PURPOSE: To decide on an optimum gaseous creepage distance of insulation in a designing stage by quantizing various voltages and dirt numerically and collectively under determined constraint condition.

CONSTITUTION: The initial gaseous flashover value of a vacuum interrupter itself is estimated and measured in a step 1 and compared with the test voltage of a product in a step 2. In a step 3, whether the flashover characteristics of the vacuum interrupter itself due to dirt and dew condensation are present or not is judged. The flashover characteristics are compared with an in-use voltage ×1/31/2×1.15 in a step 4. In a step 5, a gaseous flashover distance satisfying the test voltage VT is set. Then whether the flashover characteristic of the set distance due to dirt and dew condensation are present or not is judged in a step 6 and the flashover characteristics are compared with the in-use voltage ×1/31/2 ×1.15 in a step 7. In a step 8, gaseous flashover distance satisfying the dirt and dew condensation is set. Then, the insulation distance of the vacuum interrupter itself is compared with necessary interface distances by the in-use voltages on product gurantee condition.