An experimental study of arcing is made, by making use of time-resolved optical and electrical measurements. Photomicrographs are made, and surface features of two parallel rail limiters are examined visually. Arc phenomena are accompanied by X-ray emission from the limiter. Many arc tracks are observed on the limiter surface which is exposed to both runaway electrons and the scrape-off plasma, but hardly any arc is observed on the limiter surface exposed to scrape-off plasma only. The most probable cause of unipolar arcing is the fact that runaway electrons supply a sheath potential sufficient to produce arc tracks on the electrically isolated target or limiter in the tokamak scrape-off layer. This model explains well the unipolar arcing observed in the non-equilibrium phases such as the current rise and end phases as well as during plasma disruptions.