Ungrounded to HRG

The concern over the safety aspect of ungrounded systems when experiencing a ground fault is noted in IEEE 242-2001 Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems. Clause 8.2.5 Ungrounded Systems has a number of notes on ungrounded systems:

• Ungrounded low-voltage systems employ ground detectors to indicate a ground fault. These detectors show the existence of a ground on the system and identify the faulted phase, but do not locate the ground fault, which could be anywhere on the entire system. The system operates with the ground fault acting as the system ground point.
• If this ground fault is intermittent or allowed to continue, the system could be subjected to possible severe overvoltages to ground, which can be as high as six or eight times phase voltage. Such overvoltages can puncture insulation and result in additional ground faults.
• A second ground fault occurring before the first fault is cleared results in a phase-to-ground-to-phase fault, usually arcing, with current magnitude large enough to do damage, but sometimes too small to activate overcurrent devices in time to prevent or minimize damage.
• Ungrounded systems offer no advantage over high-resistance-grounded systems in terms of continuity of service and have the disadvantages of transient overvoltages, difficulty in locating the first ground fault, and burndowns from a second ground fault. For these reasons, they are being used less frequently today than high-resistance-grounded systems, and existing ungrounded systems are often converted to high-resistance-grounded systems by resistance-grounding the neutral if it exists or, if the system is fed from a delta source, creating a neutral point with a zigzag or other transformer and then resistance-grounding it.
• Once the system is high-resistance-grounded, overvoltages are reduced; and modern, highly sensitive ground-fault protective equipment can identify the faulted feeder on first fault and open one or both feeders on second fault before an arcing burndown does serious damage.

In an ungrounded system, it is possible for destructive transient overvoltages to occur throughout the system during restriking ground faults. These overvoltages, which can be several times normal in magnitude, result from a resonant condition being established between the inductive reactance of the system and the distributed capacitance to ground. Experience has proven that these overvoltages may cause failure of insulation at multiple locations in the system, particularly at motors. Transient overvoltages

from restriking ground faults are the main reason why ungrounded systems are no longer recommended and grounded systems of some form are the predominant choice. To reduce transient overvoltages during restriking ground faults, one should ground the system using either solid or impedance grounding.