A laboratory study on the dielectric strength of air gaps with lower oxygen constituent under switching impulse voltage

Inside a building, the risk of fire maybe reduced when oxygen constituent of air decreased. However, the dielectric strength of air may also be changed. To study the dielectric strength of air with a lower constituent of oxygen (14.6%), in comparison with ordinary air with 20.9% oxygen, laboratory tests have been performed. Tests were first carried out over a short rod-plane gap, 100 mm, inside a gas test chamber. By flowing in nitrogen the oxygen constituent could be decreased inside the chamber. Test voltage was standard switching impulse of positive polarity. Through these tests, comparisons could be made between atmospheric air and synthetic air with and without flowing in more nitrogen. The oxygen level was measured to 20.9 % as in the natural conditions without adding nitrogen. The oxygen level was controlled to be at 14.6% after nitrogen was added. No significant difference in dielectric strength was observed between atmospheric air and synthetic air. However, a clear reduction in dielectric strength is observed with the lowered oxygen level for both atmospheric air and synthetic air. The study was then carried out by testing longer gaps. Air gaps with spacings up to 2 meters were tested inside a plastic enclosure. Nitrogen was flowed into this enclosure to decrease the oxygen constituent of ordinary air in room temperature. These tests were also performed with both oxygen levels of 20.9% and 14.6%. The gap geometries tested included rod-plane, sphere-plane, and toroid-plane gaps of different spacing's. It was observed that the degree of the influence on the dielectric strength of air gaps with different oxygen constituent are related to both the gap spacing and the gap geometry. The influence seems smaller for gaps of a longer spacing and for gaps with higher uniformity. In this paper, the observations and results from this laboratory study are presented.