It has been shown that in semi-insulating films, the characterization of charge transport phenomena, such as dielectric relaxation, by resistance and capacitance only is insufficient. The independent roles played by additional transport parameters, including charge injection, trapping and field dependent mobility, are elucidated by a first principle treatment of charge transports in both the closed circuit and open circuit modes. Experimental results from typical electrophotographic paper and charging roll samples are compared with the results of mathematical simulations. The advantage of open circuit measurements, such as Electrostatic Charge Decay (ECD), is noted.