This work aims to develop an analysing technology to predict the pressure characteristics for medium voltage metal clad switchgear in case of an internal arc fault. We have been developed the numerical simulation method which has the following features. (1) It enables to consider the influence of the flow of exhaust gases by applying simple CFD method. This method is able to predict speedily the time-dependent pressure distribution inside the switchgear. (2) For calculation of the arc energy is or heating energy, its electric field acquired by experiments and arc power are used to analyze. Furthermore, the validity of the developed analyzing technology has been checked with experimental results.