Distribution Cutouts, Switches, Reclosers, and Circuit Breakers

A distribution cutout is used to protect the distribution system or the equipment connected to it. Distribution cutouts are used with the installation of transformers (fig. 2-6), capacitors, cable circuits, and at sectionalizing points on overhead circuits. Two types of switches used in power distribution are the air switch and the oil switch. Both devices are used to connect or disconnect a portion of the power distribution system. The air switch is used for the overhead section of the distribution system, and the oil switch is used with underground portions. Reclosers are for overload protection and are designed to open a circuit in an overload condition and then automatically reclose the circuit. Reclosers come in single-or three-phase models and can either be pole mounted or installed in a substation.

Oil, air, gas, and vacuum circuit breakers are used to switch electric circuits and equipment in and out of the system. They may be operated manually, by remote control, or automatically under predetermined conditions or when electrical failures in the system occur.

The purpose of installing a lightning arrester (fig. 2-6) on primary lines is twofold: first, to provide a point in the circuit at which a lightning impulse can pass to earth, through a ground wire, without injuring line insulators, transformers, or other connected equipment; and second to prevent any follow-up power current from flowing to ground Lightning arresters must be installed on the primary side of all substations, distribution centers, distribution transformers, and capacitor banks.

WOODEN POLES. Wood poles are available in various types, depending upon species of trees available in the area. For example, yellow pine is commonly used in the eastern United States. The length and circumference of poles also vary. Poles are available in 5-foot incremental lengths and with top circumferences varying in 2-inch increments. Therefore, we have poles that 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 is determined from the circumference of the pole measured at a point 6 feet up from the butt. The class determines the strength of the pole, which is the ability of a pole to resist loads applied 2 feet from the top of the pole. Pole classes are numbered from 1 to 10, with 1 being the strongest. A Class 2 pole, for example, will withstand a force of 3,700 pounds and a Class 4 pole will withstand 2,400) pounds of force.

Wood poles are used mostly in distribution systems and light-duty transmission lines. The class of pole used depends on what the pole is used for. In other words, is the pole to be used as a line pole, corner pole, or transformer pole? The length of pole used is determined, in part, by the clearances required for the voltage of the circuits on the poles, the number of circuits, and the location of the pole in relation to streets, railroads, buildings, and so forth. Clearances are also required to provide safe working conditions for linemen working on the lines. All clearances have minimum requirements that are set by the American National Standards Institute (ANSI) and the National Fire Protection Association (NFPA). These requirements are specified in the National Electrical Safety Code (NESC), ANSI C2-87, and the most recent edition of the National Electrical Code (NEC ).

Engineers also consider local conditions when determining the length of poles. For example, poles located in densely popular high-traffic areas need to be higher than those located in sparsely populated rural areas. In the Navy, the MINIMUM height of a wooden transformer pole is 35 feet and of all other wood poles, 30 feet. Other guidance regarding the heights and classes of poles is found in Power Distribution Systems, MIL-HDBK-1004/2.

CONCRETE POLES. Concrete poles are preferred where the life of wood poles is shortened by local conditions. Concrete poles may be solid or hollow. Solid concrete poles are made in a trough form with steel reinforcing rods running lengthwise. The hollow type of pole is made by placing the concrete and reinforcing rods into a cylinder of the desired length and taper and then revolving the cylinder in a lathelike machine. The hollow type is lighter than the solid type and, in addition, provides a means for making connections through the pole to underground cables or services. This technique allows wires to be concealed from view and protected from the weather.

The exterior form of concrete poles can be changed to meet almost any need. Gains (cut notches) for crossarms and holes for bolts are cast in the pole. Either metal pole steps are solidly cast into the pole or prethreaded holes for the steps are installed. Although concrete poles last longer and are stronger than wood poles, they are also expensive to make and install. However, the rising cost of wood poles and their treatment and maintenance plus better landscaping have brought on an increased use of concrete poles.

METAL POLES. Metal poles used in the Navy are either steel or aluminum. Steel poles are not used in ordinary power-line distribution circuits except for unusual circumstances, such as where there is a high stress or heavy load placed on the pole. Aluminum poles are used for lightweight distribution, such as street-lights.