In this paper we propose a new method of data analysis to interpret results obtained by Thermally Stimulated Depolarisation Current (TSDC) measurements on glassy materials. We give a new law to link the evolution of the relaxation time τ with the temperature T. While the classical law (compensation law) is only convenient in a small range of temperatures (around the compensation temperature Tc), a calculation of the depolarisation current, taking into account this new law, leads to a curve I vs. T very close to the real depolarisation current measured on a polymer in the whole range of temperatures. In addition this new law allows the determination of the compensation temperature Tc from the complex spectrum which needs single scan of measurements. To our knowledge it is the first time that this determination is proposed from the complex spectrum.