Answer / vinod

NGR capacity should allow the mininmum earth fault current
so that system with minimum earth fault current can be
detected and isolated by earthfault relay.
ie NGR fuction is to reduce the earth fault to the minimum
level in the range of earth fault relay

Answer / raghavendran.r

As far I know the formula is Rngr = Vln/Ig.

where Ig is the current flowing through the Neutral Grounding resistor(let through fault current), which depends upon the type of system you haver chosen, i.e High impedance grounding or Low resistance earthing.

The let through ground fault current through the neutral is low for High resistance earthing and its High for low resistance earthing.

But the important condition is that the let through fault current should be greater than the system capacitive charging current to avoid transient over voltage.

why do we provide metallic enclosure to bus duct and then ground it?

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