The losses due to corona effect increase very rapidly with increasing pressure beyond the critical voltage.
The magnitude of the losses as well as the critical voltage is affected, by atmospheric conditions, hence they probably vary with the particular locality, and the season of the year. Therefore, for a given locality, a voltage which is normally below the critical point, may at times be above it, depending upon changes in the weather.
Such elements as smoke, fog, moisture, or other particles (dust, snow, etc.) floating in the air, increase the losses; rain, however, apparently has no appreciable effect upon the losses. It follows then that in the design of a transmission line, the atmospheric conditions of the particular locality through which the line passes should be considered.
Ques. How should live wires be spaced?
Ans. They should be so spaced as to lessen the tendency to leakage and to prevent the wires swinging together or against towers. The spacing should be only sufficient for safety, since increased spacing increases the self-induction of the line, and while it lessens the capacity, it does so only in a slight degree.
The following spacing is in accordance with average practice.
| SPACING FOR VARIOUS VOLTAGES | |||
|---|---|---|---|
| Volts | Spacing | ||
| 5,000 | 28 ins. | ||
| 15,000 | 40 ins. | ||
| 30,000 | 48 ins. | ||
| 45,000 | 60 ins. | ||
| 60,000 | 60 ins. | ||
| 75,000 | 84 ins. | ||
| 90,000 | 96 ins. | ||
| 105,000 | 108 ins. | ||
| 120,000 | 120 ins. | ||
Resistance of Wires.—For quick calculation the following method of obtaining the resistance (approximately) of wires will be found convenient:
1,000 feet No. 10 B. & S. wire, which is about .1 inch in diameter (.1019), has a resistance of one ohm, at a temperature of 68° F. and weighs 31.4 pounds. A wire three sizes larger, that is No. 7, has twice the cross section and therefore one-half the resistance. A wire three sizes smaller than No. 10, that is No. 13, has one-half the cross section and therefore twice the resistance.
Thus, starting with No. 10, any number three sizes larger will double the cross sectional area and any wire three sizes smaller will halve the cross sectional area of the preceding wire. This is true to the extreme limits of the table, so that the area, weight and resistance of any wire may be at once calculated to a close approximation from this rule, intermediate sizes being obtained by interpolation.