Aluminum busbar

Istas [20] described an aluminum busbar system in a mercury-cell plant. The design-basis current density for the buswork was about 900kAm operation was close to 800kAm. The buswork joints were clamped. Joints between aluminum and copper or steel were through 19 ftm of nickel plate. Exposed aluminum surfaces were covered... 

We can extend the comparison between aluminum and copper buswork to the stability of joint resistances. Aluminum flows more easily under stress than does copper, and so the problem of loss of contact pressure as buswork temperature cycles is more severe when using an aluminum bus. Aluminum oxidizes more easily and more rapidly than copper and forms an oxide with a higher resistance. Furthermore, copper oxide has a negative temperature coefficient of resistivity, so that its contribution to joint resistance actually declines as temperature increases. Aluminum oxide, like the conductors themselves, has a positive temperature coefficient For these reasons, welded butt joints are often a better solution for aluminum busbars than clamped or bolted overlap joints. 

TTie rate of heat generation in a conductor is proportional to the square of the current. Both radiation and convection dissipate the heat. Compact arrangements of busbars restrict the opportunities for radiation, and convection usually is more important. Polished copper may be more attractive visually, but keeping the surface free of oxide is counterproductive. The emissivity of a polished copper surface is about 0.05 the emissivity of an oxidized surface is 10 times higher. Aluminum tends to have lower emissivities ( 0.02), but again, heavy oxidation produces a 10-fold increase. Outside surfaces of conductors sometimes are painted to increase the emissivity even more. 

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