Low Resistance Grounding
Low resistance grounding (LRG) is a system-grounding method where the neutral of a transformer or generator is intentionally connected to earth through a low-ohmic resistor, allowing a relatively high ground-fault current—typically high enough to operate protective relays and trip the faulted circuit quickly. It is commonly used on medium- and high-voltage systems to reduce damage from the first ground fault while enabling selective fault clearing.
On a shop floor or in a manufacturing electrical room, LRG is applied at the source neutral of a generator or transformer through a neutral grounding resistor (NGR). When a single line-to-ground fault occurs, the resistor limits the fault current to a controlled value, enabling protective relays and current transformers to detect the fault and disconnect the affected equipment. This approach is used where the plant wants to limit equipment damage and maintain relay coordination, rather than allow the very high fault currents of a solidly grounded system. In Canadian practice, LRG is a form of system grounding at the source neutral, distinct from general equipment grounding or bonding under the Canadian Electrical Code, and is typically associated with systems where ground-fault current exceeds 10 A or voltage exceeds 5 kV.
Why use LRG instead of solid grounding?
To reduce the magnitude of ground-fault current, limit arc-flash and equipment damage, and still keep the current high enough for relays to trip quickly.
What happens after the first ground fault in an LRG system?
The system is generally expected to be taken offline so the fault can be located and corrected; it is not typically a 'keep running indefinitely' scheme.
Where is LRG most common?
On medium-voltage and high-voltage industrial systems, often at transformer or generator neutrals in plants with substantial capital equipment.