As a bus (lets say.25" x 6") approches maximum load, does the heat generated by the load begin to concentrate on the center line of the bus?
Is there any reason to believe that there is more electrical flow at the center line of the bus?
Or is it more typical to see even heat distribution across the width of the bus regardless of the % of Maximum load?

This is relevant to an analysis I am currently performing.

I guess what I am really asking is if the edges of the bus have any greater ability to radiate heat than the flat portion of the bus.

I would guess yes, but only marginally. If you consider a cross section of the bus, the long flat portions are exposed to ambient for 180 degrees of space, but the corners are exposed to 270, and the edges nearly 360 degrees.

Since nobody else responsed I'll get the ball rolling with some very wandering comments

I can't offer any comment from experience - We don't do bus inspections at our plant yet simply because these locations are a little more difficult to get to at our plant from an electrical safety perspective. I'm sure others who have examined buses may have some more relevant comments than me.

I can offer some guesses of how to begin to analyse the situation using theory but even there I am not sure. Two aspects: 1 - current distribution; 2 - thermal conductivity/conduction thru copper

1 - current distribution

There are two effects which work in opposite directions - one tends to push current towards the outside and one tends to push current towards the inside.

The skin effect will generally tend to push current toward the outside of a conductor and is more pronounced on larger conductors.

The self-inductance effect will generally try to push current toward the center of the conductor. The reason is that this minimizes the total energy associated with the magnetic field. You may remember in physics textbooks if you have two wires carrying current in the same direction they attract and if carrying current in opposite direction they repel. The same principle could be applied within the bus - if you had current at top of the bus and bottom of the bus flowing the same direciton they would tend to attract and come toward the center. This migration towards the center is limited to a certain extent by the higher resistive voltage drop which would occur when current increases in the center (i.e. if you had a superconductor, the movement of current toward the center would be much more than for a resistive bus).

Which of those two effects would dominate for your particular dimensions I really don't know. I'm sure there are a lot of people including bus manufacturers that study this type of thing carefully. Also I'm pretty sure the ideal rectangular infinite-length single conductor can be solved analytically but again not by me.

The current distribution may also be affected by external fields, especially from adjacent conductors.

Resistance of the conductor to a small effect counteracts the above effects (the current distribution would be distorted more easily on a superconductor than on a resistive conductor).

2 - thermal conductivity / conduction within the bar.
Are these copper or aluminum bus bars? With very high thermal conductivity especially for copper maybe the temperature rise you are seeing is unreasonable regardless of the current distribution.

The thermal conductivity of copper is around K = 400 watts/m/C

Let's say we take a lenght L of your bar. The dimensions are your bar are L x w (6") x t (0.25")

Let's assume the heat is all being produced at the exact center and transferred directly outward a distance w/2 = 3"

How much heat q would be required to cause a 1C temperature difference between center and outside?

q = dT * K * A / [w/2] = dT * K * [L * t] / [w/2]
divide through by L to put everything on a "per length" basis
q/L = dT * K * t / [w/2]
subsitute in the values
q/L = 1C * 400 watts/m/C * 0.25" / 3" = 33 watts / m
Hmmm, that's not as large as I thought.

Still my gut feel is that if you are seeing any significant dT accross the bus than it is probably either a cooling effect as you mentioned (maybe the air tends to be flowing accross the edges and stagnant towards the center?) or else a measurement effect.

=====================

With all the knowledge of the other participants here, it might help if you post images of the specific situation you are interested in.

OK Pete, I didn't want to get into the specific case here, but since you asked...

This is a look at the back of an automatic transfer switch, the busses "enclose" the switch.
MMM

Bus_bar.doc (178 Kb, 39 downloads)