Comparison of standards requirements for clearances in aerospace

Abstract

High voltage electrical equipment used on power transmission and distribution systems is usually tested to ensure the insulation system is robust and not likely to suffer from electrical discharges, which if not avoided, could cause damage. Testing of high voltage equipment used in ground based power systems is reasonably straight forward and well documented in numerous standards. In comparison, the design and testing of high voltage equipment for use in aerospace applications is not well defined as only now are aerospace power systems beginning to operate at voltage levels likely to cause partial or disruptive discharges. The complication of designing and testing aerospace systems largely stem from the different atmospheric conditions that they are exposed to. The term `high voltage' is used loosely in aerospace, as the voltages that are classified as high voltage for aerospace purposes might not necessarily be classified as high voltage on systems used on land. However, it is certain that the use of equipment in a lower pressure environment leads to problems at lower voltage levels than is expected in conventional power systems. In addition, weight and volume are not drivers for power system component design in the same that they are for components used in aerospace applications. There is a call for reduction in both weight and volume while components have to be able to perform in harsh thermal, mechanical and environmental conditions while maintaining an excellent reliability. This paper summarizes the conflicting guidance that exists to allow the specification of safe clearances in aerospace systems. Existing standards and literature are reviewed. Several areas relating to the selection of safe clearances are analyzed and compared (pressure, temperature, frequency, et.) and a method for calculating the clearance d