Recently, an arcing on satellite solar array due to interaction between space plasma and the array threatening safety of spacecraft is a big issue. The arcing causes degradation of solar array at malfunction of instruments on satellites. The discharge is caused by differential potential between satellite body and insulator surfaces like coverglass of solar array, which are charged by ambient plasma. This single shot discharge is called "primary arc". The primary arc can evolve to so-called "sustained arc" that permanently short-circuits adjacent solar cells or a solar cell and conductive substrate. In order to prevent arcs on the surface of solar array, it is necessary to carry out arc tests simulating discharge phenomenon on solar array. In this paper, we investigated the effect of plasma environments on sustained arcs. GaAs solar cells were used for the test. Laboratory tests were carried out with an external circuit simulating a spacecraft power system. Solar array coupon panels simulating the hot and return ends of a string circuit were tested under various combinations of string voltage and string current. We revealed that the threshold conditions for sustained arc formation were different in test plasma environment even when the string voltage and the string current are same