A Neutral Grounding Resistor (NGR) is a resistor connected in the grounding system to limit fault current. Beyond restricting fault current levels, it also ensures protection of healthy phases against overvoltage conditions, supports the effective operation of protective devices, and safeguards conductors from thermal disturbances caused by faults.
Therefore, the design and selection of an NGR is critical to ensure the safety of equipment and personnel, as well as the continuity of power supply.
Before discussing how to size an NGR, it is essential to understand the fundamental nature and necessity of NGRs.
As the name suggests, a Neutral Grounding Resistor is a resistor connected between the system neutral and earth to limit the current during a fault. Anyone with a basic understanding of physics knows that higher resistance leads to lower current in any circuit.
So, wouldn’t it be better to not connect an NGR at all? In other words, operate an ungrounded system, theoretically having infinite resistance (or practically, very high impedance) between neutral and ground?
While it’s true that by not connecting an NGR, the fault current in an ungrounded system is minimized, this introduces another issue—overvoltage.
To analyze this, consider an ungrounded system. Even though there’s no direct connection between the neutral and ground, as illustrated above, an ungrounded system is effectively connected to ground through its natural capacitance.
The insulation (of various electrical components like surge capacitors, cables, motors, etc.) acts as a dielectric between two voltage levels—the system voltage and ground reference.
As a result, every system inherently possesses some natural capacitance due to its components. Thus, even an ungrounded system is capacitively coupled to ground.
In a healthy and balanced system, all three phase voltages are equal and separated by 120 degrees, and the neutral voltage is zero. Since the leakage currents are capacitive in nature, they lead their respective voltages by 90 degrees. These are known as capacitive charging currents.
When a ground fault occurs on phase A, the insulation is bypassed, and the current now flows through the ground path and equals the vector sum of currents and . These capacitive currents, still leading their respective voltages by 90 degrees, now increase in magnitude—۱.۷۳۲ times their nominal value—due to a rise in terminal voltage.
This behavior can be observed in the equivalent circuit, phasor diagram, and current flow chart, where the faulty system’s phasors are overlaid on the healthy system’s faded phasors for comparative analysis.
Unless the ground fault is cleared at the point of zero voltage, a certain DC offset voltage remains on the neutral. Since there is no discharge path, this offset voltage persists on the neutral conductor.