A method of determining a reduction of remaining service lifetime of an electrical device during a specific time period of operation of said electrical device, said electrical device being connected to a power distribution network, said electrical device having a maximum operating temperature and a maximum operating voltage, said method comprising the steps of: providing a measurement system comprising a temperature measurement device, a current measurement device and a voltage measurement device, measuring a temperature value corresponding to the temperature of said electrical device during said specific time period by using said temperature measurement device, measuring a set of voltage values corresponding to the voltage applied to said electrical device by said power distribution network during said specific time period by using said voltage measurement device, measuring a set of current values corresponding to the current applied to said electrical device by said power distribution network during said specific time period by using said current measurement device, determining an amount of harmonic load on said electrical device on the basis of said set of current values, determining a reduced maximum operating temperature of said electrical device on the basis of said amount of harmonic load and said maximum operating temperature, determining an amount of transient over-voltages on said power distribution network at said electrical device on the basis of said set of voltage values and said maximum operating voltage, determining a transient aging factor corresponding to the relation between said amount of transient over-voltages and said set of voltage values, determining a temperature dependent aging factor on the basis of said temperature value and said reduced maximum operating temperature, and determining said reduction of remaining service life on the basis of said specific time period, said transient aging factor and said temperature dependent aging factor. providing a measurement system comprising a temperature measurement device, a current measurement device and a voltage measurement device, measuring a temperature value corresponding to the temperature of said electrical device during said specific time period by using said temperature measurement device, measuring a set of voltage values corresponding to the voltage applied to said electrical device by said power distribution network during said specific time period by using said voltage measurement device, measuring a set of current values corresponding to the current applied to said electrical device by said power distribution network during said specific time period by using said current measurement device, determining an amount of harmonic load on said electrical device on the basis of said set of current values, determining a reduced maximum operating temperature of said electrical device on the basis of said amount of harmonic load and said maximum operating temperature, determining an amount of transient over-voltages on said power distribution network at said electrical device on the basis of said set of voltage values and said maximum operating voltage, determining a transient aging factor corresponding to the relation between said amount of transient over-voltages and said set of voltage values, determining a temperature dependent aging factor on the basis of said temperature value and said reduced maximum operating temperature, and determining said reduction of remaining service life on the basis of said specific time period, said transient aging factor and said temperature dependent aging factor.