Follow the relevant national electrical code.

First of all, are the insulators you describe rated for 400kV? If not, then they will not do. Then, check the manufacturer's specifications for the insulator. That should tell you the creepage distance. Talk to the manufacturer about your specific application. Their insulator may be acceptable for your situation, or they may have an alternative that is better.

If you are worried that the skirts of the insulators will get contaminated too heavily or quickly so that they will flash over, then you may need to use a higher voltage rating insulator. That gives you a greater creepage distance. The alternative is to to plan on more frequent maintenance outages to clean the insulators, or to use one of the services that does insulator cleaning on-line using dry ice pellets or ground walnut shell/corn cob grit blasting.

Dir sir

Thanks for the prompt reply

we dont have any specifications for the insulator and we dont know the rated for these insulator .also i dont know Is there a difference between post insulator which used in 400kv substation & another post insulator used in 132kv substation

regards

The answer (including insulator de-rating factors for various pollution levels) is in your local electrical and applicable substation insulator standards.

If you don't have access to these then try the insulator suppliers or an internet search (you may be lucky enough to find it).

If you are not qualified (and hence not legally allowed) to perform this type of substation design and equipment specification then please leave it to the professionals (you are asking questions that lead us to believe you are not).

I would suggest you leave this job alone before you kill yourself or more regrettably someone else!

Thanks for all