The aim of this paper is to show that Tan delta HFAC technique is applicable for longer cables. As underground cables are in service for years, it is necessary to carry out diagnostic test to rectify the possible problem that may occur in a cable so that precautionary action can be taken to avoid unnecessary in-service failure of the cable. One of the diagnostic testing techniques, known as Tangent Delta, is a method of testing cables to define the insulation condition of the underground cable. The phase shifts caused by the lossy current due to the existence of impurities in cable insulation is measured in this method. As most of distribution cables are long lengths, it becomes necessary to measure the tangent delta for these longer cables. In this paper, HFAC technique on longer length of good condition 11 KV power cables were used as samples to determine the tangent delta. Higher frequencies ( >100 Hz) were used to determine the tangent delta of cables at voltage of 1kV. The current (I) and phase theta were obtained in this experiment using HFAC measurement technique. CST Cable Suite software was used for simulating the experiment to verify the HFAC experimental setup by attaining sufficient dielectric current obtained through simulation and compare the leakage current values with the experimental values. The tangent delta results were attained for good cable with the length of 40 m, 70 m and 140 m and compared with 20 m length of good condition cable. The tangent delta results show that the HFAC technique is also applicable for the longer cable as well.