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Altitude Derating

Altitude Derating

Altitude Derating


Can anyone explain the logic (physics) behind the need to derate at altitide.

To my twisted way of thinking, it is the opposite to what I would have thought.

At altitude, air is less dense, and I would have thought that this would have made it a better insulator - apparently it does not, it is the opposite - why??

Also, there is the issue of different derating requirements between LV and MV.  For LV, derating only applies above 2,000 metres, while for MV, derating applies above 1,000 metres - why is this so??

I haven't been able to find a suitable explantion why - any explanation would be appreciated (because for my poor old mind, if you use this logic, a vacuum should be a good conductor).


RE: Altitude Derating

I'll take a stab at that.

One common type of derating I am familiar with is power derating of a motor of transformer due to altitude.  If the air is less dense it is less effective at removing heat.

It sounds like you're talking about reducing voltage due to altitude. I haven't heard of that, but I do believe that the breakdown voltage reduces as air pressure decreases.  As to why a vacuum interruptor works well.... breakdwon strenght is no a monotonically increasing or decreasing function of pressure.  As we decrease pressure from atmospheric the breakdwon strength starts to go down, then hits a minimum and starts to go down. Similar to Paschen's curve.

RE: Altitude Derating

MV and LV equipment are built to different standards and the standards specify the altitude derating.  See relevant NEMA, IEEE, ANSI, IEC, etc standards for the equipment you are using or specifying.

RE: Altitude Derating


Yes, i am talking about voltage derating.  The rated one-minute power frequency withstand voltage, the impulse withstand voltage and the maximum voltage rating must be corrected (derated) for altitudes above 1,000 metres for MV applications.

The continuous current rating must also be corrected.

For the site I am at, in the Andes at 3,600 metres, the altitude correction factors for MV are 0.75 for voltage and 0.95 for current.

As you say, as the air becomes less dense, it becomes less effective at removing heat - doesn't this make it a better insulator - why does this not apply electrically.

We use the correction factors as per the code, but I am just curious as to why - what are the physics involved.

RE: Altitude Derating

I have dealt with corona (partial discharge) management.  The physics are that as voltage is applied, you begin to ionize the air molecules.  When you have a conductive path between two potentials, you will get a flashover.  At lower pressure (higher altitude) and lower humidity (water vapor ahs to be ionized too), it takes less voltage to cross a given gap.  IEEE Std 4 has the formula for this.  Corona was first noticed in WWII airplanes at flying attitude - everything checked out onthe ground, but lit up big time as the planes climbed.


RE: Altitude Derating

At atmospheric pressure you need thoudands of volts to
generate a .1 inch spark/discharge. Around 1mm hundreds
are enough ( see: neon/ glow discharge lamps)


RE: Altitude Derating


You mention that PFWV, BIL, Maximum Voltage, and continuous current ratings are affected by altitude.

In my experience (mainly instrument transformers and distribution transformers) only the BIL rating is generally affected by altitude, mainly because it is more dependant on strike distance than the others and since the strike distance is through air...

The altitude (outside air pressure) has no affect on anything under oil (i.e. inside a tank or housing) in sealed type apparatus. Therefore, I do not see why it would affect max voltage or continuous current rating (at least not at any reasonable altitude).

RE: Altitude Derating


The voltage derating only applies for anything exposed to air - power line insulators, t/f bushings, switchgear busbar support insulators, etc.

At altitude, air becomes a less effective insulator electrically (as I have just discovered), hence the voltage derating.

Also, at altitude air becomes a more effective thermal insulator, hence current derating.

There is quite a good article on:


RE: Altitude Derating

I guess I can buy the thermal rating for current. I actually remember now that in IEEEC57.13 (instrument transformer spec.) maximum continuous current is derated 0.3% for every 100m over 1000m altitide.

However, I still don't understand the need to derate operating voltage. I can tell you that in equipment I deal with, operating/maximum voltage rating is not reduced with increased altitude. The air distance/strike distance should only affect the insulation levels and in that regard the transient ratings, i.e. BIL. If you have an apparatus where the rated voltage is affected by the strike distance/air distance, then I would question the integrety of that design!

IEEE C57.13 also gives a dielectric correction factor of 1% for every 100m over 1000m. This is the same as in the IEC44-XX specifications for instrument transformers. Neither of these specifications mention the need to de-rate the operating voltage rating as a function of altitude.

RE: Altitude Derating

Maybe I can add some.  As the altitude increases, there is less air, which acts as an insulator.  Dry and clean air is actually quite a good insulator.  Dirt and moisture degrade the insulation (air).  There is a curve that shows the insulating qualities of air vs. pressure.  As the air presure drops, the breakdown voltage drops.  At a vacuum you again have good electrical insulation, but there is a belly in the curve at fairly low pressure - lower than we'd ever see on earth.

I've seen the effects of the lower insulation qualitis of air at higher altitudes.  If you're not expecting it, you stand around scratching your head until you figure it out.  It IS real.  A 35 kV large interface elbow will not work in Rock Sprngs, Wyoming, even though along the east coast they use them all the time without any problems.  You get a flash about at least 10% of the time when you operate them.

Mark in Utah

RE: Altitude Derating

Not sure if you're still interested in this dialog, it's been over a month.  But I
believe the answer you're looking for has to do with the pressure-altitude-voltage
derating related to corona discharge, which increases (gets worse) with higher
altitude and peaks at around 80-100,000'

Typically this is considered to occur at votlages above 300 V peak, but be sure
you are accounting for all transients and internal voltages.

RE: Altitude Derating

The derating factor for altitude is base on the loss of dielectric strength of the air as the density decrease with the altitude.

There are several graph in ANSI C37.30 and IEC/DIN DVE 0111 that describe this factor in similar maner as follow:

                         Altitude Derating Factor
  Altitude:               Dielectric(pu)     Current (pu)
   >3,000 ft (1000 m)         1.00              1.00
    4,000 ft (1200 m)          0.98             0.992
    5,000 ft (1500 m)          0.95             0.980
   10,000 ft (3000 m)          0.80             0.96
   12,000 ft (3600 m)          0.75             0.95
   14,000 ft (4200 m)          0.70             0.935

EX. Determine the equipment rating at sea level to operate at 3,600 m (12,000 ft) if the design insulation is 900 kV BIL.

Per the above table, the dielectric derating factor for   3,600 m  is 0.75.
  External Insulation: 900kV/0.75=1200 kV BIL.

The internal insulation for appartus not direct exposed to the air could such as circuit breaker in SF6 or transformer inmerse in oil, there are not need for external derating. In this case only the bushing could be affected by the altitude.

Note that cables, surge arrester, creepadge distance and safety requirement are ruled with similar but different  requirements.

Other factor such as temperature, moisture, contamination level, should be take in consideration for determine the appropiate dielectric strength.

RE: Altitude Derating

I think that basically for say motors and generators at altitude, as the air is thinner or less dense it's ability to remove heat decreases. It is the mass flow rate of air across say motor or generator windings that is important rather than the volume flow rate.

RE: Altitude Derating

Heat dissipation would be a concern at high altitudes. But the largest reason equipment is derated is because the dielectric strength of air decreases as the altitude increases. As the dielectric strength decreases then a potential of 500 volts at sea level may perform fine but at 10000 feet it may flashover. The key to desinging electronics for high altitudes is to have larger spacing between potentials, use solder masked boards, and use conformal coating. Also be aware of the limitations of your IC's etc in the circuit. There are charts available that will tell you how much spacing is required given the potential difference. I was able to work on a high altitude baloon in college (20000 feet) and experienced first hand the problems with high altitudes (of course at this altitude, moving the heat away was not a problem, it was trying to keep the electronics warm enough). Good Luck.

RE: Altitude Derating

For the lighter air, you have less number of particles in the air. To expalin in a very simple way, I can you an exmple of a needle to be inserted in a soft material and a hard material. Say foam: it is easier to pierce with lower force. For dense foam, you need more force to pierce.
Now replace foam with say air (or for that purpose any insulating gas), the needle with electron. Apply same analogy. You will need more force (electro motive)as you have dense air (gas).
Now that we are on this subject, I give some mild socks to your old mind. More humidity (water vapour, a gas, will increase breakdown voltage of air (gas). Keep im mind only condensed water is killer not the humidity! For higher humidity, the voltage correction factor is +ve.
Hope this will explain you the science of breakdown of gaps.

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