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MOUNTING TRANSFORMERS ON THE POLE
Most transformers are fastened directly to the pole with bolts that run through the pole. When more than one large transformer must be used at a single location, a platform should be constructed to hold their weight. The hoisting of the transformers is done by means of block and tackle. One set of blocks is supported at the top of the pole, and the other is hitched to a rope fastened to the transformer itself. The pulling line runs through a snatch block tied to the pole near the ground. With the transformer hoisted to the proper position on the pole, it is then bolted to the pole, or a bracket is installed on the pole and the transformer is attached to the bracket. The method installation depends on the design of the transformer itself.
CONNECTING THE TRANSFORMER TO THE LINE
Figure 4.29 shows how a transformer is mounted on the pole and connected to the lines. Notice that a ground rod is driven at the base of the pole, and a ground wire is run up to the low voltage side (220 volt side) of the transformer. This is done to "ground" the transformer itself and also to provide a ground wire to run into the individual buildings. This ground rod should be a S/8" x 6 ft. long copper rod driven into solid ground until it is driven completely below ground level. The earth around the top of the ground rod is then removed so that six inches of rod is exposed. A wire clamp is then installed on the rod and the ground wire (running up the pole) is attached with this clamp to the ground rod. This ground wire should be no smaller than #2 wire. The rod is then recovered with earth. Fuses should be installed in the high voltage wires (440 volts) running down to the transformer to protect the transformer and the rest of the distribution system in case of electrical trouble. Selecting the proper fuses size is covered in the preceding chapter. A type of fuse used by many electric utilities is shown in Fig. 4.30, When the fuse link blows only the link needs to be replaced at very low cost. From the low voltage side of the transformer, wire is run to an insulator bracket mounted on a "crossarm" below the transformer. "Service drops" are then attached to these low voltage wires near the bracket and are then run into the buildings or houses. "Service c;,*ops" consist of two insulated wires wrapped around a bare "messenger wire". This type of wire is called self-supporting service drop cable. This bare wire is used to support the two insulated wires and also serves as the ground wire to the building.
DISTRIBUTION SYSTEM POWER SOURCE
There are two possible sources of power for a distribution system. Then system can be powered directly from a generating plant, or the system can receive its operating voltage from a substation of a transmission system. The installation and operation of a generating plant for a distribution system is beyond the scope of this manual. The attachment of a distribution system to a generating plant or d- substation is essentially the same. The connection should be made by extensively trained personnel only. The trainee who has constructed the distribution system needs to understand the connection. The considerations are outlined below. These are not all of the considerations, they will be left for the trained personnel responsible for the connection. These considerations will give the trainee the necessary understanding of the task.
SYSTEM POWER REQUIREMENTS
The power source must be able to supply the power demanded by the system. Therefore the demand of the system must be considered before the connection is made. If the demand is greater than the power source can supply, either an additional power source must be obtained or the loads on the system reduced until the power required is less than or equal to the power supplied.
There are two considerations to be considered in the connection of the power source.
A. Disconnection The system must be able to disconnect from the power source. The system cannot be maintained in case of failure unless the power can be positively removed from the system, by use of a switch. B. Over current Protection There must be over current protection to protect the power source from the damage that would result if more current was demanded than could be delivered. Also there must be protection of the distribution system so that it will be disconnected if it tries to carry more current than it is designed to carry.
(This page is based on information copied from Rural Electrification Systems prepared for the United States Peace Corps By: Volunteers in Technical Assistance, Inc. (VITA) 3706 Rhode Island Avenue Mt. Rainier., Maryland 20822 USA In accordance with Contract PC 251709 April, 1969.)