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Note on Losses in Cable Sheaths

Published online by Cambridge University Press:  24 October 2008

F. W. Carter
F. W. Carter
Affiliation:
St John's College
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Extract

In a recent communication to the Society, the author referred to cable-sheath losses, and gave formulae for computing them in certain cases. These appertained to power cables in which were comprised a group of conductors, arranged symmetrically and encased in a single conducting sheath. In some distribution systems, however, the conductors for the several phases are encased in separate lead sheaths, which are either laid in proximity as separate cables, or grouped and comprehended in an outer sheath. The analysis previously given does not include such cases directly. Moreover, it is common practice either to lay the elementary cables with sheaths in contact, or to bond the sheaths together at the ends of suitable sections, in order to prevent differences of potential between them; and, when this is done, a circulating current flows in the circuit of the sheaths and bonds, sufficient to maintain equality of potential between the several sheaths. This current, to which reference was made in the former paper, is additional to the eddy current discussed therein, the integral of which over the cross section of the sheath is zero. It is for convenience here referred to as the “circulating current,” to distinguish it from the “eddy current,” although there is no such distinction between them as the names imply.

Type
Research Article
Information
Mathematical Proceedings of the Cambridge Philosophical Society , Volume 24 , Issue 1 , January 1928 , pp. 65 - 73
Copyright
Copyright © Cambridge Philosophical Society 1928

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References

* Proceedings, vol. 23 (1927), pp. 901–906.Google Scholar

† L.c., note on § 3, p. 903.Google Scholar

* The outer sheath shown in this figure and the following is not now under consideration. It was discussed in the former paper, and is herein referred to only in section 6.Google Scholar

† The factor ejpt is omitted throughout, for the sake of brevity.Google Scholar

* For lead sheaths, and for a frequency of 50 cycles, m is generally between 0·04 and 0·1.Google Scholar

† This formula was given, substantially, by Prof. W. Cramp and Miss N. I. Calderwood; but was wrongly assumed by them to apply to the whole sheath, instead of the point nearest to the return sheath only (θ=0). See “The use of single core sheathed cables for alternating currents,” Journal I.E.E., vol. 61 (1923), p. 477.Google ScholarSee also “Losses in grounded sheaths of single conductor cables,” by Dwight, H. B., The Electric Journal, 1924, vol. 21, p. 62Google Scholar, and “Investigation of conflicting cable-sheath loss formulae,” by Hutchison, L. C., Journal I.E.E., vol. 65 (1927), p. 903.Google Scholar

* In equations (19), (21), (23) and (24), the first term within the brackets, which arises from the circulating current, assumes the sheaths in contact or bonded together. In the alternative the term should be omitted, or suitably modified.Google Scholar