Restricting Fault current - Neutral Grounding Transformer


Thread Starter


I know that my query is a unprofessional one. But still i want to learn from you guys.

Can anyone tell me <b> "How Neutral Grounding Transformer / NGR prevents Damage of Generator by restricting fault Current?" </b>

As far as my understanding, NGR/NGT is connected to Neutral of Generator, but how by controlling the current in neutral terminal help us in restricting the fault current which flow inside the generator?

Grounding resistor in the neutral will limit the ground fault current in case of an stator ground fault. Typically the resistor is chosen to limit the maximum fault current to approx. 5 - 15% of nominal current. A Uo voltage protection measuring across the resistor will detect stator ground fault and disconnect the generator.

Good luck!

:) Jan


If there is a fault in the phase side huge amount of current will flow to the fault as their impedance is negligible. This has to return through neutral. If heavy current flows through winding that damage the winding of Generator. If you provide NGT/NGR which restrict the flow of current. You NGT NGT is a inductance which does not permit sudden change of current and I need not say about the NGR.

Hope this is enough.

Feel free for more

Yes Mr.Phil. I have searched Complete, everywhere ppl talked about choosing NGR & NGT & about their calculation. Nobody talked about how exactly it works.
Mr. Jan

Thank you for Ur Response.

I have explained in my above post, the same whatever you have explained. Everyone in this forum explained about NGT/NGR limiting the fault current to some level.

But, My Question is NGT/NGR is connected at the Neutral terminal of the generator, but how come we are able to restrict the fault current flow inside the generator coils.

Kindly understand my question & Answer..,
SR... it is true that the distribution-type transformer in the NGT scheme adds an inductive component to neutral-current its effect is nil compared to the resistive component.

Regards, Phil Corso
Simplistically, ground fault current flows out of the phase terminal, into ground, and returns through the neutral terminal. A NGR in this circuit limits the current according to ohms law.
Thank you Mr.SR.

Suppose, if we remove the neutral (Ungrounded System instead of Impedance grounded System) from the generator, so then there is possibility of return of fault current to generator through neutral (because we have discarded neutral connection)

The <b> Concept (inductance of transformer prevents backward flow of fault current thru' neutral to generator) </b> that you are saying for restricting fault current is still not clear by me.

Can you pls me mail me a schematic drawing showing how current flows back to gen thru' neutral & how transformer's inductance prevents it.. mail i.d (mylgcookie [at]
Yes Phil.

During normal operation the effect is nil. During fault time it does not allow sudden change of current. By that time it reaches the relay set value it will pick up trip

RAJ and SR... I urge you to search the Control List Archives for both NGR and NGT topics. It is not adequate to just "size" the resistor to limit earth-fault current to say, 5A, 100A, or whatever value one thinks is appropriate!

Even though an earth-fault may not be present the neutral still carries the system phase-to-earth capacitive current. Its magnitude is directly proportional to the size of the installed electrical plant!

The total capacitance is one of the two major reasons that the earthing-resistor is chosen carefully for either an NGR or NGT application. If improperly sized the electrical plant may be susceptible to severe over-voltages!

The other reason is to limit damage to the iron structure of major capital assets without jeopardizing protection.

Regards, Phil Corso