The art of high resistance grounding
What is high resistance grounding?
HRG stands for high resistance grounding – an electrical supply system that is used frequently in applications that cannot afford a shutdown, or that must control ground-fault voltage on driven equipment.
When a ground fault occurs, ground-fault current will flow as in a solidly grounded system, but its magnitude is severely restricted (to a few Amperes, typically 10 or less) by a neutral-grounding resistor. This limited current has several advantages—it is sufficient to detect and locate ground faults; it will not cause escalating point-of-fault damage; it will not escalate to an arcing ground fault, and it limits touch potential (the voltage between equipment frame and earth) on portable or mobile loads to a safer level.
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. This requires a reliable insulation system not only line-to-ground but also line-to-line. Resistance grounding reduces the probability of a line-to-ground arc flash making systems safer, but it does not limit line-to-line arc-flash energy.
High resistance grounded systems cannot depend on overcurrent protective devices such as circuit breakers and fuses 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.
It is necessary to install a ground-fault detection system since the ground-fault current is not interrupted due to overcurrent. When properly designed, such systems can also quickly allow locating the faulted branch feeder, switchgear or load. Such systems however do rely on the integrity of the neutral grounding resistor.
Benefits of HRG systems
Resistance grounding is used for several purposes:
These are also stated in the IEEE Standard 142-2007: Recommended Practice for Grounding of Industrial and Commercial Power Systems (Greenbook).
HRG utilizes the best attributes of both solidly grounding and ungrounded systems while remaining cost efficient. Solidly grounded systems might be the most common form of power supply system, but in terms of hazards it is also a very susceptible one. The ungrounded system is more rarely used in some industries and countries, even though it has some advantages.
