Technologic Papers of the Bureau of Standards Volume 101-110
This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1918 edition. Excerpt: ...formed by the ground detecting device will obviate all danger from static electricity. A further qualification is that if the circuit is exposed to contact with high-voltage lines it must be grounded as thoroughly as though it were an alternating-current secondary circuit fed from a high-voltage distribution system. (b) Three-wire Systems.--In the 3-wire system of directcurrent distribution the extent of the lines is in most cases greater, and the total voltage higher than in 2-wire systems. The life and fire hazards presented by the usual 3-wire system are, therefore, appreciably greater than in the usual 2-wire system, and greater precautions are necessary to prevent accidents. On the other hand, the 3-wire system lends itself more readily to precautionary measures than the 2-wire system, in that the middle wire can be, and usually is, grounded, placing it at earth potential and making the potential difference between either outside wire and earth equal to half of the total voltage. With the circuit insulated throughout, however, an accidental ground on one of the outside wires would put the other outside wire at full voltage above ground. If an insulated 3-wire circuit accidentally becomes grounded on one of the outside wires, there is, therefore, considerable danger to life as well as property, especially if the total voltage is 440, as it is in some instances. With the middle wire permanently grounded, there is, of course, still danger to property if an accidental ground forms on one of the outside wires, but if the resistance of the ground connection is low, an accidental ground does not appreciably increase the life hazard. Nevertheless, in the case of 440-volt circuits, the question of whether the life hazard presented by 220...
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- 13 Sep 2013
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