But , i.e. .sx The line tension is estimated using this equivalent span length , and the sag for each actual span is then calculated from .sx .sx Line Erection .sx In practical erection the line is first paid out from drums by a steel running line attached to the end of the conductor .sx The hauling off is effected in steps , being stopped as the conductor end reaches each tower , when the conductor itself is raised and laid over snatch-blocks hung from the cross-arm on a level with the ends of the insulator strings .sx At the point selected for pulling up , the conductor is carried down over the last snatch-block to blocks and tackle and a dynamometer .sx By means of man-power or mechanical power , according to the size of the line , the latter is pulled up to a `killing tension' somewhat in excess of the correct tension :sx this is held for about five minutes , when the conductor is slackened off until the tension reaches the correct value for the temperature prevailing at the time .sx The sag is checked either by fixing a telescopic level to the tower at the correct height , when the lowest point of the conductor should be sighted through it , or by fixing a horizontal batten to each of two adjacent towers so that the line of sight joining them is tangential to the lowest point of the conductor .sx The line is then permanently anchored off and transferred from the snatch-blocks to the insulator clamps .sx The height from the ground of any conductor at any part of the span at 122 F. must not be less than the following :sx 20 ft .sx for voltages up to 66,000 .sx 21 ft .sx 110,000 .sx 22 ft .sx 165,000 .sx This rule governs the height of the towers employed .sx CHAPTER VIII .sx OVERHEAD LINE INSULATORS .sx Materials .sx MODERN overhead line insulators are made principally of porcelain or steatite .sx The basis of porcelain is china clay , a naturally occurring aluminium silicate , which is mixed by firing at a high temperature with exact proportions of plastic kaolin , quartz , and felspar .sx The most important supplies of these materials are found in Cornwall and Dorset .sx The insulators are carefully finished with a hard , smooth glaze ; this allows the surface to be washed clean by rain .sx The tensile strength of porcelain is about 2.5 to 3.5 tons per sq .sx in .sx , and the compressive strength from 15 to 18 tons per sq .sx in .sx Steatite is a magnesium silicate which is found native in many parts of the world .sx The relative proportions of magnesium oxide and silica vary considerably .sx The only combination suitable for electrical insulators is that obtained from the mines in Bavaria .sx Steatite has very much higher tensile and bending strengths than porcelain , and is therefore suitable for cases where the ceramic part is subjected to high tensile or bending stresses .sx Pin-Type Insulators .sx In this type the insulator is screwed on and firmly attached by hemp or other means( to a steel bolt or pin secured to the cross-arm of the supporting tower .sx The conductor is supported in a groove at the top or side of the insulator to which it is bound by annealed wire of the same material as the conductor .sx An insulator may fail electrically by a puncture through the material or by a flash-over across its surface .sx The former is prevented by providing adequate thickness of porcelain between the end of the pin and the conductor .sx Since a failure is therefore invariably due to a flash-over across the surface , the insulator must be shaped to secure a uniform potential gradient along this surface .sx This is achieved by making the body of the insulator conform to the lines of the electrostatic field .sx The flash-over voltage of an insulator when wet is always lower than when it is dry , since a wet surface is practically conducting .sx Insulators are therefore provided with one or more rain sheds , so that when the outer surfaces are wet with rainthe inner surfaces remain sensibly dry .sx In order that these rain sheds shall not alter the voltage distribution , they are shaped as far as possible perpendicular to the lines of electrostatic force , so that they lie in planes of equal potential ( Fig. 46) .sx A single-part triple-shed pin-type insulator , hemp-fixed to its pin , is shown in Fig. 47 .sx The hemp fixing is effected by winding .sx around the end of the pin a long skein of hemp , making a packing which is smoothed over and saturated with oil or red lead ; after an elastic washer of felt or asbestos has been placed in the pin-hole , the insulator is screwed on to the pin .sx The fixing may also be effected by a lead head cast on the end of the pin , or by a sheet-steel or zinc thimble fitted into the pin-hole of the insulator .sx For operating voltages greater than 33 kV .sx , the thickness of a single-part insulator becomes excessive and leads to faulty manufacture .sx Hence a multipart construction is adopted in which the insulator is made of two or more parts which are fired individually and joined together by cement or preferably by an elastic layer of hemp .sx Fig. 48 shows a two-part insulator for use on a line voltage of 55 kV .sx The dry flash-over voltage is 158,000 and the wet flash-over 140,000 .sx The mechanical characteristics of modern pin-type insulators are such that the limit of mechanical stress which may be placed on the insulator is determined only by the bending strength of the pin .sx Since the dimensions and weights of high-tension pin insulators increase fairly rapidly with the working voltage , they are not recommended for pressures above 55 kV .sx Above this value suspension insulators are used and are frequently adopted for pressures as low as 33 kV .sx Suspension Insulators .sx In this type of insulator the conductor is held in a clamp which is suspended from the cross-arm of the tower by a string of insulator units connected to each other by metal links or pins .sx The advantages of this arrangement are :sx Since each unit can be designed for about 11 kV .sx , the line can be insulated for any operating voltage by providing a sufficient number of units in the string , instead of increasing the size of the unit as with the pin-type insulator .sx This results in greater economy and reliability in manufacture .sx Increased flexibility is given to the line , the insulator string being free to swing in any direction :sx the tensions in successive spans are thus equalized .sx The conductor spacing must be slightly increased with the suspension-type insulator to allow for the possible swinging of the line towards the support .sx In one form of cap-and-pin type suspension insulator unit , Fig. 49 , the head of the porcelain disk is surmounted by a galvanized cast-iron cap to which it is cemented .sx A galvanized forged-steel pin is cemented in a hollow cylindrical cavity in the insulator head :sx the enlarged end of this pin fits into a socket in the cap of the next lower insulator forming a ball-and-socket connexion .sx In the spring-ring insulator , Fig. 50 , a spiral spring ring of steel wire of high tensile strength carried on the stem of the pin .sx is forced through the neck of the hollow cylindrical space in the head of the insulator and allowed to expand into the wider cavity , the shape of which is specially designed to conform in curvature with that of the spring itself .sx The spring is retained in position by means of a second spring and clip .sx In order to protect it from the weather , to prevent any possibility of relative movement of the components , and to secure a better distribution of the mechanical and electrical stresses over the insulator , the interspace is filled with a lead alloy .sx The substitution of this alloy for cement avoids the cracking of the insulator which frequently occurs owing to the expansion of the cement .sx It is impossible without actually breaking away the whole of the porcelain for the pin to come out of the insulator and allow the conductor to fall .sx The ball-and-socket joint is used to connect the respective units .sx In the Hewlett suspension insulator no cement is employed in the insulator assembly , each unit consisting of a single piece of porcelain :sx two lead-covered steel U-links are threaded through the insulator , each lying in one of two curved channels at right angles to each other .sx In this way the stress in the porcelain is uniformly distributed and is compressive ; if the porcelain breaks , the line is still supported by the steel links .sx Voltage Distribution over a String of Suspension Insulators .sx A suspension insulator consists of a string of alternate insulator units and metal fittings by which the units are connected .sx This constitutes , therefore , a number of condensers in series , the capacitance of each condenser being that between successive metal connectors with the porcelain as dielectric .sx There are , in addition , air capacitances from each connector to the sup-porting cross-arm or tower , i.e. to earth , and also to the line .sx The latter are usually negligible , but the former are not , and cause a non-uniform distribution of potential over the string .sx Thus , consider a four-unit suspension insulator , Fig. 51 , and let C represent the self-capacitance of each unit and kC the capacitance of each metal connector to earth .sx Then , if V1 is the voltage across the top unit , we have A transient over-voltage or surge , due to a switching or other disturbance within the system , or to lightning , may cause a flash-over :sx the danger to the insulation lies not so much in this transient voltage , which is usually of high frequency , as in the power arc which follows the short-circuit path so formed .sx An insulator may withstand a transient flash-over , but cannot withstand the intense heat of a prolonged power arc over its surface .sx Typical suspension sets are shown in Figs .sx 53 and 54 .sx In the latter , which employs double W-horns , the arc forms on the lowest point of a horn where there is a large radius and is carried to the top by convection currents of hot ionized air , and is automatically broken almost instantaneously by bowing out , without the assistance of wind .sx The provision of four points to the horn ensures that the wind , whatever its direction , will not blow the arc on to the insulators .sx Testing of Insulators .sx The following basic tests are usually carried out :sx Dry spark-over voltage test .sx The dry and clean insulator is mounted in a manner approximating as closely as possible to practical conditions .sx A power-frequency voltage is applied and gradually increased until flash-over occurs .sx Wet spark-over voltage test .sx This is usually carried out with a spray of water of resistance 20,000 ohms per cm .sx cube at 15 C. , falling at an angle of 45 with the vertical at the rate of 1 in .sx in five minutes .sx Puncture test .sx The insulator is immersed in oil to eliminate surface spark-over , and the voltage is applied gradually until puncture occurs .sx Impulse and/or high-frequency tests .sx The most important source of over-voltage is lightning , the voltages induced being of an impulsive nature , although by reflection from terminal apparatus oscillatory voltages may also be built up .sx An impulse is characterized by its very rapid increase of voltage to the maximum value , followed by a less rapid decrease to zero .sx The voltage may rise from zero to maximum in one-millionth of a second , and is said to have a steep wave front .sx Higher spark-over and puncture figures are usually obtained with impulsive voltages than with those of power frequency , due to the .sx Hence , if k = 0.1 , we have .sx Voltage across top unit = V1 = 0.198E , .sx second = 1.1V1 = 0.217E , .sx = 1.31V1 = 0.259E , .sx bottom = 1.651V1 = 0.326E. .sx Total voltage E = 5.061 V1 .sx It will be seen , therefore , that in this case the earth-end insulator takes only one-fifth of the total voltage , while the line-end insulator takes nearly one-third .sx The insulator string will therefore flash over when the voltage across the line-end insulator reaches its flash-over voltage , i.e. 0.326 E in the case of a four-unit string .sx The line voltage will then be E and not 4 x 0.326 E as would have been expected if the potential had been uniformly distributed across the string .sx