Fig. 2,352.—General Electric 2,200 volt arrester in the act of discharging, and shunting the line current. The figure shows an actual discharge taking place. It will be seen that the heavy line current passes across only four of the gaps, and then goes through the resistance rods; while the static discharge passes straight across the entire series of thirteen gaps. When the gaps of an arrester are shunted by even a low resistance, discharges of very high frequency find it relatively difficult to pass through the resistance rods, owing to the impedance of the rods, but comparatively easy to pass across all the gaps, owing to the capacity effect in breaking down the gaps. The higher the frequency, the more pronounced is this effect, hence the discharges select different paths through gaps and resistances depending upon the frequency. By frequency is meant, not the frequency of the line current but the lightning frequency, which may run into hundreds of thousands, or into millions of cycles. The equivalent needle gap for this arrester is shown by tests to be nearly the same for all frequencies and quantities of discharge; that is, the arrester is equally responsive to all frequencies.
Figs. 2,353 to 2,355.—Oscillograph record of the phenomena that take place in the different circuits or selective paths of a multi-gap arrester during a discharge such as shown in fig. 2,352.
As the spark crosses each successive gap, the voltage gradient along the remainder readjusts itself.
How the Arc is Extinguished.—When the sparks extend across all the gaps the line current will follow if, at that instant, the line pressure be sufficient. On account of the relatively greater line current, the distribution of pressure along the gaps becomes equal, and has the value necessary to maintain the line current arc on a gap.
The line current continues to flow until the voltage of the generator passes through zero to the next half cycle, when the arc extinguishing quality of the metal cylinders comes into action.
