A system grounding or earthing method provides the safety, reliability, and continuity of service required by the electrical power distribution system. In the past, ungrounded systems were preferred when reliability and continuity service were very important. Ungrounded systems, however, provide no control over a destructive transient overvoltage. Moreover, experience with multiple failures due to arcing ground faults has caused a change in philosophy about the use of an ungrounded system. Solidly grounded and high resistance grounded systems have become the standard, particularly for large industrial systems.
Electrical Faults
A great majority of electrical faults (almost 95%) are of the phase-to-ground type. A high resistance grounding will insert an impedance in the ground return path. The impedance will limit the ground fault current leaving insufficient fault energy thereby helping reduce the arc flash hazard.
International Standards
Below is a list of standards that provide basis or advantages of high resistance grounding over ungrounded systems.
IEEE Std 141-1993 (Red Book)
7.2.1 Accumulated operating experience indicates that, in general-purpose industrial power distribution systems, the over-voltage incidents associated with ungrounded operation reduce the useful life of insulation so that electric current and machine failures occur more frequently than they do on grounded power systems.
7.2.2. High-resistance grounding provides the same advantages as ungrounded systems yet limits the steady-state and severe transient over-voltages associated with ungrounded systems. Continuous operation can be maintained.
IEEE Std 242-1986 Recommended Practice for the Protection and Coordination of Industrial and Commercial Power Systems
7.2.5. Ungrounded systems offer no advantage over high-resistance grounded systems in terms of continuity of service and have the disadvantages of transient over-voltages, locating the first fault and burn-downs from a second ground fault. For these reasons, they are being used less frequently today than high-resistance grounded systems”
IEEE Standard 242-2001 (Buff Book) Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems
8.2.5 If this ground fault is intermittent or allowed to continue, the system could be subjected to possible severe over-voltages to ground, which can be as high as six to eight times phase voltage. Such overvoltages can puncture insulation and result in additional ground faults. These over-voltages are caused by repetitive charging of the system capacitance or by resonance between the system capacitance and the inductance of equipment in the system.
FM Global 5-18 Protection of Electrical Equipment Single Phase and Other Related Faults
In ungrounded systems a phase to ground fault often produces dangerous overvoltage – Sustained arcing faults in low voltage apparatus are often initiated by a single-phase fault to ground which results in extensive damage to switchgear and motor control centers.
FM Global 5-10 Protective Grounding for Electric Power Systems and Equipment
2.3.3.1 Unlike the ungrounded system the high resistance grounded system prevents transient overvoltage which can cause the potential failure of insulation.
2.3.4.1 Convert ungrounded systems to high resistance grounded systems.