BIL or Basic Insulation Level Definition Table and Calculation

Basic Insulation Level Definition

In IEEE practice, basic impulse insulation level, usually shortened to BIL, denotes the crest value of the specified standard lightning-impulse test voltage that the equipment insulation must withstand. Surge-arrester operating voltage and insulation breakdown voltage are separate characteristics.

Insulation coordination for a transmission system or substation starts with the overvoltages that can reach each item of equipment. Lightning, switching and temporary power-frequency overvoltages have different waveforms and durations. The design then selects phase-to-earth, phase-to-phase and longitudinal withstand levels for the applicable equipment and installation.
The highest voltage for equipment, earthing method, insulation type, altitude, environmental conditions and surge-protection arrangement affect the choice. A sound electrical power design uses the basic insulation level or BIL values and test rules in the current equipment standard. Nominal system voltages alone do not select one universal BIL.

Insulation is not designed to withstand every possible overvoltage indefinitely. Instead, insulation coordination compares expected stresses with standard withstand levels and the protective characteristics of surge arresters. A metal-oxide arrester limits a surge to a protective level, but the voltage at the equipment can also include lead inductance and travelling-wave separation effects. The selected BIL must retain the required coordination margin. It is normally stated as the peak kilovolt value of a standard 1.2/50 microsecond lightning-impulse withstand test.

Higher insulation ratings can increase the size and cost of equipment such as transformers. Standards therefore provide preferred withstand levels and selection methods instead of requiring the highest possible BIL. The selected value must come from the current standard for the equipment type and voltage class, followed by the insulation-coordination study. A laboratory impulse generator produces the specified voltage wave so different designs can be tested under repeatable conditions. The test wave represents a lightning impulse for dielectric testing; it is not a prediction of the exact waveform from every natural lightning event.

Impulse Voltage

The 1.2/50 microsecond designation gives the defined front time first and the time to half-value on the tail second. These parameters differ from a simple zero-to-crest time and total event duration. The page’s former 1.5/40 American value and 1/50 BIL description are obsolete or incorrect for the standard full-wave test.

During a withstand test, the specified insulation must meet the pass criteria in its equipment standard at the rated crest voltage. In service, a lightning arrestor is selected and installed so its protective level, connection voltage and separation effects remain coordinated with the equipment withstand. Flashover of self-restoring insulation, breakdown of non-self-restoring insulation and arrester operation are different events, so they should not be treated as one threshold. The table below lacks standard editions and equipment classes, so it must not be used for current design or procurement.

Legacy Basic Insulation Level Table