This report deals with two problems of primary importance in connection with the transmission of three-phase power by-high voltage lines, viz.:—(a) The effects of transposing the line conductors on the operation characteristics and power losses.†(b) The impedance of a line conductor and earth return when one phase is accidentally earthed.Item (a) affects the design and construction of the line and its normal operation, while item (b) is the principal factor in determining the magnitude of the “failure” current returning through the earth (under fault conditions) upon which the amount of interference with communication circuits is dependent.Transposition makes the power lines electrically balanced, equalizes the phase impedances and reduces the induction from normal operating currents and voltages. On single circuit lines transposition entails practically no additional cost and ought to be adopted as standard in all cases. On double circuit lines, however, special “transposition towers” (costing in the case of the “grid” lines several hundred pounds extra per tower) have to be employed at each transposition point (to prevent the conductors touching in the span), and this factor renders it desirable to avoid transposition whenever practicable. On long lines (fed entirely from one end) transposition appears essential both on account of inductive interference and to avoid large differences between the phase impedances.On short lines (say up to 50 miles long, which covers most practical cases in this country), the latter effect is negligible compared with the transformer impedances, and transposition is unnecessary when, in addition, interference with communication circuits arising from normal operating currents and voltages is unimportant. Thus on short double circuit lines no general recommendation regarding transposition can be made and each case must be judged on its merits.On all double circuit lines,whether transposed or not, the time-phase sequence of the conductor currents in the two circuits must be co-ordinated to obtain the best results from the standpoint of operation of the system.For transposed linesthe maximum reduction in phase impedance occurs when the top conductor of each circuit conveys a current of the same time phase as the bottom conductor of the other, the two circuits being transposed at the same frequency and in the same direction of spiral. This gives a small reduction in phase impedance (3 per cent for the “grid” lines) compared with that of a single circuit at similar spacings, but allows a small interchange of power (by transformer action) between the circuits which, however is negligible in most practical cases (0-28 kW per mile at 220 amperes on the “grid” system).For non-transposed linesthere are two practicable current sequences. In the first method, which gives minimum phase impedance, the current sequence is the same as that given above for transposed circuits. In the second method, conductors in the same horizontal plane convey currents of the same time phase and have equal impedances, thus giving equality of loading of the two circuits when operated in parallel. The ultimate choice depends on the magnitudes of the power transference between the circuits, the phase impedances and the differences between them. These in turn depend upon the particular conductor spacings, etc., employed, but a few simple calculations by the formulae herein soon indicate which is the better method in a given case. For the “grid” lines the same phase sequence already given for transposed circuits is recommended.As regardspowerlosses, the effects of transposition are negligible except on “long” lines fed entirely from one end, a condition which will seldom, if ever, arise in this country.If one phase is accidentally short circuited to earth the interference caused to communication circuits is proportional to the magnitude of the current returning through the earth. The percentage of the total short-circuit current returning in this way may be reduced by. the employment of an earth conductor bonded to the towers, which provides an alternative path for part of the failure current. The principaldesignfactors affecting the reduction obtained are:—(a) Impedance of the earth conductor.(b) Method of “earthing” the earth conductor. (“Multiple” earths.)The earth current is reduced in magnitude by increasing the cross-sectional area of the earth conductor, but not in proportion, because the impedance of the latter depends on the mutual inductance from the earth and short-circuited phase. For this reason after a certain point it is impracticable for economic reasons further to increase the cross-sectional area as the additional reduction obtained thereby is negligibly small, and a second earth wire in parallel with the first should be employed. For example, making the “earth” conductor on the “grid” lines of the same section as the line conductors, which corresponds to increasing the cross-sectional area of aluminium from 0–114 to 0–285 square inch (i.e. by 115 per cent) only decreases the percentage of failure current carried by the earth from about 66 per cent to 58 per cent (for an earth resistivity of 20 000 ohms per centimetre cube, see Table 6 and Fig. 8), whereas a second wire equal in cross-section to the first and suitably positioned below the phase conductors would give a much greater reduction.On existing systems the use of an all-steel earth conductor is common practice, primarily for mechanical reasons, but because of the high self-impedance of such conductors the above considerations lead to the condemnation of this method in favour of a steel-cored aluminium cable, which combines the advantages of great mechanical strength and high electrical conductivity and protection of the steel from corrosion.With “multiple” earthing (i.e. earthing the earth conductor at every tower) the percentage of failure current returning via the earth is still further reduced, but the extra reduction is only a few per cent except in the immediate vicinity of the ends of the short-circuited section (see Fig. 9). For most practical purposes (i.e. except when the short-circuited section is very short in length, perhaps one or two kilometres) the effect of “multiple” earths may therefore be ignored. The general adoption of multiple earthing is, however, advocated to ensure the earth conductor being “earthed” close up to a “fault” wherever it occurs on the system; thereby preventing thewholeshort-circuit current passing through the earth for any appreciable distance.These conclusions are based largely on theoretical and experimental work by the authors.