The aim of this article is to determine the critical breakdown voltage or U50 -corresponding to the 50% probability of breakdown for a single impulse application - of longair-gaps under positive switching impulse voltages, using predictive dynamic model basedon equivalent electrical network. This model is similar to that developed previously by ourgroup. It is based on equivalent electrical network, gas discharge theories and physicallaws. The investigated electrode gap geometries are more complex than that modeled byothers and one can find in literature; the relationships used in literature for the calculationof electric field are analytical relationships for simple geometries, those enabling to betterapproach the actual geometries. To size more accurately the insulation structures, it isnecessary to take into account the actual geometry of these structures. For that purpose,we developed a model of discharge for a rod-rod gap. Then we determine the U50 voltageusing the developed model and the up and down method. The simulated results arecompared to experimental data and to those provided by empirical formula.