The follow short circuit currents occure in the spark gapbased surge arresters (HS 45, HS 55, HS 60-20, HS 50-50, HS 50-16, HS 50-3) after their activation by an impulse discharge current, whereas their magnitude is restricted by an arised short circuit overall impedance of the energetic power network. These follow currents are spontaneously extinguished by an overpressure acting in an individual operative spark-gap chambers during the first net’s half-period transit through a zero. The amplitude of a prospective short circuit current, in place of a defined application, partly depends on the type of power system and partly on a lead distance of given application from a distribution transformer.
The following diagram describes the results of measurement evaluation of these prospective follow currents performed in 2325 three-phase network of 29 distribution plants, from that 315 were made on an overhead lines, 1215 on a combine lines and 715 on a cabel network. This graf shows that the value of prospective short circuit current never exeeds the value of 3kArms applicable for any power net type (overhead, combine or cabel network) for example while using an application distance of 600 m from a distribution transformer.
The above mentioned diagram is a favourably applicable in a project practice for a qualified estimation of a suitable spark-gap selection for a defined application and first of all for Ifi parameter determination (self-extinguish follow current at Uc). The eventual doubts, caused by a defined application specification, depends totaly on a designer to cover them by a reasonable safety coefficient (the exact measurement is from the financial and technical point of view rather demanding). For example, if the value of a prospective follow current according to the qualified estimation is 3kArms then it is suitable to choose the surge arrester with an approximately twice higher parameter of Ifi.