This report presents a study carried out in the field of steep fronted impulse wave. The survey studied SFa behaviour by means of two test arrangements employing a coaxial electrode the geometry of which is representative of a busbar set in a metal clad substaion. The tests were performed with fast transient waves （20 to 90ns） and lighting impulse （1.2/50μs ） in the case of a faultless geometry and with a peak type fault on the central conductor. These tests permitted the tracing of breakdown voltage-time cuves in the SF6 for SF6 absolute pressure of O.lMPa. The results demonstrate that fast transient waves are less constraining than the lighting impulses.