# Current carrying capacity of a copper busbar in an Aircraft

#### huytergan

Summary
In what ways an aircraft ,at over 35000 feet, imply methods to dissipate the heat generated in a busbar? What's done to get away from the heat with ease?
"The surface of copper naturally oxidises, forming a thin hard layer on the surface which normally prevents further oxidation. " I've seen this in a book, higher level oxygen on busbar, means lower the heat and get away with ease, Doesn't it? So, Can I say a thin hard layer musn't be used?

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#### berkeman

Mentor
Does the hammer deform the anvil, or knock it up in the air? Is there a reason you smack the anvil on the way up instead of just on the way down (put the anvil on the right side)?

#### huytergan

Does the hammer deform the anvil, or knock it up in the air? Is there a reason you smack the anvil on the way up instead of just on the way down (put the anvil on the right side)?
Well I must confess you made me smile. Any way, I think effects and conditions are quite different than you thought. Let me say what I think, If it goes up to the thousand feets, it means less oxygen, and air density is lower. Lower air density should mean slower movement in copper. Assume it's DC and constant current, its level isn't changing but its time are going to be different with regard to either it has more oxygen or less. Do we want it to act slower or faster as much as possible to not get a quite heat loss? I say, we must want it act faster. So in my opinion, we don't need it has a thin hard layer since it normally prevents further oxidation.

#### berkeman

Mentor
I'm so sorry! I meant that reply for a different thread, and somehow it ended up here! I was wondering where my reply went! Doh!

I did do some searching on your question, to try to find out what the difference in heat conduction is for copper oxide versus clean copper, but I wasn't able to find anything helpful.

Are you asking about the oxidation rate of the copper versus time at altitude, or about the heat dissipation differences for the busbar once it is oxidized?

#### huytergan

I'm so sorry! I meant that reply for a different thread, and somehow it ended up here! I was wondering where my reply went! Doh!

I did do some searching on your question, to try to find out what the difference in heat conduction is for copper oxide versus clean copper, but I wasn't able to find anything helpful.

Are you asking about the oxidation rate of the copper versus time at altitude, or about the heat dissipation differences for the busbar once it is oxidized?
That's okay! Yes, about the heat dissipation differences for the busbar once it is oxidized, and relationship from over the altitude difference. Do you think I'm wrong about what I've written in the previous post?

#### berkeman

Mentor
Do we want it to act slower or faster as much as possible to not get a quite heat loss? I say, we must want it act faster.
Sorry, what is "it" in this quote?

#### huytergan

Sorry, what is "it" in this quote?
I meant current, so do electrons. DC source will pomp the electrons, but electrons' speed must be variable depends on amount of oxygen in my opinion. Low oxygen, low speed of act of the electrons that's what I thought.

#### berkeman

Mentor
In the busbar? No, the electron drift velocity in the metal busbar depends on the current, not on anything else.

#### anorlunda

Mentor
Gold Member
Summary: In what ways an aircraft ,at over 35000 feet, imply methods to dissipate the heat generated in a busbar? What's done to get away from the heat with ease?

means lower the heat and get away with ease, Doesn't it?
You can coat a bar with some material that has higher thermal resistance than the copper. But if the layer is thin, it only makes a small difference.

All heat generated in the bar due to electrical current will escape into the surroundings. If you put more thermal resistance, as in a coating, it just means that the temperature difference between the bar and the air will be higher.

There is no need to talk about oxygen or electrons in this question.

#### Tom.G

A quick search shows Thermal conductivity of CuO around 20 to 80 W/mK. The Thermal conductivity of pure Cu about 400 W/mk. Since the Oxide layer is so thin I would expect little temperature difference in free air at sea level.

In free air most heat loss is due to convection, that is the air moving past the busbar is heated by conduction and the heat carried away as the air moves away.

In a vacuum, heat loss is mostly by radiation to the surroundings, and some by conduction through anything touching the busbar.

'Emissivity' is how well a material radiates energy compared to how well a 'perfect' emitter would radiate energy at the same temperature. An emissivity of 0.1 indicates the material radiates 1/10 the energy that a 'perfect' (emissivity = 1) radiator would radiate.

Polished, bare Copper has an emissitivity around 0.035. A thick Black Oxide layer raises the emissitivity to 0.78, over 20 times greater.

At high altitudes (lower air pressure, closer to a vacuum), heat transfer by radiation gets larger than heat loss by convection. So you would like a layer of Oxide on the Copper to help keep it cool.

Cheers,
Tom

#### huytergan

A quick search shows Thermal conductivity of CuO around 20 to 80 W/mK. The Thermal conductivity of pure Cu about 400 W/mk. Since the Oxide layer is so thin I would expect little temperature difference in free air at sea level.

In free air most heat loss is due to convection, that is the air moving past the busbar is heated by conduction and the heat carried away as the air moves away.

In a vacuum, heat loss is mostly by radiation to the surroundings, and some by conduction through anything touching the busbar.

'Emissivity' is how well a material radiates energy compared to how well a 'perfect' emitter would radiate energy at the same temperature. An emissivity of 0.1 indicates the material radiates 1/10 the energy that a 'perfect' (emissivity = 1) radiator would radiate.

Polished, bare Copper has an emissitivity around 0.035. A thick Black Oxide layer raises the emissitivity to 0.78, over 20 times greater.

At high altitudes (lower air pressure, closer to a vacuum), heat transfer by radiation gets larger than heat loss by convection. So you would like a layer of Oxide on the Copper to help keep it cool.

Cheers,
Tom
I appreciate, Tom. That's what I wanted to hear.
Cheers.

#### Baluncore

Copper conductors are often tin or zinc plated to prevent surface oxide formation.

In an aircraft I would expect the conductor to be made, not from copper, but from aluminium, plated with a conductive metal.

Heat radiation and conduction are not dependent on air density or pressure. Convective cooling is dependent on air density.

Outside the cabin, at 35,000 feet, the air is only about 30% of the density at sea level, so it provides less convective cooling. But the temperature at 35,000 feet is stable and close to –57°C, which lowers the resistance of the conductor.

Inside the cabin, air pressure is maintained to be equivalent to a maximum altitude of about 7,000 feet. Convective cooling and heatsinks then work normally, as specified.

#### huytergan

Copper conductors are often tin or zinc plated to prevent surface oxide formation.

In an aircraft I would expect the conductor to be made, not from copper, but from aluminium, plated with a conductive metal.

Heat radiation and conduction are not dependent on air density or pressure. Convective cooling is dependent on air density.

Outside the cabin, at 35,000 feet, the air is only about 30% of the density at sea level, so it provides less convective cooling. But the temperature at 35,000 feet is stable and close to –57°C, which lowers the resistance of the conductor.

Inside the cabin, air pressure is maintained to be equivalent to a maximum altitude of about 7,000 feet. Convective cooling and heatsinks then work normally, as specified.
I'm talking about a busbar or a cable. They're with the engine part. I'm not sure you're thinking this through. You can't expect a cable to feel -57C. Convective is a natural way of cooling at a sea level. At higher altitudes around 35000 feet, Radiation looks like the only way. AND, I don't get why you expect the conductor to be made from Aluminium, Copper's higher conductivity and much more durable.

#### Baluncore

The conductivity of Cu is not much better than Al, but Cu weighs much more than Al, so the A380 uses aluminium power cables instead of copper for weight reduction.

#### essenmein

I'm talking about a busbar or a cable. They're with the engine part. I'm not sure you're thinking this through. You can't expect a cable to feel -57C. Convective is a natural way of cooling at a sea level. At higher altitudes around 35000 feet, Radiation looks like the only way. AND, I don't get why you expect the conductor to be made from Aluminium, Copper's higher conductivity and much more durable.
Um no. Convection cooling happens not just at sea level, it will happen anywhere as long as there is a fluid present that changes density with temperature and as long as there is gravity or some other source of reasonably constant acceleration (without external acceleration there is no convection).

If the plane can "fly" at 35k ft, then that means there is air, and since the plane will fall without engines, that means there is constant acceleration, so at 35k ft you have both the required conditions for convection cooling, why would you think it doesn't happen?

Then the ambient air temperature at that altitude is as mentioned quite low, I don't know how accurate the -57C is, but if it is and the bus bar is not in the heated cabin or right next to the engine, then -57C will be its ambient.

"Current carrying capacity of a copper busbar in an Aircraft"

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