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Partial discharge (PD) testing in a low pressure environment is a diagnostic method for evaluating the integrity of electrical insulation in aircraft and spacecraft capable of operating at high altitudes. This evaluation is essential in order to ensure the reliable performance and proper functioning of power system components of aerospace equipment under operating conditions. The paper outlines an experimental PD investigation program covering various electrode systems, using air, argon and helium, at pressures in the range of 101.3 kPa (760 Torr) to 0.27 kPa (2 Torr) with 60 Hz ac energization. This pressure range corresponds to a range of flight altitudes from sea level to about 60 km. At low pressures (high altitudes) PD pulse characteristics are different compared to the values typically encountered at terrestrial pressures. In most instances, these sub-atmospheric characteristics imply a need for changes of commonly used test procedures. The paper suggests modifications and/or additions to common test procedures while observing the basic principles of the IEC 60270 standard.