Neutral Grounding Resistors

http://www.bing.com/search?q=wikipedia+neutral+grounding+resistors&form=TSHMDF&pc=MATM&src=IE-SearchBox

They can be used in LV systems. It depends on what your ground fault detection / protection philosophy and scheme are, and how your system is configured.

"Neutral Grounding Resistors not used in LT system but required in MV systems" – this is the most common case but it not a true statement. NGR is also used in LV system especially in the process industries. In most cases of LV system, single phase (line to neutral) load is used and, in the system where line to neutral is used for single phase load, NGR is not permitted to limit the fault current. If the LV system has all three phase loads, NGR is be used for fault current limiting and service continuity.

For further help see:

http://cr4.globalspec.com/thread/56612/Neutral-Grounding-Resistor

http://cr4.globalspec.com/comment/537781/Re-Neutral-Ground-Resistor-NGR

- MS

Because, LV System is also handled by common public. And, hence, in case of an earth fault, quickest possible clearance - without the need for any special earth fault and/or earth leakage protective device - is mandatory. This is possible, only in case of Solid Grounding of the Neutral, wherein, during a line-to-earth fault (which is the most probable fault), the fault current would be equal to the three-phasy symmetrical short circuit current - and, thus, even a fuse or a circuit breaker would clear the fault in much quicker time.

Whereas, if an NGR is used, the fault current would be reduced and hence the operating time of any overcurrent protective device and/or any sgort circuit protective device would be unacceptably large, so as to harm the personnel.

If quicker isolation is required, it calls for a special ELCB sort of arrangement, which, in many cases, common public, would not install.