Low resistance grounding systems limit the fault current when a phase of a power system unintentionally goes to ground. The majority of systems limit the fault current to 200-400 amps in the USA, based on the guidelines contained in IEEE-142. When a fault occurs, the neutral grounding resistor (NGR) limits the current that can flow from ground back to the transformer. This limits the fault current on grounds throughout the entire system. Although limited, the current is still high enough to require protective relays to trip a breaker and remove power from the faulted system.
