Arc Flash and Altitude

[Darrell Wilson]

First time posting on this forum.

My question is "does altitude affect arc flash calculations in any way"

The energy released would not change as it is the result Voltage*Current*time, which is not affected by altitude, to my knowledge.

However, I do know that the dielectric strength of or the air changes at higher altitudes. It gets smaller before it it gets larger. (High-Altitude considerations for Electrical Power Systems and Components, IEEE Transactions on Industry Applications, Vol 1A-20, No. 2, March/April 1984)

So the question becomes is the arc flash boundary affect in any way due to altitude? My guess is no as the energy is not increased, and it is still spread equally across an expanding sphere.

 

[FPelec]

Hello Darrell Wilson,
you are correct, the arc voltage is likely to be affected by atmospheric pressure and, eventually, by site altitude.
To my knowledge there are no standards considering this effect. However, the arc voltage and energy will be reduced if compared to sea level, so that it is safe to consider the classic formulae also at higher altitudes.


Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[cuky2000]

Considering that several software use site altitude to determine the minimum approach distance of the arc flash may require further investigation to see how this will impact the safety distance and PPE requirement.
Since he dielectric strength is reduced with the altitude for any energized part expose to air, the impacted equipment and component clearances will be derated accordingly. This may imply that the probability of arc flash occurrence may increases with the altitude.

 

[dpc]

Altitude is not considered in any arc-flash calculation methodology that I am aware of. There is no mention of altitude in IEEE 1584. Once an arc is initiated, the "air" that the arc is flowing through is ionized gas or plasma so the impact of air density may not be that significant. Other factors such as humidity, air velocity, etc could also impact the arc, but are also not considered in the calculations.

Just as an aside - arc flash calculations are not all the accurate. There are far too many variables to account for. The goal is to find a reasonable level of PPE to protect the worker under reasonable "worst case" conditions.

 

[cuky2000]

Does the high temperature of the arc flash can create an arc blast?
If so, how the pressure wave of the blast will be impacted by the altitude?

 

[dpc]

As far as arc blast - this is a qualitative term - there's no definition of what constitutes an "arc blast" compared to an arc-flash event. And no measurements or calculations to quantify it. My hunch is that there is not sufficient difference in air density due to altitude to have a major impact on either, but the safest answer is that nobody knows.

 

[cuky2000]

IEEE and NFPA and others are researching the effect of arc blasts, as well as arc flashes. The goal is to eventually produce more accurate ways of calculating and predicting the intensity of the arc blast pressure. As the research progresses, we may see someday arc flash warning labels on equipment that provide values not only for arc flash but also for the arc blast pressure.

It should be noted that PPE clothing may protect workers from the thermal effect within flash protection boundaries but not to the arc blast. Below is an illustration to think further about the impact of arc flash/blast combined.

 

[che12345]

Dear Mr.Darrell Wilson

Q. " My question is "does altitude affect arc flash calculations in any way..."
A1. I am of the opinion that this question is more as a "academic discussion " rather than a pressing problem. The answer at this juncture is "we don't know" , as the subject is neither discussed/reported in any Standards bodies nor the testing stations.
A2. Noticed that most Standards are written based on generally NOT exceeding 1000m above sea-level. BTW. Motor Standards are in general applicable up to 2000m . Except for the electrical strength of insulation, where the attitude which affects the humidity and atmospheric pressure are reported. On the other hand, for temperature-rise tests, the ambient temperature shall be reported.
A3. Majority of the equipment installation location are <1000m about sea-level.
A4. Most testing stations are located not exceeding 1000m above sea-level.
A5. Major Standards organisation viz. IEC, ANSI, UL , NFPA ... do not state that the altitude, humidity and atmospheric pressure shall the reported for the arc-flash testing.
A6. This is also evident in the early stage; the five PEHLA recommendations did not stress the altitude effect, affecting the arc-flash calculations/behaviour/characteristic.
Che Kuan Yau (Singapore)