BiRKS AND Webb. — Resistance of Earth Connections. 465 



cross-arms, and carried on ironbark poles standing 36 ft. out of the groiind 

 and sunk 6 ft. into the ground. There are 864 poles in the northern linn 

 and 873 in the southern line. The standard pole-spacing is 6 chains (396 ft.) 

 — i.e., about thirteen poles to the mile. In order to obviate ih.Q burning of 

 poles and cross-arms, it was decided to earth the whole of the high-tension 

 insulator-pins; and owing to the large inductance with high-frequency cur- 

 rent and considerable capacity of a ground wire, not to mention the greater 

 cost, it was decided to earth each pole with an individual earth-strip. In 

 addition to this, the apparatus in telephone depots along the line required 

 to be thoroughly earthed, while the stations at either end, and the nume- 

 rous transformers in the distribution system, all required very efficient 

 earths. The station lightning-arresters in particular depend for their effec- 

 tiveness on a low-resistance connection to ground. 



The substance of the British Board of Trade specification in regulations 

 concerning station earths and earths for tramways and general power 

 purposes is that the resistance shall be less than 1 ohm between two 

 plates of copper, cast iron, or galvanized iron, packed in coke, and 60 ft. 

 apart. 



Regulations under the Coal-mines Act (Great Britain), 1911, specifies 

 that '" All conductors of an earthing system shall have a conductivity at 

 all parts and at all joints at least equal to 50 per cent, of that of the largest 

 conductor used solely to supply the apparatus a part of which it is desired 

 to earth." In practice this would involve an earth-resistance never greater 

 than 10 to 15 ohms, and usually not more than 2 or 3 ohms. " Sparks " 

 (journal of the Institution of Electrical Engineers), 15th March, 1915, recom- 

 mended packing-plates placed vertically at 6 ft. or more beneath the sur- 

 face, with a foot of coke on each side : "' Tests made on the earth plates 

 constructed on the lines recommended by ih.Q General Regulations, the 

 plates being bui'ied in excavations 4 ft. by 2 ft. by 8 ft. deep in clay, under 

 favourable conditions as to moisture, vary from 1-8 to 2-2 ohms, while 

 the resistance of similar earth plates in another district, in excavations 

 4 ft. by 2 ft. by 6 ft. deep in well-consolidated marl and clay, the earth 

 plates resting on clay in damp positions, reached 2-6 and 2-7 ohms. . . . 

 If the conditions are not favourable a nuich higher resistance to earth will 

 be found, as the resistance to earth is directly affected by the nature and 

 temperature of the surrounding strata and by the amount of moisture." 



It is now fairly common knowledge that the Board of Trade test is 

 seldom obtained, and that values such as those quoted above are decidedly 

 the exception rather than the rule. 



Some tests Avere made locally on plate earths of the Christchurch City 

 Council. These plates were \ in. copper, 2 ft. square, and were placed 

 horizontally in a bed of coke 1 ft. thick above and below \h.Q plate, and 

 at a depth of 6 ft. to 8 ft. These were tested against the city high -pressure 

 water-main, and (assuming the water-main as of zero resistance to earth) 

 these gave the following results : Beckenham transformer -house plate 

 earth = 51 ohms (loam and shingle) ; Sydenham transformer-house plate 

 earth = 17-2 ohms (clay) ; Montreal Street transformer-house plate earth 

 = 13 ohms (clay). 



The Christchurch Tramway Board also had some considerable difficulty 

 with earth connections. Finally two large copper plates 8 ft. square were 

 placed some 10 ft. below the surface (60 ft. apart), and well packed with 

 coke. Tests on these plates failed to reach that required by the Board of 

 Trade. To avoid further trouble and expense the ground wire was earthed 



