Ignition of explosive gas-air mixtures in HV machines

The author has undertaken a number of studies for the UK Health and Safety Executive (HSE) concerning the possibility of explosions occurring in HV machines when operating in hazardous areas. A study in 1994 included tests on a 420 kW, 6.6 kV, two-pole cage rotor induction motor. The motor was started and run on no-load with a gas/air mixture in the motor enclosure. The objective was to determine if ignition would occur. The test programme included starts and operation at a range of voltages up to the rated voltage, and with various gas/air mixtures, from methane/air to hydrogen/air. In none of these tests did an explosion occur. The report to the HSE included a number of recommendations for further study, including increasing the voltage between the stator winding and earth in the test motor and hence increasing the consequent discharge level until ignition occurred. This work was completed and reported to the HSE in 1996, and this paper is based on that study