts. (Note: The neutral earth voltage during normal operation at 5th floor is below 3 volts only). How to reduce the neutral to earth voltage.

Answer 1

looks like a bad connection in the neutral.

Answer 2

just check any of the equipment at seventh floor may have connection with phase & earth in stead of   phase & neutral

Answer 3

You know that the neutral line will be connected to earth at your main substation distribution.

The p.d. between neutral and earth at the far end can only come about because of a voltage drop along the neutral line, which is carrying current.  

The earth line should not normally carry current and will therefore not have a voltage drop along it.

Why is there such a voltage drop along your neutral line?  Is it a bad connection at one particular point, or is the current flowing through it bigger than it was designed for?

Either way, if you want to reduce the neutral to earth voltage, it is the neutral line that needs fixing.

Answer 4

Have you checked the neutral current? I know you said all three phases are balanced and that means the neutral current should be nearly zero, but switched power supplies (common to IT equipment) can produce large amounts of third order (triplen) harmonics.

In a 3 phase wye system triplen harmonics don't cancel in the neutral. If the neutral is only sized for the unbalanced load, as it frequently is, then the neutral could be overloaded and causing the large neutral to ground voltage you're seeing.

The solution frequently is to install a "super neutral" that is sized up to 173% of the phase conductor size or install a parallel run on the neutral.