NGR Panel

http://www.google.co.uk/search?sourceid=navclient&aq=0&oq=neutral+grounding+&ie=UTF-8&rlz=1T4SKPT_enGB409GB409&q=neutral+grounding+resistor

Thank u but Not helpful.

Search Neutral Grounding resistor or NGR in this forum, there are lots of threads on the subject.

TonyS is correct. GA

(Here is one of the comments for a similar question in the past: http://cr4.globalspec.com/thread/56612/Neutral-Grounding-Resistor)

NGR protects the electrical system and equipment by limiting the ground fault currents. Because of the use of resistance between the neutral and the ground, the ground fault current is minimized for the single line to ground fault (most common type of fault) and hence there is less damage to the system as well as the faulty equipment.

There are two types of NGR: High Resistance Grounding (HRG) and Low Resistance Grounding (LRG)

High Resistance Grounding (HRG): It is used where service continuity is vital, such as process plant motors. With HRG, the neutral is grounded through a high resistance so that very small current flows to the ground if ground fault occurs. In the case of ground fault of one phase, the faulty phase goes to the ground potential but the system doesn't trip. This system must have a ground fault monitoring system. The use of line to neutral (single phase) is prohibited (NEC, 250.36(3)) in HRG system, however, phase to neutral is used with using the additional transformer having its neutral grounded. When ground fault occurs in HRG system, the monitoring systems gives alarm and the plant operators start the standby motor and stop the faulty one for the maintenance. This way, the process plant is not interrupted.

Low Resistance Grounding (LRG): This is used for limiting the ground fault current to minimize the impact of the fault current to the system. In this case, the system trips for the ground fault. In this system, the use of line to neutral (single phase) is prohibited here as well.

See this link for further help:

http://ecmweb.com/power_quality/electric_ground/?smte=wl

- MS

The formula for calculating the single-line-to-ground fault current is:

{(3 x Vph) / [Z1 + Z2 + Z0 + 3(Zn + ZGP)]}

where,

Z1 = the system positive sequence impedance

Z2 = the system negative sequence impedance

Z0 = the system zero sequence impedance

Zn = the neutral-to-ground impedance

ZGP= the impedance of the ground return path

In case of soild grounding & the ground return path being a conductor (as in the case of TN-C & TN-S Systems, which are in vogue in industrial power systems), Zn & ZGP are negligible. The fault current will be limited only by the system sequence impedances which are quite low and hence the fault current will be quite high.

If one wants to limit the fault current, one cannot control the system sequence impedances which are pre-fixed; then one can only change the neutral-to-ground impedance. The simplest way of doing it is by inserting a resistance in the ground-to-neutral path of the source. In such a case, the value of Zn will no more be negligible, but will be a few hundred Ohms. This will greatly reduce the fault current and thus protect the system/equipment.