Pressure testing flameproof equipment intended for extremely low temperatures

International requirements for flameproof equipment are contained in International Electrotechnical Commission standard IEC 60079-1. An important test for flameproof equipment is the overpressure test. This is normally based on pressure determination tests which involve measuring the maximum pressure from a number of explosions using a representative gas/air mixture. The highest figure pressure obtained is then multiplied by a factor to obtain the pressure figure to be used for the overpressure test. It is known that the pressures obtained during pressure determination tests can be expected to rise as the temperatures drop. If temperatures drop as far as −60 °C this increase becomes significant. The standard provides several options for overpressure testing of equipment that is intended for use in temperatures below −20 °C. But for some equipment, such as motors, where pressure piling is possible, the options are restricted to two. One of the options that can be applied to any equipment is that the reference pressure can be determined at normal ambient temperature using the defined test mixture, but at increased pressure. The absolute pressure of the test mixture (P), in kPa, is calculated by a formula given in the standard. However, there is a lack of published data that supports this approach, in particular where pressure piling might occur. This could particularly be an issue for motors. This paper initially examines data from tests done some years ago with gases at extremely low temperatures. It then reports on recent tests using an enclosure producing pressure piling and the correlation of the pressures produced using the formula and those produced at actual temperatures. The paper concludes with recommendations for the next edition of IEC 60079-1 in regard to the application of this approach for testing equipment intended for extremely low temperatures.