and second to prevent any follow-up power currentfrom flowing to ground Lightning arresters must beinstalled on the primary side of all substations,distribution centers, distribution transformers, andcapacitor banks.CONDUCTOR SUPPORTSAn important element in any overhead electricaldistribution system is a structure that is designed tosupport the weight of the conductors and all equipmentmounted on the structure. The structure is also designedto provide required clearances from the ground to theconductors and between conductors. Common types ofstructures used for this purpose are wood poles,reinforced concrete poles, metal poles, and metaltowers. The following text discusses poles.Types of PolesPoles used in the Navy can be wood, reinforcedconcrete, or metal (steel or aluminum). However,concrete and metal poles should be used only when theyare more economical or when special considerationswarrant their use.WOODEN POLES.— Wood poles are available invarious types, depending upon species of trees availablein the area. For example, yellow pine is commonly usedin the eastern United States. The length andcircumference of poles also vary. Poles are available in5-foot incremental lengths and with top circumferencesvarying in 2-inch increments. Therefore, we have polesthat measure 30, 35, 40 feet, and so on, in length and 17,19, 21 inches, and so on in top circumference.The classification (or class) to which a wood pole,of given length and top circumference, belongs isdetermined from the circumference of the polemeasured at a point 6 feet up from the butt. The classdetermines the strength of the pole, which is the abilityof a pole to resist loads applied 2 feet from the top of thepole. Pole classes are numbered from 1 to 10, with 1being the strongest. A Class 2 pole, for example, willwithstand a force of 3,700 pounds and a Class 4 polewill withstand 2,400) pounds of force.Wood poles are used mostly in distribution systemsand light-duty transmission lines. The class of pole useddepends on what the pole is used for. In other words, isthe pole to be used as a line pole, corner pole, ortransformer pole? The length of pole used is determined,in part, by the clearances required for the voltage of thecircuits on the poles, the number of circuits, and thelocation of the pole in relation to streets, railroads,buildings, and so forth. Clearances are also required toprovide safe working conditions for linemen workingon the lines. All clearances have minimum requirementsthat are set by the American National Standards Institute(ANSI) and the National Fire Protection Association(NFPA). These requirements are specified in theNational Electrical Safety Code (NESC), ANSI C2-87,and the most recent edition of the National ElectricalCode®(NEC®).Engineers also consider local conditions whendetermining the length of poles. For example, poleslocated in densely popular high-traffic areas need tobe higher than those located in sparsely populated ruralareas. In the Navy, the MINIMUM height of a woodentransformer pole is 35 feet and of all other wood poles,30 feet. Other guidance regarding the heights andclasses of poles is found in Power Distribution Systems,MIL-HDBK-1004/2.CONCRETE POLES.— Concrete poles arepreferred where the life of wood poles is shortened bylocal conditions. Concrete poles may be solid or hollow.Solid concrete poles are made in a trough form with steelreinforcing rods running lengthwise. The hollow type ofpole is made by placing the concrete and reinforcingrods into a cylinder of the desired length and taper andthen revolving the cylinder in a lathelike machine. Thehollow type is lighter than the solid type and, in addition,provides a means for making connections through thepole to underground cables or services. This techniqueallows wires to be concealed from view and protectedfrom the weather.The exterior form of concrete poles can be changedto meet almost any need. Gains (cut notches) forcrossarms and holes for bolts are cast in the pole. Eithermetal pole steps are solidly cast into the pole orprethreaded holes for the steps are installed.Although concrete poles last longer and are strongerthan wood poles, they are also expensive to make andinstall. However, the rising cost of wood poles and theirtreatment and maintenance plus better landscaping havebrought on an increased use of concrete poles.METAL POLES.— Metal poles used in the Navyare either steel or aluminum. Steel poles are not used inordinary power-line distribution circuits except forunusual circumstances, such as where there is a highstress or heavy load placed on the pole. Aluminum polesare used for lightweight distribution, such as street-lights.Guying of PolesAs poles must be strengthened sufficiently to carryheavy conductors and pole-mounted equipment, the2-6
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