Flashover and breakdown characteristics in low pressure environments

A present trend in aircraft design is the increasing use of 270-Vdc power circuitry. Also, significant voltage spikes may be superimposed on the dc level. In many situations involving unsealed equipment enclosures, electrical insulation is provided by a combination of gaseous medium and spacing between conductors. Consequently, flashover/breakdown characteristics of electrical wiring and printed-circuit-board traces depend upon the environment in which these components operate. In an aerospace environment, breakdown/flashover characteristics may change as air pressure changes due to an aircraft changing altitude. This is especially true for circuitry located outside of a pressurized environment. Prudent design of aerospace components, therefore, requires that conductor spacing be sufficient that gaseous breakdown be avoided for all pressures that may exist during aircraft operation. Unfortunately, existing standards and design guidelines may impose confusing design constraints. In the effort described here, a series of experiments in a low pressure chamber experimentally determined the breakdown characteristics for wire-plane and printed-circuit-board configuration as a function of pressure and electrode spacing. The pressure range of interest was that corresponding to ground level (101 kPa) and 30 km (1.1 kPa). The environments were both dry and humid air. Results indicate a limiting Paschen-like characteristic of voltage breakdown/flashover vs. pressure-spacing product. Results were compared to some standards that are often used in the design, manufacturing, and testing of aircraft components.