fig2 the insulating layer 3 of the high resistance spark gap 5 is considerably thicker than the insulating layer 4 of the low resistance spark gap 5 39 however the performance of the present preferred embodiment is not degraded as a result because the great difference in the specific resistances of the layers 3 4 the prevents the consequent voltage drops from being noticeably influenced by the thicknesses of the insulating layers another feature which contributes to the above noted advantages is the difference in the lengths of the sparkover paths 6 6 39 these spark over paths are indicated in the form of sliding discharge gaps 6 6 39 in the details a and b illustrated in fig4 and 5 in the case of detail a fig4 the length of the part d of the insulating layer 5 projecting upwards above the electrode 2 is decisive for the sliding discharge gap 6 occurring there in the case of detail b fig5 the distance e between the edge 21 of the electrode 1 39 and the edge 2 39 of the electrode 2 is decisive for the size of the sliding discharge gap occurring there the distance e i e the length of the arc 6 39 on the low resistance spark gap 5 39 is greater than the part d and accordingly the length of the arc 6 on the high resistance spark gap 5 in this regard it should be noted that it is within the scope.