Study of Electric Transients in the Vacuum Switch by Means of the Cathode Ray Oscillograph

The opening of an electric circuit by means of a vacuum switch is a complex performance which is analyzed into a sequence of stages according to the phenomena involved. First, the voltage difference between the opening contacts will increase more rapidly than the electrode spacing, producing successive discharges, which physically are field currents. If the energy delivered is sufficient, an arc is formed; if not, the transients persist for a time. The arc, if formed, continues until a minimum current is reached, when the current suddenly drops to zero. Immediately before and just after this drop occurs, the current is unsteady, many surges being superimposed on the otherwise simple characteristic. The sudden drop to zero current induces a damped sinusoidal voltage oscillation of appreciable magnitude in the external circuit. Such an oscillation of small magnitude follows the field currents when the interrupted current is too small to form an arc. As can be seen, the circuit is normally cleared in the first half cycle. Hayward’s work disclosed, but left incomplete, the question of an apparent power storage in the arc. This thesis presents considerable new data from which it appears we may conclude that there is no storage of energy in an arc such as appeared to be the case. While several hypotheses may explain why Hayward’s results were not duplicated, it is probable that his current measurement was erroneous, due to a time lag in his current coil deflections. While the equipment available did not permit the determination of the kva. limit of the vacuum switch, the tests made indicated that it will handle its tasks well.