BIL

" BIL" means? in Electrical engineering?

BIL is Basic Impulse Level or sometimes some other referred to as "Basic Insulation Level".

It represents the "strength" of equipment's (transformers,motors)insulation for a high-voltage impulse. The impulse wave form is often 8 by 20 milliseconds -- An 8 ms rise and 20 ms in duration.

Here is a quote from C57.12.00:

"Transformers shall be designed to provide coordinated low-frequency and impulse insulation levels on line terminals and low-frequency insulation levels on neutral terminals. The primary identity of a set of coordinated levels shall be its basic lightning impulse insulation level (BIL)."

Each voltage class of equipment has a ANSI standard BIL rating. For example: 15 kV is the most common distribution voltage class in the US. Equipment installed on a 3-phase, 4-wire circuit in the 15 kV class with have a BIL rating of 95 kV. (The 15 kV class includes voltages from 11.8 kV through 14.4 kV. The most common actual system voltages are 12.47, 13.2, and 13.8 kV.)

The appropriate rating depends on whether the equipment is effectively grounded and/or connected to a 3-wire or 4-wire system. Some of the more common BIL ratings used on distribution equipment in North America are 60, 95, 125, 150, and 200 kV.

Transmission and substation equipment has a minimum BIL rating of 110 kV. Here are some of the minimum BIL ratings for various transmission voltages:

System - BIL
138 kV = 450 kV
230 kV = 650 kV
345 kV = 900 kV
500 kV = 1130 kV
765 kV = 1800 kV



Source(s):
C57.12.00 - IEEE Standard General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers