PD Doctor brings precise and reliable diagnostics and online monitoring of isolation conditions at medium (and low) voltage lines and devices in transformer and switchboard stations.It increases the reliability of the power supply, as it detects the power line damage, and immediately identifies and reports operational states and anomalies. The system saves online data and plays the role of a black box, where it is possible to analyze the history of operational states, including incidents just right prior to the failure.Partial discharge online monitoring enables to observe of the degradation process of power lines and devices and indicates the necessity of intervention for better repair planning. PD Doctor allows to decrease actually necessary prophylactics costs by thousands of USD annually and lowers field visits.PD Doctor helps to protect every electrical device – generators, electric motors, transformers, cable lines, overhead cables, switchboards and switchgears. Online measuring significantly decreases costs of new devices installation, as it allows to use a technical lifetime of devices and lines maximally. They can thus be safely and reliably operated until the physical (and not only theoretical, and statistical) end of their lifetime.