464: Transactions. 



Art. XLYlll.—Investigation into the Resistance of Earth Connections. 



Ey L. BiRKS, B.Sc, M.Inst.C.E., M.I.E.E., and Eric Webb, Lieut. E.E 



(A.I.F.). 



[Read before 'the Philosophical Institute of Canterbury, 7th July, 1915.] 



The subject of this investigation is one whicli is being constantly referred 

 to by the electrical engineer, and called for by innumerable regulations 

 framed for the protection of life and plant ; but in practice it is often so 

 vague and uncertain, and so few engineers actually measure their earth- 

 resistances, that there is room for much investigation before we can claim 

 to understand the principles involved or their effect when applied to any 

 particular set of circumstances. The electrical engineer, in practice, early 

 admits the difficulty of the problem, and realizes the absence of any com- 

 prehensive treatise on the subject. There is such diversity in the data 

 extant, and so strange a variation between his own experience and the 

 stated results of others, that he is only too glad to escape further atten- 

 tion from official inspectors by obtaining permission to " earth " to a water- 

 pipe. And in this way the matter, for the most part., is quietly shelved. 



The present paper aims primarily at the publication of data obtained 

 during an investigation of earthing devices, for use on the Canterbury 

 Plains (New Zealand), in connection with the high-voltage electric-power 

 transmission-line recently erected by the Public Works Department of New 

 Zealand for the supply of Christchurch and district from Lake Coleridge, 

 a distance of sixty-two miles. Some description of the country traversed 

 is essential to an intelligent apprehension either of the data itself or of the 

 difficulty experienced in attaining the objective. 



The Lake Coleridge power-house is situated in the Kakaia Eiver valley, 

 sixty -two miles west of Christchurch, and from this point two 66,000-volt 

 three-phase transmission-lines are run to the city on independent pole-lines. 

 For the first sixteen miles from the power-house the lines are located about 

 3 chains apart, and pass along the terraces and low hills of the river-valley, 

 in many places over screes or fans of shingle detritus. At Windwhistle 

 Point they divide. In the next . fourteen miles the north line continues 

 over low hills and rolling downs to Glentunnel, while the south stretches 

 almost directly across the thirty-two miles of shingle-beds which constitute 

 the Canterbury Plains. At the city end the lines meet again at Eealey 

 Road, seven miles from the substation, and continue about 1 chain apart. 

 For this distance the soil is largely of a sandy to peaty nature, the remains 

 of a marsh formed by the sandhills of a former seashore and in the bed of 

 an ancient river. It is thus evident that, although the major part of the 

 line runs through dry shingle country, there is at the same time extensive 

 variety. In many places across the plains water in ordinary soakage wells 

 is. almost unobtainable, not being met till below 60ft., and in some cases 

 120 ft. to 140 ft. Evidently the permanent water-level may thus be at a 

 very great depth, leaving at the top a considerable thickness of dry shingle. 

 The shingle is, in the main, a hard, highly crystalline, bluish-grey sandstone, 

 commonly known as greywacke. In size it ranges from sand to boulders 

 18 in. in diameter. 



The line conductors consist of 7/-135 bare aluminium wires, and are 

 carried on Thomas No. 4000 pin insulators, mounted on wooden (jarrah) 



