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Apparatus Ratings at High Elevations

In most electrical apparatus, the surrounding air acts as both insulator and coolant. Both functions depend upon air density. In general, the denser the air, the higher its breakdown voltage and the greater its heat transfer capability. Since air density decreases with increasing elevation above sea level, both insulating and cooling effectiveness will also decrease.

In mountainous regions of the world, notably in the mining industry, large electrical systems operate at more than 3,000 meters above sea level. Equipment manufacturers and published standards offer various formulas and tables relating required reductions in apparatus temperature rise at sea level to meet thermal ratings at higher elevation. Most temperature adjustment criteria consider that no change is necessary for elevations up to 1,000 meters.

Motors, transformers, switchgear, circuit breakers, fuses, and individual conductors are all subject to such variation. Of greater utility, however, are similar "derating factors" that define the reduction in load-carrying capability for high altitude service.

For motors, as an example, the factors can be used in two ways. One is in choosing a design having a lower temperature rise at sea level so it will not overheat at the altitude. An alternative is to select a motor of higher output rating to carry the load safely at the altitude. These approaches require knowledge of the relationship between winding temperature rise and power output.

Similar factors, in even greater variety, apply to voltage ratings based on dielectric breakdown of air. As atmospheric pressure decreases with increasing elevation, impulse withstand or BIL ratings for switchgear, transformers, or circuit breakers must either be increased at sea level, or operating voltage reduced at the higher elevation. Rotating machines are not assigned any BIL values, however, and industry standards offer no guidance in adjusting motor or generator operating or test voltages to compensate for altitude.

The relationship between air gap breakdown voltage, gap size, and atmospheric pressure follows Paschen's Law. As air density decreases directly with pressure, the number of gas molecules in a given volume also decreases, lessening the opposition to development of an arcing path. That process reverses, however, when elevation is high enough for the atmosphere to more closely approach a perfect vacuum. Air molecules then become so diffuse that an ionized path cannot readily form, and the breakdown voltage then trends sharply upward. Except for vacuum interrupter components, land-based electrical apparatus is not found in that region.

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