Why coated copper carry more current than bare bar?

Question

From Rittal Technical Information Rated currents of busbars E-Cu (DIN 43 671) :

In the table, it says bare bar carry less current than coated bar. What is the reason of that ? and what kind of coated or paint should be used ?

Answer 1

The reason is given in the norm you cite, the German DIN 43671:

Blanke Stromschienen teilweise oxidiert, so daß der Emissionsgrad 0.4 ist;

Stromschienen gestrichen, so daß der Emissionsgrad 0.9 ist.

That is bare bars have a surface of oxidized copper with an emission coefficient of 0.4, while the bars coated in paint have a much higher emission coefficient of 0.9.

With the higher coefficient of emission the painted bars can radiate heat much more efficiently and maintain the allowed temperature difference to ambient of 30 K at a higher current.

The type of paint is not specified. The coating should be thin to not affect heat dissipation by its own heat conductivity. The color doesn't matter - at 80° Celsius the thermal emission is entirely in the infrared range, mainly at wavelengths between 5 and 15 µm. Any paint with an emission coefficient of at least 0.9 in this range will do.

The norm also makes clear that the values given only apply for indoor installations of single bus bars. The rating changes outdoors (additional heat influx from solar radiation and different oxidation behavior), with airflow (better heat transfer by convection) and if bars are grouped (less efficient radiation).

As a side remark, one should also wonder how the cross section is calculated as it doesn't match the dimensions given. The reason is that bus bars as defined in DIN 46 433, section 3 don't have a rectangular cross-section but rounded corners.

Answer 2

Those are ratings, you need to find out what the limiting factors are when making them.

There may be a couple of possibilities

1 - Are the dimensions including or excluding the coating? If excluding, then the coating would make the bar bigger, so increase the cooling surface area available, thus increasing the current needed to maintain the bar temperature of 65 C.

2 - What's the emissivity or surface roughness of the coating? Improved cooling by greater infra-red emission or by breaking up laminar flow to convection would increase the current to maintain bar temperature.