Assuming you are talking about 480V assembly in switchgear, the recent manufacturer I worked for used insulated standoffs that were about 1" thick. The nominal clearance in air we used between uninsulated bus bar and a ground plane was 1" at 480V. When this clearance was not achievable, we would use a phenolic insulating material or insulating fiberboard such as Tufnol.
See page 26, product on center of page on the following link.
Ever stop to think, and forget to start again?