Underground cables have a much more expensive investment value for both installation and maintenance. The high cost of underground cable installation is a reflection of the high costs of equipment, labor, manufacturing time, dredging and storage using back fill and in the installation of the cable itself. Because the price is expensive, the installation of high voltage ground cables is installed in urban areas, power plants and substations. Large costs associated with the installation and also need attention to the use and selection of types and sizes to serve the installation burden. In this study the author has the aim to be able to compare the leakage current from the calculation results with the results at the time of testing and will be adjusted to the IEC standards that have been set. In this thesis study discusses the Performance Study of Connected Underground Cables in the 150 kV Gejayan Substation. In this final assignment study comparing the value of leakage current test results directly with the calculation using thermal failure to be able to determine the feasibility of the components contained in it such as cables and arresters so that when damage occurs it can be addressed early, the results of this final assignment have been get the results by measuring the direct measurement of the leakage current found in T phase arresters in the joint box 1 the leakage value is 0.3 mA and when the calculation uses the thermal failure method the result is 0.45 mA and from the test and measurement the difference in leakage current value is 15%. And from the results of the measurement of the value of the leakage current it meets the IEC standard because the value of the leakage current is still below 1 mA.