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This paper describes the important aspects that diagnostics play in reducing “customer minutes lost” in medium-voltage networks. The strategic approach of different diagnostic methods e.g. local methods like PD measurements at Damped AC voltages, integral methods like RVM (Return Voltage Measurements), IRC (Isothermal Relaxation Current) and tan δ will be discussed. All of these diagnostic methods have the same goal; assessing the condition of cable insulation and cable accessories of aged and newly installed medium-voltage power cables. The main difficulty is to interpret the diagnostic data and transform them into knowledge rules to make reliable decisions. Based on field experiences from several countries a lot of practical data has been gained and transformed into knowledge rules which can support the asset manager's decision process.