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Lifetime estimation of power cables insulations is a very important issue for development of a preventive maintenance policy of cables in order to avoid their premature breakdown and interruption of power supply to consumers. In this paper a lifetime estimation method of a low density polyethylene insulated cable is presented. For this purpose, accelerated tests on model cable samples immersed in a NaCl solution (having a concentration of 0.1 mole/l) and subjected to a sinusoidal voltage (U = 5 kV and variable frequencies between 1 and 5 kHz) for different ageing times τ (τ = 24...500 h) were performed. The water trees lengths were measured and the variation curves of maximum lengths 1wtm with the ageing time (1wtm (τ)) for three frequencies (1, 2 and 3 kHz) were drawn. Considering three end of life criteria (according to current standards), the lifetime lines were determined and the lifetime corresponding to an electric field of frequency 50 Hz was computed. Finally, it is shown that the tested polyethylene insulation is not recommended to be used for medium voltage underground cables.