The effect of the initial average degree of polymerization (DP0) of cellulose macromolecules on the electrical properties of industrially produced electrical insulating paper (EIP) was studied. A comparative assessment was conducted on the short-term electric strength, as well as on the changes in the degree of polymerization and the mechanical tensile strength of the EIP with different DP0 in the process of accelerated thermal aging. It has been confirmed that the currently accepted paper operability limits are unreasonable. The analysis of the experimental results was carried out relying on the micrographs (obtained with SEM), which clearly illustrates structural changes in the EIP cellulose base. The results of the study correct the notion of the limit of the physical existence of cellulose fibers and the relationship between the EIP resource and DP0. On the basis of the information obtained, the assumptions about the reasons for the discrepancy between the experimental and calculated values of the DP and the mechanical strength of the paper are formulated, as well as the factors causing the discrepancy between the time when the material reaches a limit state using criterial characteristics: DPcr = 200 units and Рd 50 % (the reduction of the initial mechanical strength by 50 %). A reasoned conclusion is given on the need to adjust the views on the resource of cellulosic dielectrics and paper-impregnated insulation.