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This paper will begin with a brief history of low voltage grounding which includes ungrounded, solidly grounded and resistance grounded systems. As part this discussion, benefits and limitations of each system will be covered. Reliability, continuity of service and personnel safety are important issues and will be discussed. It will be shown that the use of high resistance grounded low voltage systems makes good sense in the petro-chemical industry. Design, construction, operation and maintenance factors for high resistance grounded systems will be discussed and analyzed.
A debate has existed in the petroleum and chemical industry for years concerning grounding of low voltage (480-600 Volt) power systems. Since reliability and continuity of service are important in the petro-chemical industry, some engineers in the past preferred using an ungrounded system. The practicality of such ungrounded systems becomes questionable as the size and extent of coverage increases. Many of these systems are still in existence, but few ungrounded low voltage systems are presently being designed and constructed due to the possible destructive nature of transient overvoltages resulting ·from an arcing ground fault, especially in a large configured facility. Most systems now utilize either a solidly grounded or high resistance grounded source.
The use of a high resistance grounded system when three phase, four wire loads are present will be discussed. Finally, operational problems and solutions will be discussed where significant variable speed drive loads are utilized. The use of the high resistance grounded system provides so many benefits that it should become a standard of the industry, and the solidly grounded system should be used only in applications where the high resistance grounded system becomes impractical.
Reliability has always been an important aspect in the design and operation of a low voltage power utilization system, and especially in the petro-chemical industry. (In this paper, the term low voltage will typically reference the use of voltages in the 440-600 Volt range.) Loss of low voltage power can cause a complete upset to a plant, create personnel and equipment safety problems, have an adverse environmental impact, and can result in substantial economic losses. As such, the need for a safe, reliable low voltage power source is essential.
In the past, an ungrounded system was extensively used with the assumption that the system was more reliable. However, experiences with multiple failures due to arcing ground faults on an ungrounded. system have resulted in a change in philosophy about the use of the ungrounded system. Solidly grounded and high resistance grounded systems have become the standard for large industrial complexes. Solidly grounded systems may have their place where redundant equipment is installed and are the norm for 3 phase, 4 wire systems.
|High Resistance Grounding of 480 V Systems - A Standard for the Petroleum and Chemical Industry|