The　objective　of　this　paper　is　to　propose　a　breakdown　model　based　on　the　Cranberg　＂clump＂　hypothesis　for　an　impulse　voltage　breakdown　under　a　non-uniform　field　in　high　voltage　vacuum　interrupters.　It　is　assumed　that　the　micro-particles　of　loosely　adhering　material　are　instantaneously　detached　from　a　parent　electrode　surface　at　a　critical　voltage　given　by　the　Cranberg　hypothesis.　The　influence　of　non-uniform　field　distribution　on　the　micro-particle　charge　has　been　considered.　And　the　voltage　rise　rate　in　impulse　voltage　occurring　during　the　transit　time　of　the　micro-particles　to　the　opposing　electrodes　has　also　been　considered.　For　an　impulse　voltage　of　constant　rise　rate　applied　on　the　micro-particles,　the　breakdown　voltage　of　a　vacuum　gap　depends　upon　the　0.74　power　of　the　gap　length　for　the　plate-to-plate　contacts　with　contact　gaps　10　to　50　mm.　Finally,　the　hypothesis　has　been　validated　by　the　experiments.　And　the　experimental　results　show　that　the　impulse　breakdown　voltage　was　related　to　a　0.70　and　a　0.79　power　of　the　contact　gap　depending　on　a　different　contact　surface　roughness.　So　the　experimental　results　support　the　Cranberg　hypothesis　in　its　application　to　the　impulse　breakdown　under　non-uniform　field　in　vacuum　interrupters.　©　2011　IEEE.