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Incipient faults detection on underground power cables mitigate the high cost of reparation and the economic losses caused by the interruption in the supply of electrical energy if a fault occurs. On the other hand, with the ability to detect incipient flaws it is possible to establish a program of corrective actions in a scheduled way, which reduces extra cost and prevents overtime while accelerates repairs. Partial discharges (PD's) measurement is a tool of great utility for underground cables on-site diagnosis that we apply extensively together with the power supplied by a portable resonant power supply. To realize this measurement one needs a resonant system with the reactive power necessary to evaluate installed cables lengths to its working voltage. These resonant systems are designed to tune in resonance with the cable capacitancia, according to its length. The application of this diagnostic technique on-site represents a challenge for the testing engineer because it has to face such problems like electromagnetic interference that affects measurement sensibility, inadequate ground systems and do forth. Acquired experience on underground cable diagnostics has allowed us to correlate PD plots to different deterioration mechanisms. On the other hand, cable diagnosis requirement allowed us to develop resonant systems that fulfill the requested conditions of power and voltage. In this article, some design and testing experiences are presented for cables rating from distribution 15 kV to transmission level 161 kV.