General expression for positive switching impulse strength valid up to extra long air gaps

A new algebraic expression has been established for the critical sparkover voltage (CSOV) afair insulation submitted to positive switching impulses. By using the concept of the gap factor, the expression is found to be valid for all practical gap configurations in the distance range 1 to at least 30 m. Based on these considerations, an absolute limit of 2.4 MV is foreseen for the rated voltage of AC systems. Sparkover voltages under A.C. or lightning impulses are also predictable. Complementary investigations concerning the physics of discharge are needed for further improvements.