For a given electrical equipment at a rated voltage of say 11kV, how is the Basic Insulation Level (BIL) determined

Ekow,

There's usually some person in your organization who performs an Insulation Coordination Study that determines the BILs for all the major components and voltage levels of your grid. The BIL depends upon many factors; lightning intensity, earthing resistance, soil conditions, equipment ratings, transmission line configuration, type of equipment, switching surges, etc., etc.

It's usually a lifetime career for those that are really good at it, there's plenty of info available on the web from the IEEE, CIGRE, etc., certainly more than can be related in a few words here.

Alan

Basic technical values & design criteria are determined as per recommendations of
IEC 60038 , 60071 Part 1 & 2 and other relevant IEC publications.

BIL for 11kV system reads as 12 / 28 / 75kV -
Highest system voltage, ph-ph , rms : 12kV
Power frequency withstand voltage ( rms ) : 28kV
Lightning impulse withstand voltage ( peak ) : 75kV

From memory, IEC standards do not use BIL, it does not exist. It is a North American usage. Since 11 kV is an IEC standard voltage, not a North American voltage, I presume that the question originates in the IEC world, it does not make sense to use the term BIL.

Krishna's response above seems correct for 11 kV if you cross out the first line starting "BIL...".

Alan, your response is good for where you live, but does not apply to the rest of the world.

Gregory Speer, please review your comments carefully. You have made a typo error. you may also want to delete.

RGDS

I have studied some IEC standards such as ones for HV transformers and HV switchboards as I have specified these many times in contracts. There is no definition for "BIL" and it does not appear in the text. A voltage group for 11 kV such as "12 / 28 / 75kV " is not a set that defines "BIL". Can you can quote an IEC standard that defines BIL?.