Lightning and Switching Impulse: Standard Waveshapes

Short answer

A standard waveshape is a test voltage with times fixed by the standard, so that the same test can be repeated in any laboratory. IEC 60071-1 defines three for insulation: short-duration power frequency, the 250/2,500 µs switching impulse and the 1.2/50 µs lightning impulse. The generation and measurement methods are in IEC 60060-1.

The three standard shapes

For very-fast-front overvoltages there is no standard test shape in IEC 60071-1: the product committee of the equipment must specify one, if needed (clause 3.37, note 3).

How to read the name of an impulse

A voltage impulse rises quickly to a peak and falls more slowly. The two numbers in the name are times:

• First number: the rise time. In the lightning impulse it is called the front time (T1) and equals 1.2 µs. In the switching impulse it is called the time to peak (Tp) and equals 250 µs.
• Second number: the time until the voltage falls to half the peak value, measured from the start of the wave (T2). It is 50 µs for lightning and 2,500 µs for switching.

A lightning impulse reaches its peak in a little over a microsecond; the switching impulse takes hundreds of microseconds. That is why they represent different overvoltage classes and each has its own test.

Ranges of each class

Table 1 of IEC 60071-1 separates the classes by the shape of the overvoltage that occurs in the network:

• Slow-front: time to peak between 20 µs and 5,000 µs, tail up to 20 ms.
• Fast-front: front time between 0.1 µs and 20 µs, tail up to 300 µs.
• Temporary: frequency between 10 Hz and 500 Hz and duration between 0.02 s and 3,600 s.

A real overvoltage that falls within one of these ranges is replaced, for the test, by the standard shape of its class. That is why the standard speaks of the representative overvoltage: a voltage of standard shape that produces the same dielectric effect on the insulation as the real overvoltages of the class. When an assumed maximum is used, the lightning impulse has the same peak as the maximum of the phase-to-earth fast-front overvoltages; the switching impulse, the same peak as the maximum of the slow-front ones.

Combined shapes for phase-to-phase and longitudinal

When the insulation lies between two phases, or across an open device, a single impulse does not represent the stress. The standard defines two combined shapes:

• Combined switching impulse, for phase-to-phase insulation: two components of equal peak value and opposite polarity. The positive one is a standard switching impulse; the negative one has times to peak and to half value no shorter than those of the positive one. Both reach their peak at the same instant and the peak value of the combined voltage is the sum of the peaks.
• Combined voltage, for longitudinal insulation: a standard impulse on one terminal and a power-frequency voltage on the other. The impulse is applied at the peak of the power-frequency voltage of opposite polarity.

How they are used in a test

The general conditions are in clause 6.1 of IEC 60071-1:

• Tests are usually dry, in the reference atmosphere. For external insulation without weather protection, the power-frequency and switching tests are done under rain, according to IEC 60060-1.
• All impulses are verified in both polarities, unless the product committee indicates only one, or it has been shown that one polarity or condition is the most severe.
• If the laboratory atmosphere differs from the reference one, the test voltage is corrected according to IEC 60060-1.
• What counts as a failure and how it is detected is defined by the product committee of the equipment.

The number of impulses and the acceptance criterion depend on the chosen procedure, which is explained in the guide on withstand voltage tests.

Why it matters when reading a specification

When a specification or a nameplate says “lightning impulse 1.2/50 µs” or “switching impulse 250/2,500 µs”, it names the shape with which the insulation was demonstrated. If the value is a lightning impulse, the test was done with the lightning wave; if it is a switching impulse, with the switching wave. For switchgear, see how it is expressed in rated insulation level of switchgear and how instrument transformers are tested in impulse tests on instrument transformers.

To go deeper, see Insulation coordination: what it is and what it is for, Slow-front overvoltages: switching and faults and Lightning overvoltages in a substation.

When to call us

At Tiza Group we carry out dielectric field tests, in Colombia or elsewhere on request, and review the type and routine test reports of your equipment remotely. Learn about our testing and commissioning service and get in touch for a quote.

Related guides

• Insulation coordination under IEC 60071: the steps
• Withstand voltage tests: types and acceptance criterion
• Insulation coordination: what it is and what it is for
• Overvoltages in the system: types and origin
• Self-restoring and non-self-restoring insulation

References

• IEC 60071-1:2006, Insulation co-ordination - Part 1: Definitions, principles and rules: clauses 3.18, 3.19, 3.37, 5.2 (Table 1) and 6.1.
• IEC 60060-1, High-voltage test techniques - Part 1: General definitions and test requirements (cited by the standard).
• IEC 60071-2:1996, Insulation co-ordination - Part 2: Application guide: clause 2.3.4.1.

Frequently asked questions