This paper gives the definition of correlated software defects, gets some property of correlated defects by experiment, and then proposes an approach named testing approach with defects replacement, to detect correlated defects.

There are many reasons for undetected defect such as unsuitable parameter selection of probe, the influence of interference wave and the unclear judgment of defect echo and burr echo.

The paper discusses the defect echo feature when applying different K value s probe detects different depth artificial reflectors in ultrasonic detection,and makes comparison among them,thus puts forward the significance of applying different K value probe on different detected parts,meanwhile it provides theoretical basis for practical detection work.

The process of Xingren-Baoan HVDC fault on May 5,2008 is introduced and the behavior of DC protection is analyzed in detail by using digital wave replay system.

According to the principle of the ultrasonic pulse echo method used by JTUIS (Jiao Tong University Ultrasonic Imaging System),the cause that thin electric switch contact brazed quality cannot be well evaluated using the ultrasonic flaw echo testing method is researched.