why neutral earthing is required for DG.

Diesel generator, or for that purpose any device, generates a voltage difference between two terminals (for simplicity i am assuming single phase and not talking about EMF)

To have a proper working environment, that include equipment and personnel safety, one of the voltage has to be tied to a value (V_ref) and the other is now (V_ref+V_gen).

Obviously the best value of v_ref is zero in normal circumstances ie the earth voltage. To make two components (Earth and Neutral point of generator) you must electrically connect them - either directly or through a resistor/ impedance (NGR) depending on the system requirement and configuration.

Second question is not clear however for any system the neutral is forom line and never from earth pit.

As mentioned a few have the neutral connected through NGR etc to limit fault currents.

In case the earth pit is used as neutral, the ELCB can not be used and in case a earth fault occurs, the isolation will not be possible.

In case the earth pit is used as neutral, the ELCB can not be used............Can you elaborate more on why it is so..??

As always... things are not quite that simple. Many factors will determine how and where a generator neutral is tied to ground. This is a complicated subject and could introduce a life safety hazard if not addressed properly. Please contact someone who can analyze your system and advise you properly.

Neutral earthing forces one terminal of the output of the generator to ground. This leaves only the Hot wire as having a lethal potential. This also allows chassis ground or equipment ground to force a breaker trip if Hot accidentally contacts grounded metal on the equipment. It also reduces the possibility of the equipment chassis rising to the Hot potential and becoming dangerous to anyone who may touch it.

I'll agree, unless it's a generator and a single piece of equipment, grounding and neutral bonding can become complex. Better bring someone in who can review your whole system and make an educated assessment.

Simplifying:

1- The Neutral is Grounded so that there will be a return path for any fault involving a phase line and the gounded body of the equipment supplied with electric power. Protection circuitry are usually made to detect such fault, using the current leaking back to the DG alternator, via the earth and back to the Neutral point.

2- From DG terminal box Only.

The earth pit is to ground the neutral of the DG at the panel only, for the reasons elaborated in 1- above.

Note: You are not very aware of the topic. Therefore consult before executing any electrical installation or at least let it be checked by a professional!!

Understanding the below will also good for you.

1.Do you know why neutral is earthed in a distribution transformer?

2. Have you seen any Generator without a neutral terminal unless it is built for speciall purpose?...

3. Have you studied anywhere that neutral is taking from the earth pit?..please send some documents if any.

Refer to the article on earthing systems in Wikipedia.

I suggest that you attend to our two days' duration training programme on Earthing; pl. mail me thro this forum's mail box for further details.

Please follow Indian Standard 3043. If in doubt, consult a qualified local Electrician.