Basic Insulation level (BIL)@345-kV

[shocker23]

Not having access to ANSI 57.1200 (a new engineering office) however I know in ANSI the BIL levels cover 3-different ranges 900, 1050 & 1175 at 345-kV. How does one choose the correct level?

 

[Jraef]

First off. BIL stands for Basic Impulse Level. It has to do with the dielectric strength of the insulators and standoffs of the gear and transmission equipment; essentially their ability to safely withstand high potential pulses or surges from things such as lightning strikes or switching transients. The available levels are usually dictated by the voltage classes of the equipment involved, but there are often a few choices in each class as you have undoubtedly noticed. The higher the BIL, the better the dielectric strength is. But that is not to say you should always go with the highest level. It is something that needs to be carefully coordinated with the lightning and switching transient protection scheme you are using.

This is the realm of PT&D Engineers (of which I am not a member) and if you are not experienced in this, I respectfully suggest you solicit the help of someone who is.