Sixty-cycle and impulse spark-over of large gaps

TESTS AT 60-cycle frequency were made by using five 350-kv testing transformers in cascade connection. The rated voltage at this connection is 1,750 kv. However, the useful test voltage was limited to 1,360 kv rms by clearances to ground of 24 feet. The 5-unit cascade is shown in Figure 1, as used for the calibration of a 200-centimeter sphere gap. The 60-cycle calibration of the sphere gap was extended to 88-centimeter spacing and checked within 2.5 per cent of the negative impulse values of this sphere gap. The output voltage of the cascade connection was measured by reading the voltmeter coil voltages of the individual units and adding them arithmetically. The output voltage of a cascade connection of five units is very sensitive to relatively slight changes in load capacitance.