What is the importance of the BIL rating of a MV and HV equipment? Can you share me the formula how to get the BIL rating of a certain equipment?

BIL is the level of a lightning strike equipment can withstand, and is not calculated. Equipment is built to certain standards and rated with a BIL in kilovolt value based on historical statistics and construction methods that have exhibited the capability to withstand a particular impulse of high voltage.

In sizeing the rating of a certain equipment cb, swgr, transformer panel board do we have to cosider the BIL rating in our design.

Yes, since end users such as utilities, generation, distribution, and consumers using HV equipment will have specification requirements of BIL. BIL rating will be dependent on the dielectric strength of the insulation materials used a design. Standards for supply voltages are set by IEEE and ANSI, may be best to refer to these standards or which ever standards that are applicable to you equipment or country.

Can we have a free access to the ANSI or IEEE website?

Thanks

There may be some limited access to standards, but these are standards organizations and membership will be required to access the details, as their publications is copyrighted .

If you or your company is manufacturing HV equipment, I would think membership or consultation to some sort of standards be a prerequisite to the design of this equipment.

Try:
IEEE - the world's leading professional association for the advancement of technology
American National Standards Institute - ANSI

For international consideration try:
IEC - International Electrotechnical Commission > INTERNATIONAL STANDARDS AND CONFORMITY ASSESSMENT

is there any difference using IEEE with IEC standard? Do they have the same testing equipment parameters.

Assuming we calculate the interrupting current, do we calculate the value and select the product based on NEMA/IEEE standard or IEC standard? Say the products will be imported from Japan, Europe, and America what standard of calculation do I use?

Depends on the country the product will be used in and which standard that country subscribes to.

For a 13.8 KV system , BIL will be 95 KV for 1.2/50 Micro second standard lighting impulse testing