Why Neutral Grounding Resistor (NGR) is used in Neutral Earthing?

Updated: Feb 27, 2021

What is Neutral Grounding Resistor?

NGR Neutral Grounding Resistors is used in generating stations,power transformers and long line shunt reactors. The NGR is made of the material which has high temperature coefficient and its resistance remains fairly constant when large current flow through it.

Why NGR is used?

The earthing of the neutral point is done in order to ensure the tripping of the upstream breaker in case of the earth fault. This process of earthing of neutral point is called the system grounding. The system grounding can be done in three ways, solid grounding,resistance grounding and the reactance grounding. Each type of the grounding system has its own pros and cons.

When solid grounding is used the the enormous fault current flows through the equipment. But the current transformer senses the current quickly and trips the breaker to isolate the faulty section. The enormous current may cause damage to electrical system and even it can cause catastrophic damage to electrical system.

Here comes the role of neutral grounding resistor. The enormous current flowing in the case of solid grounding can be limited if impedance is created in the fault current flowing path. The resistor resist the flow of electrons or current. Therefore, the resistance is added in the neutral grounding circuit. The value of the resistance is selected in such a way that earth fault current flowing through the equipment is not more than full rated current of the equipment.

The current transformer is mounted in the neutral grounding circuit to detect the earth fault current in order to trip the circuit to isolate the faulty section.

NGR Circuit Diagram

NGR connections

Neutral grounding resistor is connected in between the neutral point of the electrical equipment and earth point.

Read More:

1. How to Calculate NGR resistance Value?

2. What is HRG( High Resistance Grounding)?

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