Circuit for Impulse Testing of Gas-Tube Lightning Arresters

Telephone lines may be protected from lightning-induced surges by gas-tube lightning arresters. Typical arresters break down at 250-400 V dc. Breakdown voltage increases with the rate of rise of the applied voltage and may be two or three times the dc value at a rate of rise of 10 kV/μs. Lightning surges on telephone lines approach this rate of rise. The circuit described in this paper generates and measures ramps with linear rates of rise from 0.5 to 10 kV/μs chopped at 500-1500 V. In order to achieve ramp linearity and to minimize oscillations after the chop, stray capacitances and inductances must be made as small as possible. Construction and layout required to achieve this are described in some detail and typical test oscillograms at 0.5 and 10 kV/μs are presented in corroboration. Measurement accuracy is analyzed and maximum errors are estimated to be, for breakdown voltage, 3 and 10 percent at 0.5 and 10 kV/μs, respectively; and for rate of rise, 5 percent for all ramps. If greater accuracy is required, it can be obtained by correction for divider time constant errors and by further study of carbon resistor voltage coefficients.