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.