The present invention relates to a method of determining a reduction of remaining service lifetime of an electrical device during a specific time period. A measurement system is provided comprising a temperature measurement device, a current measurement device and a voltage measurement device. A temperature value, voltage values and current values are measured by using the measurement device. A harmonic load is determined based on the current values. A reduced maximum operating temperature is determined based on the harmonic load. An amount of transient over-voltages is determined based on the voltage values. A transient aging factor is determined based on the amount of transient over-voltages. A temperature dependent aging factor is determined based on the temperature value and the reduced maximum operating temperature. Finally, the reduction of remaining service life is determined based on the specific time period, the transient aging factor and the temperature dependent aging factor.