|
|
| Home |
| PPD |
| Sites |
| ABdA |
| Help |
|
|
WEEK 13: DISTRIBUTION: EQUIPMENT Distribution: Equipment: Structures Sections: Structures | Pole Lines | Conductors | Transformers | Voltage Control | Protection Structural Design of Pole Lines Pole Location In residential areas, poles generally are spaced from about 100 to 150 feet apart depending on the size of the lots. The span usually is an integral number of lot widths in length, and poles are set to provide convenient points for connection of services. Longer spans up to 300 feet or longer are used in rural areas. In either case, pole spacing should consider expected future growth and service requirements in the area. The choice of pole spacing is also a function of the load to be carried and the relative economics of longer spans. PEC Article 3.4.2 has specific requirements of poles near other poles, roads, buildings, fire hydrants, etc. Selection The height of poles is determined by required clearances over obstructions, streets, crossings; the span lengths; and the number and character of conductors or circuits to be carried. The most usual length for poles used in distribution construction have been 30, 35, and 40 feet, Armless construction and joint use tend to favor the 40-feet length. With CATV and other facilities on joint-use poles, 45-feet poles are even more commonly used. Depth of wood-pole setting start at 5 feet for a 25-foot pole, 5½feet for 30-foot pole, 6 feet for 35- and 40-foot pole, 6½feet for 45-foot pole up to 9½feet for a 75-foot pole.
Guying Guy wires are made of stranded steel cable, usually galvanized for weather resistance. Several grades are available in several diameter in steps of 1/16 inch up. When the horizontal loads to be carried by poles are greater than can be safely supported by the poles, guys and braces are required to provide additional support. Tension guys and anchors are commonly used wherever conductor tensions are not balanced, as at dead ends, corners, or where the direction of the line changes substantially. Down guys transmit force from the overhead structure system to a buried anchor system. They are located opposite the forces and use materials in tension to balance the forces. Where it is not practical to place these facilities to continue the imbalanced forces to ground, a compressive guy or pole brace must be used. Span Guy transfers some strain from one pole to another. Head Guy, a variation of the span guy, differing in that the wire runs to a point somewhere below the top of the sustaining pole, usually 10 ft from the ground level. Anchor Guy consists of a wire attached from some high point on a pole to an anchor in the ground. Stub Guy consists of a stub, a piece of wood, to which the guy wire is attached. The Sidewalk Guy a horizontal strut is attached to the pole midway down the pole; the guy wire runs angled from the top of the pole to the strut and straight down to an anchor from there. Crossarm Guy compensate unbalanced pull on crossarm such as in sidearm construction or dead ends for heavy conductors.
Clearances Minimum Vertical Clearances Above Grounds, in Feet
Minimum Vertical Clearances between Wires, in Feet
Joint-Line Construction is used where two or more utilities would otherwise maintain separate pole lines, such as where both power and communication lines are routed along the same street. Poles are used jointly under a joint-ownership agreement or under lease agreement. Under joint ownership, the cost of providing the pole is borne jointly by the companies which share in its ownership. Division of expense is, in general, made in proportion to the space allotted for respective users. Clearance space, required between power and communication circuits and between the lowest attachments and ground, is disregarded in this determination. Clearance between higher-voltage and lower-voltage power circuits is chargeable tot he higher-voltage circuits. In case of leased space, the lessee acquires only the right to occupy a specified space. The owning company installs and maintains the pole and includes all charges in the rental price. Lessees usually install their own attachments and maintain them. Generally a minimum vertical clearance of 40 inches is used between communication conductors (or open wires of 0 to 750V) and supply conductors operating between 750 and 8700 V to ground. Separations of 60 inches are required for supply conductors operating above 8700V. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
When poles with imbalanced forces are so located that tensile guys cannot practically be used, compressive guys and, often than not, pole-foundation underbracing are used. A 
22kV