Due to the difficulties pointed out, it was necessary to study frequency dependent models. This was the task of the IEEE Working Group [2], that developed a model in which the non-linear V-I characteristic is represented by two sections of non-linear resistances designated by A0 and A1. These resistances are separated by a R-L filter
For slow front surges, the R-L filter has very little impedance and the two non-linear sections of the model are essentially in parallel. For fast front surges, the impedance of the R-L filter becomes more significant. This results in more current in the non-linear section designated by A0, than in the section designated by A1. Since characteristic A0 has a higher voltage for a given current than A1, the result is that the arrester model generates a higher voltage. This model yields good results for arrester discharge voltages, when the discharge current has a time to crest within the range of 0.5 to 45µs. The major problem associated to it is to calculate its parameters. The IEEE Working Group suggests an iterative method, where adjustments are necessary to achieve better results. The initial parameters depend on electrical and physical data