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Home / Technical Articles / What Is The Basic Insulation Level (BIL)

Introduction to BIL

Insulation levels are designed to withstand surge voltages, rather than only normal operating voltages. Since the insulation lines and equipment is protected by surge arresters draining the surges rapidly before the insulation is damaged, the arrester must operate below the minimum insulation level that must withstand the surges.

What Is The Basic Insulation Level (BIL)
What Is The Basic Insulation Level (BIL) – photo credit by David Parrish via Flickr

An example is shown in Figure 1a below.

The minimum level is known as the Basic Insulation Level (BIL) that must be that of all of the components of a system.

Insulation coordination
Figure 1a – Insulation coordination

Insulation values above this level for the lines and equipment in the system must be so coordinated that specific protective devices operate satisfactorily below that minimum level.

In the design of lines and equipment considering the minimum level of insulation required, it is necessary to define surge voltage in terms of its peak value and return to lower values in terms of time or duration. Although the peak voltage may be considerably higher than normal voltage, the stress in the insulation may exist for only a very short period of time.

For purposes of design, the voltage surge is defined as one that peaks in 1.5 microseconds and falls to one-half that value in 40 microseconds (thousandths of a second).

It is referred to as a 1.5/40 wave, the steep rising portion is called the wave front and the receding portion the wave tail, Figure 2.

Surge Voltage 1.5 by 4.0 Wave
Figure 2 – Surge Voltage 1.5 by 4.0 Wave

Insulation levels recommended for a number of voltage classes are listed in Table 1. As the operating voltages become higher, the effect of a surge voltage becomes less; hence, the ratio of the BIL to the voltage class decreases as the latter increases.

Table 1 – Typical Basic Insulation Levels

Basic insulation level, kV
(standard 1.5- × 40-μs wave)
Voltage class, kVDistribution classPower class (station, transmission lines)

*For current industry recommended values, refer to the latest revision of the National Electric Safety Code.

Distribution class BIL is less than that for power class substation and transmission lines as well as consumers’ equipment, so that should a surge result in failure, it will be on the utility’s distribution system where interruptions to consumers are limited and the utility better equipped to handle such failures.

The line and equipment insulation characteristics must be at a higher voltage level than that at which the protecting arrester begins to spark over to ground, and a sufficient voltage difference between the two must exist.

The characteristics of the several type arresters are shown in the curves of Figure 3.

Sparkover Characteristics of Distribution Value / Expulsion Arresters
Figure 3 – (a) Sparkover Characteristics of Distribution Value Arresters; (b) Sparkover Characteristics of Expulsion Arresters

The impulse level of lines and equipment must be high enough for the arresters to provide protection but low enough to be economically practical.

Surges, on occasion, may damage the insulation of the protective device; hence, insulation coordination should include that of the protective devices.

As there are a number of protective devices, mentioned earlier, each having characteristics of its own, the characteristics of all of these must be coordinated for proper operation and protection.

Before leaving the subject of insulation coordination, such coordination also applies within a piece of equipment itself. The insulation associated with the several parts of the equipment must not only withstand the normal operating voltage, but also the higher surge voltage that may find its way into the equipment.

So, while the insulation of the several parts is kept nearly equal, that of certain parts is deliberately made lower than others; usually this means the bushing. Since the bushing is usually protected by an air gap or arrester whose insulation under surge is lower than its own, flashover will occur across the bushing and the grounded tank.

The weakest insulation should be weaker by a sufficient margin than that of the principal equipment it is protecting. Such coordinated arrangement restricts damage not only to the main parts of the equipment, but less so to parts more easily accessible for repair or replacement.

The insulation of all parts of the equipment should exceed the basic insulation level (BIL). Figure 1b.

Simplistic diagram illustrating Basic insulation level (BIL) and Insulation coordination
Figure 1b – Simplistic diagram illustrating Basic insulation level (BIL) and Insulation coordination

Resource: Power Transmission and Distribution – Anthony J. Pansini (Get this book from Amazon)

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More Information

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. Lebo
    May 05, 2021

    Please assist I have a 275kV line voltage and would like to know the BIL of it
    Thanks in advance for your response it means a lot

  2. taigon
    Aug 04, 2020

    Please suggest the clearance LT to HT winding and inter layer insulation of HT coil for resin casted potential transformer.

    PT BIL is 95 kV,

    we can’t achieve impulse test with 95 k, please help

  3. Seshadrik
    Mar 25, 2020

    It’s very good information for up coming young engg

    We need more and more latest upcoming
    Development update

  4. Antoine KAYIJUKA
    Jun 12, 2019

    What is the length of 110kV insulators considering BIL 650KV and Nominal creepage distance 3679mm? and how to calculate.

  5. Zosimo Bacalla
    Jul 24, 2018

    What is the correction factor in terms of insulation for a potential transformer rated at 15.5kV Insulation with 110kV BIL full wave. Being installed at the height of 3000meters.

  6. Al-Amin
    Mar 31, 2018

    nice Article.

    Bil rating will vary from operating voltage level to level ?

  7. Florian Misoc
    Jan 19, 2018

    Great article!

  8. Brian Trager
    Jan 16, 2017

    Good job Edvard. Your contribution to the industry is appreciated.

    Sep 05, 2016

    What is “BIL” Test ?
    How to do “BIL” Test ?

    • Iryani
      Nov 03, 2016

      BIL refer to lightning impulse test. can refer to IEC 60060-1

      • Nilesh
        Jun 30, 2018

        Waht Is BIL of below Rated Surge Arrester.

        How its calculate Please explain.

  10. Vu
    Mar 11, 2016

    Is insulation Levels presented in table 1 Phase-Neutral Voltage?

  11. vinod nagrecha
    Dec 11, 2015

    How would Basic Insulation Level vary as altitude where equipment is to be installed rises ?

  12. Electrical Engineer
    May 26, 2015

    good topic

  13. Jan Ledochowski
    Mar 27, 2014

    There is a mistake below title Hot Swap Controllers PDF:
    ….one-half that value in 40 microseconds (thousandths of a second).
    should be: millionth of a second.

    Feb 09, 2014

    Dear Sir/Madam,

    can you tell me abt earth falut details…

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