In these electrically operated oil switches a small motor is located on top of the brick cell that contains the contact parts, and this motor releases and compresses springs that open and close the switch. While it is not desirable to employ open-air switches to open circuits of several thousand or even hundreds of kilowatts at voltages of 2,000 or more, it is nevertheless possible to do so. This is shown by the experience of the first Niagara Falls station, where the 2,200-volt two-phase switches are reported to have opened repeatedly currents of more than 600 amperes per phase without injurious sparking. The great rise of voltage that was shown by the experiments at Kalamazoo to follow the opening of a simple open-air switch was avoided at the first Niagara switches by a simple expedient. In these 5,000 horse-power open-air switches a shunt of high resistance was so connected between each pair of contacts that the blades and jaws that carried the main body of the current never completely opened the circuit. When the main jaws of one of these switches were opened the shunt resistance continued in circuit until subsequently broken at auxiliary terminals. That no excessive rise of voltage took place when one of these switches was open was shown by connecting two sharp terminals in parallel with the switch and by adjusting these terminals to a certain distance apart. Had the voltage risen on opening the switch above the predetermined amount there would have been an arc formed by a spark jumping the distance between the pointed terminals.
Fig. 58.—Schenectady Switch-house on Spier Falls Line.
Fig. 59.—Second-floor Plan of Saratoga Switch-house on Spier Falls Line.
Safety and reliability of operation at high voltages, say of 5,000 or more, require that each element of the equipment be so isolated as well as insulated from every other element that the failure or even destruction of one element will not seriously endanger the others. With this end in view the cables from each generator to its switch should be laid in a conduit of brick or concrete that contains no other cables. The brick or stone compartment for each phase of each switch should be so substantial that the contacts of that phase may arc to destruction without injury to the contacts of another phase. Bus-bars, like switches, should be removed from the operating switchboard, because an arc between them might destroy other apparatus thereon, and even the board itself. It is not enough to remove bus-bars from the switchboard where very high voltages are to be controlled, but each bar should be located in a separate brick compartment so that an arc cannot be started by accidental contact between two or more of the bars. It is convenient to have the brick and stone compartments for bus-bars built horizontally one above the other. The top and bottom of each compartment may conveniently be formed of stone slabs with brick piers on one side and a continuous brick wall on the other to hold the stone slabs in position. Connections to the bus-bars should pass through the continuous brick wall that forms what may be termed the back of the compartments. To close the openings between the brick piers at the front of the compartments movable slabs of stone may be used. Feeders passing away from the bus-bars, like dynamo cables running to these bars, should not be grouped close together in a single compartment, but each cable or circuit should be laid in a separate fireproof conduit to the point where it passes out of the station.
Fig. 60.—Ground Floor of Saratoga Switch-house.
The folly of grouping a large number of feeders that transmit great powers together in a single combustible compartment was well illustrated by the accident that destroyed the cables that connected the first Niagara power-station with the transformer-house on January 29th, 1903. On the evening of that day lightning short-circuited one of the cables in the short bridge that connects No. 1 station with the transformer-house, and all the cables in this bridge, supplying local consumers as well as railways and lighting in Buffalo, were destroyed. This bridge contained probably more than thirty-six cables, as that number of new cables was put in position within twenty-four hours after the accident, and these cables, covered with inflammable insulation, were close together. The result was not only the loss of the cables, but also the damage to power users. If these cables had been located in separate fire-proof conduits, it is highly probable that only the one directly affected by lightning would have been destroyed.