neutral grounding resitor design for HT alternator

The NGR is designed to limit the current to a specific maximum allowable value, and is designed to handle the fault current (and generated heat in the resistive elements) for a certain time so the site's protection can operate. It isn't rocket science but it needs to be sized to match your existing protection scheme and fault current requirements, not the other way around.

If you have no idea where to start, have a look at the previous threads on CR4 regarding this subject.

What is the allowable limit, is it standard value in %. what are the design value of current in % of rated alternator current.I think alternator are design cable to feed earth fault current up its full load current under fault design. What is standard value in % of Earth fault current for any designed M/C

What is the allowable limit, is it standard value in %

No, it depends on a number of things including what fault current your alternator can handle, your existing protection scheme, your winding arrangement, local electrical standards, wiring cross-sectional area and insulation, etc. See post #1.

If this is not enough information for you to figure out what to do next then this is NOT something you should be designing by yourself. Please get a qualified electrical consultant, engineering firm, fellow engineer working at your company, original alternator manufacturer or any combination of the above to assist you. Failure to do so could result in destruction of property and/or death.

Thank.All general things I khow I can give lecture on this topic. But i hunting for practical results & core technical pesons answers.

Additionally try a google search for "designing sizing neutral earthing grounding resistor". This produced a number of links to useful general and specific design information that should be of help.