Based on the known theoretical concepts about the partial arc discharge development process and the data on the main design features of support insulators, numerical studies of their electrical strength under pollution and moistening conditions were carried out. The conditions under which the electrical strength of insulators can or cannot achieve its maximum possible value were analyzed. It is shown that the highest electrical strength of contaminated insulators is obtained at the optimal value of the ratio of the creepage distance to the height of the insulators along their insulating part (L/H)opt. The value of (L/H)opt is determined by the edge profile design features. As a rule, for insulators with a simple edge profile, the value of (L/H)opt = 2.45, and for insulators with a complex edge profile (L/H)opt = 3.1. In the region L/H ≤ (L/H)opt the effective use of the creepage distance by the discharge channel is ensured. It is shown that excessive increasing of the L/H ratio above the optimal value leads to inefficient use of the creepage distance by the discharge channel, which entails a noticeable decrease in the electrical strength of insulators. By using the proposed calculation estimates, it becomes possible to a priori determine the expediency of applying a particular insulator for being used in areas with various normalized pollution degrees.