Arc motion along arc runners enclosing different opening angles from 60/spl deg/ to 150/spl deg/, symmetrically arranged, was investigated in a model switch at currents from 1 kA to 5 kA. Wall distance, wall and runner material were varied. The measured criteria were the lengthening time and the frequency of back-strikes. Narrow slots between lateral insulating walls caused fast arc motion and lengthening at 1 kA arc currents independent of the wall material (gassing or not). Increasing slot width delayed the arc and favoured back-strikes. Immobile arc bands appeared frequently and depended on the runner material when the walls were completely removed. Increasing current from 1 kA to 3 kA decreased the lengthening time due to the increase of the magnetic self-blast field. Further increase of the current up to 5 kA did not further accelerate the arc. Back-strikes and immobile arc bands were favoured on iron runners (nickel plated or not). Variation of the opening angle did not yield any significant influence on the lengthening time under advantageous running conditions (narrow slot width copper runners). Unfavourable conditions (5 kA, 150/spl deg/) caused frequently immobile arc bands in the lengthening area even on copper runners.