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I was just looking at some DC Drives and they have an altitude rating. Am I right this is because of the density of the air and it changes the cooling characteristics of the drive? One I looked at had a 3000' rating that I thought was pretty low.
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As air gets thinner with altitude increase, it's ability to absorb heat diminishes. So when power devices inside the drive reject their heat into the heat sinks, you either need more air (logarithmically) across it to remove that heat, or you de-rate the current capacity of the power devices. It's cheaper to just de-rate. Industry standard for electrical equipment is 1000 meters (3300ft.) but some US based companies just use 3000ft to make it sound easier. Then there is usually a de-rating factor, typically 1-2% for every 100m over 1000m, which is easier to use than 1-2% for every 32.8ft over 3300 ft. Most max out at 4500m, around 15,000 ft. because at that point, it's just not worth doing because everything ELSE will not work well anyway. I have run into that issue however for a couple of gold mining projects in the Chilean Andes (Kinross Mining). One of their sites is at 4500m above sea level, they have to over size EVERYTHING by about 40%.

The bigger problem with the extreme altitudes is because if you get into Medium Voltage equipment that has to feed power to the low voltage equipment, then you also have to deal with a significant loss of the dielectric (insulating) properties of thinner air, which affects your BIL (Basic Impulse Level) rating, the ability for equipment to withstand voltage spikes without flashing over. At some point you just can't do it any more.
 

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The bigger problem with the extreme altitudes is because if you get into Medium Voltage equipment that has to feed power to the low voltage equipment, then you also have to deal with a significant loss of the dielectric (insulating) properties of thinner air, which affects your BIL (Basic Impulse Level) rating, the ability for equipment to withstand voltage spikes without flashing over. At some point you just can't do it any more.
That's interesting. I would have thought thinner air would be a better insulator (less "stuff" to conduct) the extreme idea being a vacuum breaker.
 

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Yeah, once you have NO air it is better because there is nothing to ionize, but up to that point it is worse in that aspect. The ability for gasses to ionize decreases as density of the gasses increases. Or in other words the more molecules of a gas you have packed into a space between two poles of an arc source, the more energy it takes to make enough of them break apart to allow the arc to jump the gap. So as altitude goes up, the less desne gasses that remain are easier to ionize, which means it is less insulating. So it is only when there is nothing left at all that it becomes insulating again.
 
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