In general, power supplies that meet the IEC 60950-1 safety standard are designed to operate at commercial and residential areas, where altitude can vary from slightly above sea level to as high as 2,000 meters. However, many power supply manufacturers provide units that are designed to operate at higher altitudes, up to 3,000 meters or higher so their power supplies can be used for broadcasting/ communication station and towers that are located at higher elevations.
Altitude affects the design of power supplies since air is probably the most commonly and widely used of all electric insulating mediums in the construction of power supplies. The density and dielectric strength (insulating property) of air is very good at sea level, but at higher altitudes, some of its dielectric strength drops when air gets thinner. Basically, switching power supplies operate off of high voltages inputs (100 to 240 VAC), since air becomes thinner at higher altitudes and becomes less of an insulator, the PCB (Printed Circuit Board) and component layouts of the power supplies have to be designed with sufficient safety spacing distances to prevent high voltage arcs or breakdowns between conductors and electronic components, and to protect the device and operators.
For instance, typical power supply design may allow 8 mm spacing distance between primary and secondary circuits and 4 mm spacing distance between primary and ground. These spacing distances will vary depending upon the voltage levels between conductors and components and the expected humidity, temperatures, pollution levels, and attitudes.
The base design altitude for power supplies is 2,000 meters. However, as mentioned before, as the altitude increases, the air becomes a poorer insulator and the spacing distances have to be increased per the following table (assuming an 8 mm clearance at 2000m).
|Multiplication Factor for Clearance||Resulting Clearance (mm)|
For those power supplies that must be approved per the CCC (China Compulsory Certification), the Chinese Safety Standard GB 4943.1-2011 mandates strict rules for clearance distances. From December 1, 2012 the primary-to-secondary clearance must increase by a factor of 1.48 to qualify the power supply for operation up to 5,000m, or otherwise carry a warning label designed to show that the equipment is not for use above 2,000m in altitude.
The other major effect of high altitudes on power supplies is that the less dense air does not conduct heat as well. To compensate for higher altitudes, power supplies need to be de-rated, or have increased forced air flow, or a combination of these to insure proper cooling.