Neutral Grounding Resistor
A neutral grounding resistor (NGR) is a resistor connected between the neutral of a transformer or generator and ground to limit ground-fault current and control voltage-to-ground during an earth fault. In Canada, CSA C22.2 No. 295 covers high-impedance grounding assemblies for controlling ground-fault current or voltage to ground per the Canadian Electrical Code.
On a shop floor, an NGR is used in resistance-grounded medium-voltage or low-voltage distribution systems to prevent a single line-to-ground fault from escalating into severe arc-flash energy or major equipment damage while allowing protective relays to detect and clear the fault. It is installed in the neutral-to-ground path of a transformer or generator, sized so fault current is high enough for relay operation but low enough to reduce thermal and mechanical stress on cables, switchgear, motors, and transformers. The NGR must be rated for line-to-neutral voltage and intended fault current duty, with guarded live parts, qualified-person access, heat dissipation, and warning signs per Canadian Electrical Code rules.
Is an NGR the same as a grounding resistor?
In most industrial electrical contexts, yes; 'neutral grounding resistor' is the common term for a resistor in the neutral-to-ground path of a grounded AC system.
Why not use solid grounding instead?
Solid grounding allows very high ground-fault current, increasing arc-flash, equipment damage, and blast hazard; resistance grounding via an NGR limits that current to a controlled value.
What should the NGR current be relative to system charging current?
It should be sized so the resistor current is greater than the system's capacitive charging current; industry guidance emphasizes this as a key sizing requirement.