Strain Insulator Figure 4-26.- Suspension insulator.

comers, at sharp curves, at extra long spans, at river The strain insulator looks exactly like the crossings, or in mountainous country. In such places the suspension insulator but is designed to hold much insulator must not only be a good insulator electrically heavier physical loads. Strain insulators are used when but it also must have sufficient mechanical strength to a pull must be carried as well as insulation provided. counterbalance the forces due to tension of the line Such places occur whenever a line is dead-ended, at conductors. (See fig. 4-27.)

Figure 4-27.- Strain insulator

CONDUCTORS The wires and cables over which electrical energy is transmitted are made of copper, aluminum, steel, or a combination of copper and steel or aluminum and steel. A conductor is a material that readily permits the flow of an electric current. Materials, other than those mentioned, that conduct electricity are not generally used to make wires and cables because of economic or physical reasons.

Copper Conductors Copper is the most commonly used line conductor. It conducts electrical current very readily, ranking next to silver. It is very plentiful in nature, it can be easily spliced, and its cost is comparatively low. Three kinds of copper wire are in use: hard-drawn copper, medium-hard- drawn copper, and annealed copper, also called "soft drawn."

For overhead line purposes, hard-drawn copper wire is preferable on account of its greater strength. Medium-hard-drawn copper can be used for distribu-tion lines usually for wire sizes smaller than No. 2. 

Aluminum Conductors Aluminum is widely used for distribution and transmission line conductors. Its conductivity, however, is only about two thirds that of copper. Compared with a copper wire of the same physical size, aluminum wire has 60 percent of the conductivity, 45 percent of the tensile strength, and 33 percent of the weight. The aluminum wire must be 100/ 60 = 1.66 times as large as the copper wire in cross section to have the same conductivity. When an aluminum conductor is stranded, the central strand is often made of steel that serves to reinforce the cable. Such reinforcement gives added strength for the weight of conductor. Reinforced aluminum cable called ACSR (aluminum-conductor steel-reinforced) is especially suited for long spans.

Copperweld Steel Conductors In this type of conductor, a protective copper coating is securely welded to the outside of the steel wire. The copper acts as a protective coating to the steel wire, thus giving the conductor the same life as if it were made of solid copper. At the same time, the layer of copper greatly increases the conductivity of the steel conductor, while the steel gives it greater strength. This combination produces a satisfactory yet inexpensive line conductor. Its chief field of application is for rural lines, for guy wires, and for overhead ground wires.

The conductivity of copper-weld conductors can be raised to any desired percentage, depending on the thickness of the copper layer. The usual values of conductivity of wires as manufactured are 30 and 40 percent. 

Classes of Conductors Conductors are classified as solid or stranded. A solid conductor is a single conductor of solid circular section. A stranded conductor is composed of a group of small conductors in common contact. A stranded conductor is used when the solid conductor is too large and not flexible enough to be handled readily. Large solid conductors are also easily damaged by bending.

The need for mechanical flexibility usually determines whether a solid or a stranded conductor is used, and the degree of flexibility is a function of the total number of strands. The strands in the stranded conductor are usually arranged in concentric layers about a central core. The smallest number of wires in a stranded conductor is three. The next number of strands are 7, 19, 37, 61, 91, 127, and so forth. Both copper and aluminum conductors may be stranded. 

Conductor Sizes Conductor sizes are ordinarily expressed by two different numbering methods: the American Wire Gauge (AWG) and the circular mil. The AWG con-ductor sizes are numbered from 30 to 1, then continuing with 0, 00, 000, and 0000 (or 1/ 0, 2/ 0, 3/ 0, and 4/ 0, respectively). Number 30 is the smallest size and 4/ 0 the largest in this system. As an example of the actual physical size of the conductors commonly used in transmission and distribution work, refer to figure 4-28. The circular mil is the unit customarily used in designating the cross-sectional area of wires. A "circular mil" is defined as the area of a circle having a diameter of 1/ 1000 of an inch. The circular mils of cross section in a wire are obtained by squaring the diameter expressed as thousandths of an inch. For example, a wire with a diameter of 0.102 inches (102 thousandths of an inch) has a circular mils cross section of 102 x 102 = 10,404. Conductors larger than 4/ 0 AWG are designated in circular mils. These range from 250,000 to 2,000,000 circular mils (250 MCM or 2,000 MCM).

DISTRIBUTION SYSTEMS A power distribution system is a system that delivers the energy from the generators or transmission lines to the customer. In the Seabees you will be mainly

Figure 4-28.- American wire sizes for bare copper. concerned with the construction, maintenance, and repair of the distribution system. Depending on the system, it will consist of a combination of the following components, substations, distribution transformers, distribution lines, secondary circuits, secondary service drops, and safety and switching equipment. The distribution system may be underground, overhead, or a combination of the two. 

DISTRIBUTION SUBSTATIONS Distribution substations change the transmission or generator voltage to a lower level, providing voltage sources for the distribution circuits supplying power to the customers.