There are definite advantages to incorporating high-resistance grounding in a great many applications, this being one of them.
In an ungrounded three-phase system, a phase-to-ground fault does not produce ground-fault current because of the lack of a return path to its source. As a result, conventional current-sensing ground-fault relays cannot be used on an ungrounded system. The remaining unfaulted phases will carry an increased voltage and the added stress on the insulation between phases could cause the insulation to fail - resulting in a phase-to-phase fault. Because these ground faults are difficult to locate, cannot be automatically cleared, and can cause dangerous transient overvoltages, they’re undesirable. The advantage to employing an ungrounded system is its ability to maintain service during a ground-fault.
Solidly grounded systems eliminate some of the problems caused by ungrounded systems, but introduce others. In the event of a ground fault, the magnitude of the ground-fault current is limited only by the system’s impedance. Arc flash incident energy can be very high, service during a ground fault likely cannot be maintained, and it is difficult to design ground-fault protection for a wide range of ground-fault current levels. The advantage to employing a solidly grounded system is to have a fixed phase-to-ground voltage during a ground fault.
When compared to a solidly grounded system, high-resistance grounding eliminates the risk of an arc flash on the first ground fault. Research indicates that 80 to 98 % of industrial faults begin as ground faults. High-resistance grounding limits ground-fault current to a level that is insufficient to sustain an arc or create plasma. Limiting ground-fault current also means a significant reduction in point-of-fault damage and phase-to-phase and three-phase arc-flash energy (despite the fact that this is not reflected by the NFPA 70E incident energy equations). When a phase-to-phase or three-phase arc flash occurs, ground becomes involved when the arc plasma contacts grounded surfaces (in a grounded enclosure for example). In a solidly grounded system the plasma contact with ground contributes to the flash energy by providing a low impedance path for current to flow to ground. In a high-resistance-grounded system there is not enough current to sustain an arc to ground, so there is little or no ground contribution to the arc energy.
Limiting ground-fault current to 10 A or less allows the system to continue to operate with a single ground fault. This differs from an ungrounded system in that the fault is immediately annunciated by current-sensing ground-fault relays. Faults are easy to locate, can be automatically cleared, and do not create transient overvoltages.
Following are links to two papers that further explain the benefits of high-resistance grounding.
