Discussion on “mine duty controllers” (Reed), Pittsburgh, Pa., April 9, 1914. (see proceedings for April, 1914)

F. L. Stone: There is one line of control on which Mr. Reed has touched very lightly. That is the handling of 2200 volts. This voltage is coming more in use every day, and must be controlled in a safe and efficient manner. The use of oil-immersed switches was the first method tried, but where the operation is frequent and the service severe, as in the case of the mine hoist, either of the slope or shaft type, it does not work out as well as might be hoped. The frequent rupturing of the arc carbonizes the oil, and will eventually cause an are-over. This comes without any warning to the operator, and usually puts the switch out of commission. Another cause of damage to oil-immersed switches when operated frequently is, that a body of gas will form on top of the oil and finally a hot bubble will arise, and if the proper proportions of air are present a violent explosion will occur, this frequently wrecking the tank, and putting the switch out of commission until repairs can be made. The ordinary oil switch such as used on a switchboard for control of various circuits may be called upon to operate two or three times a day. A switch to control a hoist operating two trips a minute will operate approximately 1000 times a day in eight hours. In the first case the oil-immersed switch is entirely satisfactory, while in the latter case such information as I have been able to gather seems to indicate that it is entirely unsatisfactory.