The　behavior　of　fault　arc　in　a　high-speed　switch　(HSS)　has　been　studied　theoretically　and　experimentally.　A　simplified　HSS　setup　is　designed　to　support　this　work.　A　two-dimensional　arc　model　is　developed　to　analyze　the　characteristics　of　fault　arc　based　on　magnetic-hydrodynamic　(MHD)　theory.　The　advantage　of　such　a　model　is　that　the　thermal　transfer　coefficient　can　be　determined　by　depending　on　the　numerical　method　alone.　The　influence　of　net　emission　coefficients　(NEC)　radiation　model　and　P1　model　on　fault　arc　is　analyzed　in　detail.　Results　show　that　NEC　model　predicts　more　radiation　energy　and　less　pressure　rise　without　the　re-absorption　effect　considered.　As　a　consequence,　P1　model　is　more　suitable　to　calculate　the　pressure　rise　caused　by　fault　arc.　Finally,　the　pressure　rise　during　longer　arcing　time　for　different　arc　currents　is　predicted.