voltage difference between neutral and ground

Following can result in the voltage between neutral and earth:

1) In a Y connected system with neutral grounded, presence of triplen harmonics due to non-linear loads such as VFDs, electronic ballasts, SMPS etc. This can be eliminated by installing suitable harmonic suppressors or filters.

2) Unbalance loading of the three phases. This can be reduced or eliminated by redistributing the load equally among the three phases.

3) Phase coming in contact with neutral.

4) High neutral to ground impedance.

5) In neutral grounded generator, if the winding pitch factor is other than 2/3, this can lead to a voltage between the generator neutral and the earth. This voltage will increase with increase in generator loading. Triplen harmonics in a generator has to get eliminated in the design stage itself. Running the generator with isolated neutral can be practiced to avoid neutral to ground voltage but only after fully evaluating the system consequences.

I'm not sure as to what is the acceptable level of the voltage.

It also depends on where one is measuring this voltage difference. People forget that wires have resistance. So unbalanced currents that propagate down the neutral wire will manifest a voltage across this resistance. The ground lead should only have significant current during a fault condition. So little to no voltage exists across this wire.

It is, or once was, the standard in the UK to keep the neutral at, I think, negative 10 volts.

what is the connection between the question and your answer, I am surprised sometimes with answers like this on this site. I think people should stick to the threads and not divert like this.

Voltage between the earth and the neutral, what else?

I don't know where the OP lives or knows of the subject.

Nonsense. How does one maintain <...negative 10 volts...> in the context of an AC supply (rhetorical question - NNTR)?

If there is voltage between earth and neutral, what should be the reason ? and solution

Unbalanced loads leads to this. Neutral is nothing but center point of "star" at mains step-down transformer and it grounded close to the transformer. Hence if loads are not perfectly balanced, some voltage will appear between Neutral & earth. Such voltage will be very high in case of single phase SMPS power supplies. The solution is: keep single phase loads as minimum as possible compared to 3 phase loads.

* What should be allowable limit in case, if the voltage between earth and neutral exist ? / OR what is limit for safe voltage between earth and neutral?

I do not know about any limit. The earth fault relays (if installed at sub-station) should not trip unnecessary due to this voltage. Long back (286 based PC -XT released to market with DOS ver-4) one computer engineer told me that it should be less than 2volts and troubled me a lot.

Nonsense.

At the Electrical service enterance the Neutral is bonded to Ground; the farther you get away from that point the more the the chance of a voltage differential. Magnetic fields can be RF and get on the electrical system wiring or EMI from equipment in the eletrical system and can cause the voltage differential. To answer your question 1/2V is the maximum that is recommend. The solution is to get an Isolation transformer and rebond the Neutral to Ground. To take it one step farther you can get isolation transformer based power conditioners that have a filtering circuit after the transformer that removes virtually all noise and strike transients from the Line or Neutral to Ground. One of the better brand names on the market is "Oneac".

The voltage between earth and neutral at any location is merely a function of the neutral current at that place and the earth loop impedance at that place. The earth loop impedance has to be sufficiently low in order to operate the circuit protective device(s) in the event of a fault, and will be specified during the design of the installation in accordance with some applicable standard, for example, British Standard 7671, and tested by the Qualified Electrician for compliance before a handover test record document is issued in accordance with the applicable standard.

To reduce the voltage, simply lower either or both of the neutral current and the earth loop impedance.

<...safe voltage between earth and neutral...> - <...safe...> for what purpose?

• Usually one does not come into contact with a neutral conductor. So a safety issue for the provision of protection from electric shock does not usually arise.

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