- High Resistance Grounded System (HRG)
Electrical safety for high-resistance grounded power systems
What is high resistance grounding?
The neutral point of an HRG system is connected to ground through a current-limiting resistor. This type of electrical supply system is used frequently in industrial facilities so they can continue to operate with a ground fault, or to control ground-fault voltage on driven equipment.
When a ground fault occurs, ground-fault current will flow, similar to a solidly grounded system but with a magnitude severely restricted (typically 10 A or less) by the neutral-grounding resistor (NGR). This has several advantages—there is sufficient current to detect and locate ground faults; it will not cause escalating point-of-fault damage; it will not escalate to an arcing ground fault, it limits touch potential (the voltage between equipment frame and earth) to a safer level, and it allows continued operation until the system can be shut down in a controlled manner.
It is important to note that the line-to-ground voltage of the unfaulted phases will increase during a ground fault, which increases the probability of a second ground fault—the faulted equipment should be repaired or replaced as soon as practicable. Resistance grounding reduces the probability of a line-to-ground arc flash, making systems safer, but it does not limit line-to-line current or arc-flash energy.
High-resistance grounded systems cannot depend on overcurrent protective devices, such as fuses and circuit breakers, even those equipped for ground-fault protection, to protect against ground faults. In fact, in many cases, ground faults are allowed to remain on the system until they can be repaired in an orderly and planned manner. To detect low-level ground-fault current, it is necessary to install a suitable ground-fault detection system. When properly designed, such systems can also quickly locate the faulted branch feeder, switchgear or load. Tripping systems can automatically disconnect the faulted circuit, allowing the rest of the system to continue to operate. Such systems, however, rely on the integrity of the neutral grounding resistor.