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High-tension switchboards.

In power transmission plants, where the energy of waterfalls is transmitted to a long distance, by means of polyphase currents, very high e. m. fs. are used, ranging all the way from 10,000 to 40,000 volts, and the switchboards made for such installations have to be arranged so as to safely withstand such pressure. The switches and circuit breakers used are of special construction so as to produce a long break when they are opened. In addition, these devices are separated from each other by barriers made of suitable insulating material, so that the arc produced on opening the circuit may not jump across from one to the other. The type of switchboards used for such purposes is well illustrated in Figs. 82 and 83, which show the boards at a receiving station at the end of a transmission line. The line current which is impelled by an e. m. f. of 11,000 volts is received in the board, Fig. 82, and from here passes to the primary coils of transformers, which are designed so as to develop secondary currents of 2,200 volts. These secondary currents are conveyed to board, Fig. 83, and from there pass to the distributing circuits, or to other transformers, where they are again reduced and rectified.

The switchboard, Fig. 82, handles the high-tension transmission currents that come from the power station, It consists of three panels, each one made of two marble slabs placed one behind the other. The switching is accomplished by means of plugs and sockets in a manner similar to that employed in the arc light boards already explained. The terminals connected by the plugs are mounted on the marble slabs, the latter being separated

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High tension Switchboards.

by a space of about two feet. There are two incoming three-phase lines, and three groups of three-phase transformers, and the arrangement of the sockets on the switchboard panels is such that either one of the incoming lines can be connected with any one of the groups of transformers. The incoming lines are provided with circuit breakers that have copper primary, and carbon final breaking contacts, both of which are mounted on the end of vertical arms about five feet long which when released swing out to the horizontal position, making an opening of about six feet. One of these circuit breakers is seen at the top of Fig. 82. The three arms are mounted on a single shaft, being strongly insulated from each other, and are arranged so that they may be operated independently or all together. They are operated by means of elctro-magnets, and the lever and connecting rod on the right are used to close the three arms at once, while the smaller lever higher up is for the purpose of opening them simultaneously.

The board, Fig. 83, handles the secondary current from the three-phase groups of transformers that are connected with the high-tension board, Fig. 82. This board has three panels from which three outgoing currents are controlled. Each panel is provided with circuit breakers similar to the high-tension design but of smaller size, the separation of the contact points being about one foot and a half. In addition to the circuit breakers, each panel contain three ammeters, one voltmeter, arranged to be connected across any of the three phases, two intergrating wattmeters, to measure the power passing to the outgoing circuits, and three main switches, the blades of which are protected by hard rubber shields so that they

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