The aim of this work is to propose a reliable approach to estimate values of the relaxation time constant of polymers at the glass transition temperature. This approach consists of using a new thermo stimulated depolarisation current TSDC protocol which analyzes the relaxation current kinetics obtained after submitting the sample to an electrical field shock at constant temperature. The measurements have been performed on a series of polyethylene terephthalate (PET), polycarbonate (PC) and polyethylene terephthalate glycol (PETg) samples. Different temperatures were needed to explore the range of the glass transition domain of each polymer. The results compared to calorimetric and dielectric data show a good agreement. So we confirm that the value τms = 100 s often used for characterizing the molecular dynamic of the liquid glass former at Tg is not universal.