Following the total yield approach (used to predict the sign of charging of electron irradiated insulators) a positive charging is expected when the primary beam energy Ep is situated in the energy range where the number of outgoing electrons, $(\delta + \eta) I_0$, is larger than the number of incoming electrons, I0. But a negative charging is often experimentally observed when a positive charging is predicted. The present paper is an attempt to elucidate this experimental fact. The arguments being developed are based on the use of a dynamic double layer model (+ for the secondary electron mission, SEE; − for the incident electron implantation) which explains a negative charging via the influence of an evolving S-shape potential function, V(z), which induces a partial freezing of the nominal (uncharged) SEE, δ°, combined to a progressive compression of the negative space charge below the surface. The observed large difference in the measurements of the yield by using pulse excitation methods, or by permanent irradiation methods, δ° or δ is then explained. Furthermore, the influence of an oblique incidence is also deduced.