A new method of on line measuring ac resistance at the joint of high voltage transmission line and its practice of soft ware and hardware are described in this paper.

The explosion expanding is a tube to tube-sheet joint method that used powder or electric resistance silk(foil), which can produce ultra high pressure via explosion.

Consequently, pitting corrosion resistance of the welded joint declines obviously and pitting preferentially occurs at ferrite phase in the high temperature HAZ near the fusion line.