An Acoustic Method to Measure the Length of an ESD Spark

For a given charge voltage, the spark length of an ESD will vary for approaching electrodes due to the interaction of the statistical time lag and the speed of approach. The reduction of the spark length can increase the peak value by a factor of 10 and the maximum current derivative by a factor of 1000. Therefore, spark length is an essential parameter to be captured in air discharge related research. Traditionally, spark lengths have been measured using mechanical methods. The paper presents a proof of concept for a purely acoustic method of measuring the spark length. A microphone picks up the spark noise and the impact noise to obtain the time difference. The velocity is measured by two alternative methods: By an ultrasonic pulse based distance sensor or by ultrasonic reflection based on the Doppler effect. The combined information allows the length of the spark to be estimated, assuming that the spark follows a straight path, which is a reasonable assumption at least up to 10 kV.