The effect of radio frequency electric field on the initiation of plasma induced bi-polar arc was studied with a titanium rod inserted inside a toroidal plasma. The superposition of relatively low frequency field on the voltage impulse made the arc initiation voltage lower than that without RF. On the contrary the superposition of relatively high frequency field made it higher than that. The physical mechanisms for explaining these apparently opposite results were proposed. The relation between the arcing and the impurity release in high power wave heating experiments was discussed.