Derating breaker AIC rating for altitude

[squeirdo]

We are submitting gear for a project that will operate at 480v nominal at 7500 ft. The specs require 600v rated breakers because 480v breakers can't handle 480v when derated for altitude. The one-line shows 38000A of available fault current at 480v. The breakers we have submitted have an AIC rating of 65000A at 480v. They are being rejected because they are only rated for 25000A at 600v. It looks to me like they are derating the 480v AIC of the breakers by a factor greater than .58 for the nominal system voltage. Does this seem right? Neither voltage and ampacity of these breakers would be derated more than .95 for this altitude. Why would such a high derating factor be applied to the AIC rating?

 

[topgone]

We are submitting gear for a project that will operate at 480v nominal at 7500 ft. The specs require 600v rated breakers because 480v breakers can't handle 480v when derated for altitude. The one-line shows 38000A of available fault current at 480v. The breakers we have submitted have an AIC rating of 65000A at 480v. They are being rejected because they are only rated for 25000A at 600v. It looks to me like they are derating the 480v AIC of the breakers by a factor greater than .58 for the nominal system voltage. Does this seem right? Neither voltage and ampacity of these breakers would be derated more than .95 for this altitude. Why would such a high derating factor be applied to the AIC rating?

Here's my take on this issue:

Firstly, you didn't tell us what kind of gear are you submitting in your project. Be that as it is, I assumed your breaker is of the "air-break" type, hence you need to derate the AIc rating of your breaker due to a lower relative air density at higher altitudes (the breaker on high-voltage side of your step-down trafo.) It would be best if you contact the manufacturer to verify the derating.

OTOH, low voltage circuit breakers 600V and below have a standard rating through 2,000 meters (6,560 feet). Above that, breakers are derated at around:

1. 1% per 100 meters (328 feet) for voltage and
2. 2% per 1000 meters (3280 feet) for current
   

above 2,000 meters

 

[Smart $]

Because whomever is doing the rejecting don't know how to determine the corrected AIC rating at 480V. I surmise they are probably thinking the AIC rating at 480V is the same as that at 600V.

 

[bob]

http://www.google.com/url?sa=t&rct=...7fimspd7tWk_onFd_qix86A&bvm=bv.42080656,d.eWU

Check this site. It shows a 90% rating at 16000 ft.

 

[Jraef]

Re-rating for high altitude is based on two main issues:

1. The lowered ability for the thinner air to absorb heat, meaning current ratings must drop because the equipment cools at a different rate
2. The decreased dielectric strength of the air, meaning that BIL ratings and air gaps selected for use at lower altitudes cannot insulate and/or interrupt current flow as well at high altitude.

It stands to reason then that the breaker must be designed for a higher voltage rating, but in reality, most 480V breakers (above 400A frame) are 600V design anyway.

However the INTERRUPTING current rating of the breaker has only to do with the MECHANICAL forces the device will be subjected to during operation in a fault condition. Those mechanical forces are based on let-through ENERGY during the fault. So the reason why a breaker has a lower AIC rating at 600V compared to 480V is because at the higher supply voltage, there is going to be more ENERGY in the same amount of current. Do the math; 65,000A x 480V will result in less energy than 65,000A at 600V. The relationship to mechanical forces is not linear, that's why the same breaker rated 65kAIC @ 480 is typically only 25kAIC @ 600.

But here is where they are wrong. YOU DO NOT REALLY HAVE 600V on the breakers, you have 480V. You are only responding to the voltage rating increase because of the reduced dielectric of the thin air in the breaker's air gap, but that does NOT mean that the mechanical forces during a fault will be higher. It is STILL only 480V!

So they are incorrectly applying de-rating factors to this specific issue in my opinion.
Proving it to them may be a difficult task however, because you will be looking to disprove an erroneous presumption.

By the way, here is a good resource on altitude rating factors for electrical equipment compiled from IEEE data by a PE friend of mine.
http://thomasblairpe.com/altit1.htm#Amps

 

[squeirdo]

Thanks for the input!

Thanks for the input!

Thank you all for the information! Based on these responses, it seems pretty clear that derating the AIC down from 65000A at 480V to less than 38000A at 480V is applying too large a derating factor. We'll see how this plays out ...

 

[GoldDigger]

However the INTERRUPTING current rating of the breaker has only to do with the MECHANICAL forces the device will be subjected to during operation in a fault condition. Those mechanical forces are based on let-through ENERGY during the fault. So the reason why a breaker has a lower AIC rating at 600V compared to 480V is because at the higher supply voltage, there is going to be more ENERGY in the same amount of current. Do the math; 65,000A x 480V will result in less energy than 65,000A at 600V. The relationship to mechanical forces is not linear, that's why the same breaker rated 65kAIC @ 480 is typically only 25kAIC @ 600.


So they are incorrectly applying de-rating factors to this specific issue in my opinion.
Proving it to them may be a difficult task however, because you will be looking to disprove an erroneous presumption.

By the way, here is a good resource on altitude rating factors for electrical equipment compiled from IEEE data by a PE friend of mine.
http://thomasblairpe.com/altit1.htm#Amps

The ability of a breaker to extinguish an arc as it opens depends not only on the mechanical force, which directly determines the speed at which the contacts separate, but also on the ability of an arc to continue or restart after each 1/2 cycle of current.
Just as vapor lamps will not restart when they are hot because the voltage required to start an arc at higher gas pressure is higher, the breaker may not have the same ability to prevent an arc from restarting at a given contact separation when the air pressure around the contacts is lower. The degree to which this is important will depend critically on the design of the breaker (arc chute, etc.) as well as the speed at which the contacts separate.
Unless the particular manufacturer supplies AIC derating factors for altitude, just applying either the normal current or normal voltage derating factors may not be conservative enough. But I agree that blindly using the 600 volt AIC figure is not correct either!

The paper that you cite is an excellent compilation of calculated current and voltage capacities, as well as the switching transient voltage tolerance, but I am not convinced that any of these factors are directly applicable to the AIC rating.

 

[jim dungar]

FWIW and the fact that I live around 700ft.

35+ years of dealing with molded case breakers, I have never seen 'field' adjustment numbers, of any kind, for AIC. Not for ambient, not for altitude and not for voltage.