Four-wire Unbalanced Feeder

Author: Edmund A. Laport

This is an extension of type V by adding another grounded wire (Fig. 4.7). The same general method of construction can be used except for the additional wire, either above or below the high-potential wire. The equation is derived on the basis that the height is large enough with respect to other dimensions to give very nearly equal currents in each of the three grounded wires, a condition realized in typical overhead-line constructions. The extra grounded wire increases k and decreases the current returning in the earth. When h >> a,

Fig. 4.7

A sample case, for equal-size wires, is:

p (inches)	h (inches)	a (inches)	k	Z0 (ohms)
0.064	120	10	0.685	363

Compare this with types IV, VI, and VII. These types, in succession,

increase the enclosure of a single high-potential conductor with a circle of ground wires that carry an increasingly large portion of the total return current, and the system gradually approaches equivalence to a coaxial feeder.