The model recommended by IEEE W.G 3.4.11[1]
is shown in Fig.1. In this model the non-linear V-I
characteristic is obtained by means of two non-linear
resistors (tagged A0 and A1) separated by a R-L filter.
For slow surges the filter impedance is extremely low
and A0 and A1 are practically connected in parallel
On the contrary, during fast surges, the impedance
of the filter becomes significant, and causes a current
distribution between the two branches. For precision
sake, the current through the branch A0 rises when the
front duration decreases
Since A0 resistance is greater than A1 resistance for
any given current, the faster the current surge, the
higher the residual voltage. This because high
frequency current are forced by the L1 inductance to
flow more in the A0 resistance than in the A1
resistance[1].
The comparison of the calculated peak values with
the measured values shows that the frequency
dependent model gives accurate results for discharge
currents with times to crest between about 0.5 Ps and
45 Ps.