Abstract: The article analyzes the characteristics of rotary ignition mechanism of spark test apparatus (STA) with simplified anlysis method and then gets data of STA accessing and breaking time under different rotation tolerance and different quantities of tungsten wires on the basis of writting procedures chart. STA testing scope of capacity circuit can be acquired by analyzing data. It is verified that inaccurate quantities of tungsten wires results in correspondingly changing of accessing and breaking time, which is likely to cause misjudging about intrinsic safety of capacity circuits. The article proves that estimating the time constant of capacity circuit is both safe and efficient to determine the proper quantities of tungsten wires.