The Effect of Transient Voltages on Dielectrics-IV Law of Impulse Spark-over and Time Lag

The law of time lag and impulse breakdown is derived from experimental data. This law, which appears to be rational, shows that a given amount of energy is required to cause. breakdown of a given gap and that voltage and time are interdependent. It is quite in accord with the work in the original paper and applies to breakdown on rectangular waves, on the front of slanting waves, or to overvoltages on standard waves.¹ By means of simple formulas the results for any wave can be calculated or the effects of different waves correlated. Three general types of impulse tests, dependent on the form of wave used and the spark-over point selected, are discussed as follows: 1. The so-called "rectangular" wave with spark-over on the top 2. The uniformly rising voltage with spark-over on fronts of various slopes 3. The use of a "standard" wave of logarithmic front and tail with spark-over occurring at any desired point on it. Method 3, using the full standard wave, is by far the simplest to apply and gives the most consistent results. It is also shown that it may be used in a way to give effects equivalent to the other two methods. Interesting energy relationships for impulse failures of gaps are brought out in this connection. Field experience with actual lightning waves is compared to laboratory results with the "standard" wave and found to check very closely.