Insulation Reliability - Gaseous Insulation [Dakin's Corner]

If we were to ask the question, "What is the most commonly used insulation?" the answer would have to be, "Air - a gas." Its application is so common that we may sometimes overlook its importance as an insulation material or medium in so many places: overhead power lines, spacing in household electrical appliances, numerous types of electrical apparatus, etc. For each application, however, the limitation of the dielectric strength ofthe gas must be considered, and adequate gas spacing must be designed for the service voltage, gas pressure, system geometry, and expected transient overvoltages. As with all insulating materials, the gas breakdown voltage depends on the conductor (electrode) geometry, and it decreases with electric stress concentration at edges and points of small radius. It was found many years ago by Paschen that the breakdown voltage of a gas in a uniform electric field varied consistently with the product of the pressure and the spacing, independent of the separate value of each. This indicates that the amount ofgas (number ofmolecules) between the electrodes was the controlling factor. The author argues that the data represented by the Paschen curves can be interpreted in a different way. Instead of the usual curve, the KV/mm curve is calculated from the Paschen curve by dividing each value of the ordinate (KV) by the corresponding abcissa value (bar mm), assuming the pressure is 1 bar (1 atmosphere). This leads to a declining curve of breakdown stress (KV/mm) at 1 bar pressure.