Altitude Correction of External Insulation: Ka and m

Short answer

The higher the altitude, the thinner the air, and air clearances and insulators withstand less voltage. IEC 60071-2 corrects this effect with the altitude correction factor Ka, calculated as Ka = e^(m·H/8150), where H is the altitude above sea level in metres. The exponent m is 1.0 for lightning and for power frequency in clean air, and is read from a figure for switching impulse. Only external insulation is corrected: internal insulation does not depend on altitude. This guide follows clause 4.2 of the 1996 edition and clauses 5.4, 5.5 and 5.9 of IEC 60071-1:2006.

What is corrected and what is not

The standard withstand voltages of IEC 60071-1 apply under reference atmospheric conditions:

• temperature of 20 °C,
• pressure of 101.3 kPa,
• absolute humidity of 11 g/m³.

When the equipment works in a different atmosphere, the coordination withstand voltage is corrected with the atmospheric factor Kt, and in the case of altitude with Ka. This correction applies only to external insulation. For internal insulation, the standard assumes that air conditions have no influence.

One clarification from IEC 60071-1: Ka is applied at any altitude, not only above 1,000 m. What changes at 1,000 m is the normal service condition: the standard’s tables of withstand voltages are intended for altitudes not exceeding 1,000 m.

Why only pressure counts

Low temperature and high absolute humidity improve withstand capability. High temperature and low humidity reduce it. IEC 60071-2 notes that the most adverse conditions do not usually occur together, and that at a given site the humidity and temperature corrections cancel out in practice. For that reason, only the air pressure corresponding to the altitude is considered when coordinating insulation, for both dry and wet insulation. The standard warns that this assumption is less accurate for closely spaced insulator sheds, where rain can bridge the sheds.

The formula and the exponent m

The standard bases the factor on the variation of atmospheric pressure with altitude:

Ka = e^(m·H / 8150)

• H: altitude above sea level, in metres.
• m: exponent that depends on the type of voltage.

In figure 9, m decreases as the coordination switching voltage increases. It has curves for phase-to-earth insulation, longitudinal insulation, phase-to-phase insulation and the rod-plane reference gap. The standard indicates that the figure values are conservative estimates, because the minimum discharge path is not known when specifying. The IEC 60060-1 relationships on which m is based come from measurements up to 2,000 m, and using them above that altitude calls for care.

A calculation example

With m equal to 1.0, the formula gives factors that grow faster than intuition suggests:

Take a lightning coordination withstand voltage of 100 kV. At 3,000 m and with m = 1.0, the required withstand voltage rises to about 144 kV, before applying the safety factor. With a smaller m, as happens for switching impulse, the increase is smaller.

What changes in equipment selection

The required withstand voltage results from multiplying the coordination voltage by the safety factor Ks and by the atmospheric correction. IEC 60071-1 gives two routes when the site exceeds normal conditions:

IEC 60071-1 (clause 5.5) allows choosing a Um higher than the next standard value to compensate for the lower withstand capability of external insulation, for example when the equipment is installed above 1,000 m. The alternative is to keep Um and apply the factor.

What happens in testing

Equipment with internal and external insulation is tested with the corrected standard voltage, provided the atmospheric factor is between 0.95 and 1.05. Outside that range, the standard allows alternatives, such as testing the external insulation on models or estimating it from dimensions. In general, external insulation does not need to be tested if the air clearances are equal to or greater than those in the minimum-clearance tables of IEC 60071-1. Each product standard defines the test conditions.

What not to do

• Do not apply Ka to the internal insulation of transformers, cables or GIS: the standard does not correct it.
• Do not use m = 1.0 for switching impulse without checking figure 9: it overestimates the correction at high voltages.
• Do not extrapolate the m curves above 2,000 m without analysis: the relationships were measured up to that altitude.
• Do not also correct for temperature and humidity when coordinating: the standard considers them compensated.

To go further, see External insulation pollution: levels and creepage, Safety and test conversion factors, Special service conditions for switchgear, Selection of primary substation equipment and Insulation coordination: what it is and what it is for.

When to call us

The altitude of a site can force a different insulation level, a different air clearance or a different insulator. At Tiza Group we review switchgear selection for your altitude as part of engineering design and studies, remotely and worldwide. Write to us and we will send you a quote.

Related guides

• Insulation coordination under IEC 60071: the steps
• External insulation pollution: levels and creepage
• Safety and test conversion factors
• Special service conditions for switchgear
• Altitude above 1,000 m for instrument transformers

References

• IEC 60071-2:1996, Insulation co-ordination, Part 2: Application guide, clauses 3.1.3, 4.2.1 and 4.2.2, 5.3.7 and figure 9.
• IEC 60071-1:2006, Insulation co-ordination, Part 1: Definitions, principles and rules, clauses 5.4, 5.5 and 5.9.

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