High Resistance Grounding
High Resistance Grounding (HRG) is a neutral-grounding method where the neutral point of a three-phase system is connected to earth through a current-limiting resistor, sized to keep single line-to-ground fault current typically ≤10 A at ≤5 kV. The resistor is selected so that its current exceeds the system's capacitive charging current, controlling transient overvoltages and allowing continued operation during a first ground fault.
On the shop floor, HRG is applied on 480 V/600 V three-phase systems in industrial plants like paper mills, chemical plants, and data centers to ensure process continuity. It includes a neutral grounding resistor (NGR) sized for ≤10 A fault current, often with an artificial neutral via a zig-zag transformer. Ground-fault detection relays and alarms are mandatory, allowing the system to remain energized on a first ground fault while maintenance locates and clears it. This reduces arc-flash hazards for phase-to-ground faults and minimizes unplanned outages, but requires qualified workers and strict lockout/tagout procedures per WorkSafeBC and CSA Z462.
How is 'high resistance' distinguished from 'low resistance' grounding?
In low-resistance grounding, the NGR allows hundreds of amps (50–400 A) for short periods to trip protective devices. In high-resistance grounding, the NGR allows only a few amps (typically ≤10 A at ≤5 kV; <25 A but >Xco/3), with Ro ≤ Xco, enabling the system to remain energized during a ground fault.
Why is Ro ≤ Xco so important in HRG system design?
The system has distributed capacitance to ground, creating capacitive charging current. During a ground fault, Ro ≤ Xco ensures resistor current exceeds capacitive current, allowing quick discharge and controlling transient overvoltages to phase-to-phase voltage. If Ro > Xco, the system behaves like an ungrounded system, risking dangerous overvoltages.
How does HRG reduce arc-flash hazard, and what are its limits?
For phase-to-ground faults, HRG limits fault current to ≤10 A, too low for hazardous arc-flash, as noted in CSA Z462 Annex O. However, HRG does not affect line-to-line or three-phase faults, so arc-flash risk assessments must still consider worst-case multiphase faults.