According to the requirements of standard GB 11022—2011

the basic lightning impulse insulation level（BIL）withstand voltage to earth of 24 kV insulation rod is 125 kV. Experience result shows that the insulation rod itself can pass the test with the voltage of 135 kV. After assembling the insulation rod onto product

it can only pass the test when the voltage is 120 kV（positive polarity）. In this paper

electric field distributions of the insulation rod itself and with the soft connections installed are simulated. It is found that the whole solving regions and the maximum field strength positions of the two situations above are difference. The maximum field strength point is close to the end of the grounding insert. Considering this distribution feature

improvement design and test are implemented on the insulation rod. Test results show that the new insulation rod can withstand the BIL of 135 kV on both separately and assembly

which satisfies with the design specification. In addition

electric simulation result of the improved rod shows that the field distributions of the original rod and the improved rod differ little

while the experiment results have a quite big difference. Therefore

the key factor which influences the BIL withstand capability under positive polarity of lightning impulse is not the electrical field distribution but the increase on thickness of the important parts of the rod.