Are you talking about just the resistor (ie you know the resistance value you need, or do you want to calculate the value of resistor needed?
If you know ohmic the value, you need to size it for the current and the maximum time (under worst case back-up trip times plus a bit - maybe even double it).
If you want an ohmic value, that all depends on what value you want to limit the current to. You need to think about:
1] do you want effective or resistive earthing (IEEE green book is a good reference)
2] Step and touch potential limits
3] Legislative requirements (particular in hazardous / flammable sites)
4]Communications circuits interference
5]Potential cross-connection or paralleling with other systems
6]Relay and CT sensitivities, trip times, etc
Probably people can add to this list, but theses would be the main points to consider.
These comments are centered around US-ANSI practices, which—based on listed voltages—may not apply. Low-voltage, high-resistance grounding dictates a continuously-rated wye-point-to-ground resistor sized for 1-5A fault current per MVA of transformer capacity.
Various aspects of LV High-R grounding have been extensively discussed in Thread238-3370 Thread238-6870 Thread237-7403 Thread238-9005 Thread238-10998 thread238-16620
vaishali
actually lot of factors which decide the value of NGR. one of the most important factor is at what value you want to limit your earth fault current. More the value of NGR, less sensitive will be the earth fault protection. It also depends on country. If you are in india, then it is normal practice to keep the star poin of a distibution transformer (415 V side) solidly grounded to make the earth fault protection more sensitive.
