In order to understand the behavior of arc inception and its transient propagation till complete breakdown across an insulator it is necessary to find out the maximum electric strength, max E , at the arc tip in an air gap and investigate the relationship of the dimensions of this particular geometry to max E . The most widely used model is a rod-plane one. Because of the strong non-uniformity of the field distribution it is impossible to derive an equation by using conventional electromagnetic methods such as Laplace’s equation to find max E . In fact, there is only one well-known method, Roy’s equation, which is commonly used for a rod-plane system to calculate max E , i.e. by solving Laplace’s equation based on a parabolic approximation. In this paper, the validity of the results obtained by Roy’s equation is discussed and its results are compared with those obtained by finite element methods. It will also allow obtaining a better spectrum of the rod electrode system field distribution.