Abstract: According to the practical application of intrinsically safe switching power supply,the model of electric po- tential and capacitor (EC) was proposed. The spark discharge of EC circuit with cut-off type protection was studied through explosive test based on the IEC spark test apparatus. It is found out that the circuit of cut-off type protection can directly intercept the spark discharge process of the EC circuit. The equivalent circuit of the EC circuit with cut-off type protection during spark discharge stage was given. The influence of its key parameters on the discharge power and energy was analyzed with numerical simulation tools. At spark discharge initial stage,the discharge power peak are mainly caused by the capacitor discharge. While,at spark discharge sustained stage,the discharge power are mainly caused by the power supply discharge. The shorter the response time of cut-off type protection,the better the intrinsi- cally safe performance of the corresponding devices. Experimental results show that the proposed model and theoretical analysis are valid and feasible.