The electrical breakdown and prebreakdown characteristics of uniform fields in compressed gases are described. The failure of Paschen's law and the dependence of the breakdown voltage on the material, preparation and area of the electrodes, on the dust content of the gas, and on the type of applied voltage are included. The failure of the Townsend and streamer mechanisms of low-pressure breakdown is discussed, and a summary is made of the many suggested breakdown mechanisms for compressed gases. These are divided into three groups: breakdown due only to field emission of electrons, breakdown due to electron-field emission and space charge in the gas, and breakdown due to electron-field emission and charge collection on an electrode. The breakdown and prebreakdown characteristics are interpreted in terms of the various breakdown mechanisms, and the areas indicated where further investigations are needed. The surface flashover of insulating spacers in compressed gases is also briefly reviewed. '

Abstruct-High-voltage breakdown measurements were made in two similar particle contaminated coaxial test systems, one with AC and the other with DC voltages. Information is presented on the effects of particle size, shape, and material for both SF6 and N2 gases at pressures up to 15 atm in a plain coaxial gap and a coaxial gap including a posttype support spacer. Particle motion and location were found to strong ly influence insulation performance. Measured values of electric fields which lifted and drove the particles, so that they bounced vertically and laterally, compare favorably with calculated levels. Movement into the the higher stress region at the center conductor was correlated with the initiation of sparkover. These breakdowns could be at levels more than a factor of five lower than those obtained when contamination was not introduced. Large variations in breakdown voltage of as much as 3 to 1 encountered under DC correspond to conditions where particle motion could be restricted, presumably by corona discharge, to motion near the outer electrode. AC sparkover levels were typically at the lower limits of the DC range. Both free and attached particles on the dielectric spacer surface would trigger flashover at the same low levels as were measured in the gas gap.

In the past ten years there have been significant advances in the theoretical and experimental analysis of high-voltage gas breakdown and surface flashover of insulators in compressed gases. This has probably been fostered by the recent growth in the design and application of gas-insulated high-voltage equipment. The review describes the characteristics of compressed-gas breakdown, including the effects of failure of Paschen's law; conditioning; electrode area; material and surface; breakdown-voltage distribution; particle contamination; voltage waveform; temperature; and gas mixtures. The insulator-flashover characteristics are then described, including the effects of insulator-electrode interface; insulator material; insulator shape; voltage waveform; charge generation; particle contamination; surface contamination; conditioning; flashover distribution; and dependence on type of gas. The various mechanisms proposed for gas breakdown and insulator flashover are reviewed and discussed in relation to the experimental characteristics. Future theoretical and experimental work is suggested to clarify the gas-breakdown and insulator-flashover mechanisms, and which would also help bring about the design of improved high-voltage gas-insulated systems.

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