Helium is of particular interest as a basic insulating agent in the development of superconducting electrical power equipment. This is due to the fact that contrary to the low electrical breakdown strength under ordinary conditions helium shows satisfactory breakdown strength at very low temperatures where its density is high. In fact the breakdown strength of helium in suitable states is superior to that of good transformer oil. In the paper it is shown that in agreement with breakdown theories of gases the breakdown strength of gaseous helium is determined mainly by the gas density. On the other hand the breakdown strength of liquid helium at pressures above 1 bar is determined mainly by the helium pressure. The phenomenon may be explained by the formation of gas bubbles produced by moving ions in the liquid leading to electrical breakdown. © 1972.
Gerhold, J. (1972). Dielectric breakdown of helium at low temperatures. Cryogenics, 12(5), 377–384. https://doi.org/10.1016/0011-2275(72)90113-0