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Under the action of an electric field and in the presence of water, water trees develop inside power cables insulations. Regardless of their type, shape or dimension, water trees contribute to the worsening of electrical properties, especially lowering the inception voltages of partial discharges and electrical trees leading to breakdown . In this paper an experimental study regarding the variation of permittivity and loss factor of the polyethylene with the frequency (fa) and the duration (τ) of applied electrical field is presented. The experiments were made on flat XLPE samples sliced from a power cables insulation. The samples were subjected to an electrical field with the intensity E = 4 kV/mm and frequencies fa = 3 and 5 kHz for τ = 48, 72 and 96 hours. After ageing the water trees dimensions (length lwt and diameter D) and concentration cwt were measured. The permittivity (εr) and the loss factor (tgδ) were measured (using a NOVOCONTROL dielectric spectrometer) at the temperature T = 30°C and frequency fm = 103--106 Hz. The results show that with the increase of ageing time or the ageing frequency all the quantities cwt lwt D, εr and tgδ increase, too.