The function of the serving and the necessity for clearing any defects are first considered. Field experience shows that insulation tests alone are not sufficient to indicate the presence of certain types of imperfections. The reason is given for selecting 10 kV d.c. as a standard test voltage for new cable.There follows a description of the terminal tests generally used for the localization of faults. Various types of confirmatory tests are then described, these being made at the suspected position indicated by the preliminary tests.Finally, the paper describes in detail new equipment designed specifically for the localization of serving defects.