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This paper discusses grounding practices used on electric distribution systems above 600 Volts. In particular, the paper concentrates on the three-phase fourwire, multi-grounded neutral system that is extensively used in North America. The paper addresses the benefits of the multi-grounded power system and makes comparisons with other grounding system designs including ungrounded; three-wire single-point grounded; three-phase, four-wire single point grounded neutral; and the three-phase, five-wire systems.
Advantages and disadvantages for grounding practices of each system will be discussed. Some criticism regarding stray currents and stray voltages has been made of the multi-grounded neutrals on electric distribution systems and this will be discussed. Technical responses will be made to these comments including a discussion on reasonable solutions, alternative designs, and “acceptable risks.” The three-phase, four-wire, multi-grounded distribution system has been selected by most utilities in North America as the medium voltage distribution system of choice even though many utilities started with a three-wire, ungrounded delta system.
The reasons for the development of the threephase, four-wire, multi-grounded systems involve a combination of safety and economic considerations. The three-phase, four-wire multi-grounded design has been successfully used for many years and is well documented in the standards including the National Electrical Safety Code (NESC) , and the National Electrical Code (NEC).
Have there been problems associated with this system? Yes. Are there reasonable solutions available to minimize these problems? Absolutely! Should the use of the multigrounded system be eliminated? This paper will show that the answer to the last question is absolutely not. The earth is an electro-magnetic circuit with north and south magnetic poles and with an ionosphere made up with charged particles. During electromagnetic storms caused by sunspot activity, observations have been made showing potential gradients (stray voltages) on the earth’s surface of one to ten volts per kilometer. These voltage gradients have occurred since the origin of the earth and will continue to occur in the future. Man and animals have lived with these stray voltages and associated stray currents with no apparent adverse reactions. And, if found that there were hazards associated with them, there is little that can be done about stopping it at its source, the sun. Therefore, we live in a world where stray voltages and stray currents are natural.
|The Grounding of Power Systems Above 600 Volts: A Practical View Point|