I think that you guys are missing something. High resistance grounding is a preferred method in modern plant power distribution systems, as opposed to ungrounded or grounded.
Here is an excerpt from IEEE:
IEEE Standard 242-1986 Recommended Practice for the Protection and Coordination of Industrial and Commercial Power Systems 242-1986 section 7.2.5 offer the following perspective:
"Ungrounded systems offer no advantage over high-resistance grounded systems in terms of continuity of service and have the disadvantages of transient overvoltages, locating the first fault and burndowns from a second ground fault. For these reasons, they are being used less frequently today than high-resistance grounded systems"
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There are many benefits from grounding the electrical distribution system including:[/font]
- [font=Arial, Helvetica, sans-serif]Reduced magnitude of transient over-voltages[/font]
- [font=Arial, Helvetica, sans-serif]Simplified ground fault location[/font]
- [font=Arial, Helvetica, sans-serif]Improved system and equipment fault protection[/font]
- [font=Arial, Helvetica, sans-serif]Reduced maintenance time and expense [/font]
- [font=Arial, Helvetica, sans-serif]Greater safety for personnel [/font]
- [font=Arial, Helvetica, sans-serif]Improved lightning protection[/font]
- [font=Arial, Helvetica, sans-serif]Reduction in frequency of faults.[/font]
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The choice for many engineers is focussed on what grounding technology to use.[/font]
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A solidly grounded system is one in which the neutral points have been intentionally connected to earth ground with a conductor having no intentional impedance and this partially reduces the problem of transient over-voltages found on the ungrounded system.[/font]
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While solidly grounded systems are an improvement over ungrounded systems, and speed the location of faults, they lack the current limiting ability of resistance grounding and the extra protection this provides. The destructive nature of arcing ground faults in solidly grounded systems is well known and documented and are caused by the energy dissipated in the fault. A measure of this energy can be obtained from the estimate of Kilowatt-cycles dissipated in the arc:[/font]
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Kilowatt cycles = V x I x Time/1000.[/font]
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In the same IEEE Standard as reference above, section 7.2.2 states that:[/font]
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"one disadvantage of the solidly grounded 480v system involves the high magnitude of ground-fault currents that can occur, and the destructive nature of arcing ground faults."[/font]
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Since the vast majority of arcing faults start their life as single-phase faults, the key to reducing their impact is to use technology that either significantly reduces the fault current level thereby reducing the magnitude of the arc hazard and/or using technology that prevents transient overvoltages that can lead to single-phase faults escalating into arcing faults.[/font]
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The answer in both cases is high resistance grounding, as recognized in the Canadian Electrical Code section 10-1100, and the National Electrical Code section 250-36. [/font]
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High resistance grounding of the neutral limits the ground fault current to a very low level (typically from 1 to10 amps) and this is achieved by connecting a current limiting resistor between the neutral of the transformer secondary and the earth ground and is used on low voltage systems of 600 volts or less, under 3000 amp. By limiting the ground fault current, the fault can be tolerated on the system until it can be located, and then isolated or removed at a convenient time. [/font]