The electric conductivity σ in highly insulating materials is determined by the equilibrium thermally generated carriers and by the injected carriers. The injected excess electrons will dominate the thermally generated electrons when the total number of injected electrons substantially exceeds the total number of initially empty electron traps existing in the material. Under these circumstances the electrical charge transport mechanism is no longer ohmic. In order to analyze the dependence of σ upon injected/trapped charge, isothermal and non-isothermal currents in Teflon FEP have been investigated at various temperatures, field strengths, in a vacuum or in ambient air conditions. At temperatures below 413 K, for charging times longer than about 10 s but shorter than about 600 s, the electric conductivity is almost electrical field strengths independent proving that the injected charge plays a minor role. For these conditions the charge is mostly trapped in superficial traps. At higher temperatures σ is field dependent. The final thermally stimulate discharge current has a peak around 500 K with a mean apparent activation energy around 1.35 eV. For a well conditioned sample the peak current is strongly dependent on the charging electric field and on the mean trapping depth of the injected charge. The relaxation time of the trapped charge is around 106 s at 523 K, proving that the injected charge is very stable, a fact of significant importance for applications.