Rating of busbars
Posted 01 November 2005 - 04:41 AM
I read somewhere once that silver plated busbars can be uprated by a factor due to the fact that the surface does not oxidise.
Does anyone know what this factor is?
Why?..Is it due to the skin effect with ac current or simply the impedence increases due to the oxidisation.
And I would like some kind of reference or standard that I can quote to back this up if this is the case.
Some flexible busbar manufacturers have much higher current ratings that you would expect for the csa of the copper, and I have seen references to that being due to this oxidisation effect.
And finally would tinning have the same effect if this is true?
Posted 01 November 2005 - 05:43 AM
At very high frequencies, the skin effect is such that sliver plating will reduce the effective resistance, but at the frequencies that we are dealing with, I do not believe that silver plating will make much if any difference because the skin effect will be relatively minor.
My understanding for the plating of bars, is to prevent the corrosion of the copper and that can be an issue.
I was recently asked to repair a large soft starter, and when I stripped it down, I found that a lot of the tracks on one of the snubber pcbs had been eaten away. The tracks wer coated with soldermask and conformal coating and the corrosion got through the lot. This starter was being used on an irrigation pump and I suspect that it managed to suck some super phosphate dust through when the farm was being fertilised.
Copper corrosion can occur anywhere, not just in Rotorua.
Adding a good high temperature sleeving will increase the surface area of the bars and proveded that the long face is in the verical plane, the thermal resistance will be reduced and the current rating will increase.
The rating of the busbar is based on the temperature rise of the bar and this is in turn a function of the power dissipated in the bar and the thermal resistance of the bar. The thermal resistance is proportional to the vertical component of the surface area and to a lesser extent to the horisontal component of the surface area. The power dissipated is proportion to the square of the current and the effective cross sectional area.
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