Effects on a cable at sea level compared to 35,000 feet

[joe465]

Hi Guys,

Struggling to find any information on the changes to a cable at altitude compared to at sea level. The initial task was to calculate the resistance of generator wires and then change them from copper to aluminium cables to save weight. I have done this leading me onto describing the effects of the cable at 35,000 feet compared to at sea level.

All i can think of is a decrease in temperature causing a reduction in resistance but would ideally like to provide an example but struggling to find a formula to fit. I know copper has a temperature coefficient of resistance of 3.9 * 10^-3 and aluminium is the same.

Are there any other major effects to the cable at altitude?

Cheers,

Joe

 

[mfb]

The temperature depends on the environment. 35000 feet above sea level is a typical altitude for an airplane, so temperature will depend on the position of the cable in the airplane.
You also get a bit more radiation, but that should be negligible for cables, and I don't see any effect of a changed air pressure.

 

[joe465]

Thanks, it is for an aircraft, it is generator cables for a 747. No environment stated so i could possibly just go with the 2 degree decrease every 1000 feet?. What would you recommend?

 

[mfb]

If temperature is important, I would recommend to check the design, and the expected temperature range for the cable.

 

[Carl Pugh]

I don't have any answers, but do have some questions.
At 35,000 feet the breakdown voltage of air is much less than at sea level.

Would there be a problem with cable terminations breaking down at 35,000 feet?
Would cable have corona at 35,000 feet? (Very short life)
Cooling of cables will be less at 35,000 feet. What is temperature rise?
Will cables outgas at 35,000 feet? Is this a problem?

 

[mfb]

It is impossible to answer those questions in general.

 

[SirAskalot]

To the OP, have you done some research?

E.g:
http://ieeexplore.ieee.org/xpl/articleDetails.jsp?tp=&arnumber=6440678&queryText=cables+airplane

 

[joe465]

Thankyou for the link, finally came across http://www.ehow.com/info_10062949_effects-altitude-aircraft-wiring.html with some basics on it, should be enough to tie me over combined with your link. Thanks very much guys.

 

[berkeman]

To the OP, have you done some research?

E.g:
http://ieeexplore.ieee.org/xpl/articleDetails.jsp?tp=&arnumber=6440678&queryText=cables+airplane

Thanks for the link. I kept asking myself, "What is the big deal? It's easy to select a power cable." I wasn't understanding the part about how important weight is in a plane design.

Makes much more sense now!

 

[AlephZero]

No environment stated so i could possibly just go with the 2 degree decrease every 1000 feet?. What would you recommend?

If this is a "homework" exercise, I think using the International Standard Atmosphere would be defensible set of design conditions. Or use ISA + or - 20C, whichever is the worst design conditions.

If it is "for real", you need to find the relevant section of the Joint Airworthiness Regulations (JARs) - sorry, I'm not an "electrical" guy so I can't tell you which section you want, but don't expect it to be easy bedtime reading!

Remember your generator probably has to work after sitting on the ground all day in the Middle East in air temperatures of +50C, as well as at -80C on a cold day at 35,000 ft!