The art of high resistance grounding
What is high resistance grounding?
High resistance grounding (HRG) is when the neutral point of an electrical system is connected to ground through a current limiting resistor, detecting ground faults when they occur. In many applications, this type of electrical supply system can continue to operate with a ground fault, and controls ground-fault voltage on driven equipment, preventing hazards. HRG provides the best attributes of both solidly grounded and ungrounded three-phase power systems while remaining cost efficient.
Overcurrent protective devices, such as fuses and circuit breakers, even those equipped for ground-fault protection, cannot protect against ground faults in an HRG system. A suitable ground-fault detection system will detect ground-fault current in the low-ampere or milliampere range. When properly designed, such a system will also quickly locate the faulted branch feeder, switchgear or load. Tripping systems (including second-ground-fault protection systems) can automatically disconnect the faulted circuit, allowing the rest of the system to continue to operate.
Ground-fault current is limited
When a ground fault occurs, ground-fault current continues to flow, similar to a solidly grounded system but typically restricted to 10 A or less by the neutral-grounding resistor (NGR). This has several advantages—there is sufficient current to detect and locate ground faults; escalating point-of-fault damage is prevented; arcing ground faults cannot occur; touch potential (the voltage between equipment frame and earth) is limited to a safer level; continued operation until the system can be shut down in a controlled manner is allowed; and transient overvoltages cannot occur.
Can a faulted HRG system run indefinitely?
As with ungrounded systems, during a ground fault the line-to-ground voltage of the unfaulted phases increases (from line-to-neutral to line-to-line voltage), which increases the probability of a second ground fault because of increased stress on insulation. The faulted equipment should be repaired or replaced as soon as practicable.
While resistance grounding reduces the probability of a line-to-ground arc flash, making systems safer, line-to-line current and line-to-line arc-flash energy are not affected.
The NGR is a vital component
Resistance-grounded systems rely on the integrity of the NGR, which should be continuously monitored. NGR failure in open mode changes the system to an ungrounded condition, defeats current-sensing ground-fault detection and allows the possibility of a transient overvoltage; in short mode the system is solidly grounded with consequentially high prospective ground-fault current and increased arc-flash hazard. NGRs should be continuously monitored to detect these conditions and also detect ground faults (including during an NGR-open failure mode). Bender NGRM500 and NGRM700 Neutral-Grounding-Resistor Monitors provide all three required protective functions defined by the 2021 CE Code Section 10—a ground fault in the current-carrying conductors, a shorted NGR, and an open NGR.
HRG system advantages
Resistance grounding has several advantages over solid grounding:
These advantages are noted in IEEE Std 3003.1: Recommended Practice for System Grounding of Industrial and Commercial Power Systems, and its predecessor publication 142-2007, the Green book.
Solidly grounded systems are common, but they are susceptible to arc-flash hazards and uncontrolled shutdown due to ground faults. Low-resistance grounding is sometimes used for medium-voltage systems. Ungrounded systems are used in some industries and countries, and have some advantages including the ability to operate with a ground fault. Bender has monitoring solutions for each of these power system types.