Tuesday, April 28, 2009

Busbar Ratings

Busbar ratings are based on the expected surface temperature rise of the busbar. This is a function of the thermal resistance of the busbar and the power it dissipates. The thermal resistance of the busbar is a function of the surface area of the busbar, the orientation of the busbar, the material from which it is made, and the movement of air around it. The power dissipated by the bus bar is dependent on the square of the current passing through it, its length, and the material from which it is made.
Optimal ratings are achieved when the bar runs horizontally with the face of the bar in the vertical plane. i.e. the bar is on its edge. There must be free air circulation around all of the bar in order to afford the maximum cooling to its surface. Restricted airflow around the bar will increase the surface temperature of the bar. If the bar is installed on its side, (largest area to the top) it will run at an elevated temperature and may need considerable derating. The actual derating required depends on the shape of the bar. Busbars with a high ratio between the width and the thickness, are more sensitive to their orientation than busbars that have an almost square cross section.
Vertical busbars will run much hotter at the top of the bar than at the bottom, and should be derated in order to reduce the maximum temperature within allowable limits.
Maximum Busbar ratings are not the temperature at which the busbar is expected to fail, rather it is the maximum temperature at which it is considered safe to operate the busbar due to other factors such as the temperature rating of insulation materials which may be in contact with, or close to, the busbar. Busbars which are sleeved in an insulation material such as a heatshrink material, may need to be derated because of the potential aging and premature failure of the insulation material.

The Maximum Current rating of Aluminium Busbars is based on a maximum surface temperature of 90 degrees C (or a 60 degree C temperature rise at an ambient temperature of 30 degrees C). If a lower maximum temperature rating is desired, increase the ambient temperature used for the calculations. i.e. If the actual ambient temperature is 40 degrees C and the desired maximum bar temperature is 80 degrees C, then set the ambient temperature in the calculations to 40 + (90-80) = 50 degrees C.

The Maximum Current rating of Copper Busbars is based on a maximum surface temperature of 105 degrees C (or a 75 degree C temperature rise at an ambient temperature of 30 degrees C).

The Busbar Width is the distance across the widest side of the busbar, edge to edge.

The Busbar Thickness is the thickness of the material from which the Busbar is fabricated. If the busbar is manufactured from a laminated material, then this is the overall thickness of the bar rather than the thickness of the individual elements.

The Busbar Length is the total length of busbar used.

The Busbar Current is the maximum continuous current flowing through the busbar. The power dissipated in the busbar is proportional to the square of the current, so if the busbar has a cyclic load, the current should be the RMS current rather than the average. If the maximum current flows for a considerable period of time, this must be used as the current to determine the maximum busbar temperature, but the power dissipation is based on the square root of the maximum current squared times the period for which it flows plus the lower current squared times the period it flows all divided by the square root of the total time. For example, a busbar carries a current of 600 Amps for thirty seconds, then a current of 100 amps for 3000 seconds, then zero current for 3000 seconds. The power dissipation is based on an RMS current of sqrt(600x600x30 + 100x100x3000 + 0 x 3000)/ssqrt30 + 3000 + 3000) = 82.25 Amps.

The Ambient Temperature is the temperature of the air in contact with the busbar. If the air is in an enclosed space, then the power dissipated by the busbar will cause an increase in the ambient temperature within the enclosure.

To calculate the rating of a busbar, enter in the width and thickness of the bar, and the ambient temperature around the bar. Select the units as either metric or imperial, and the temperature as Celsius or Fahrenheit. The program displays both the current rating of an aluminium bar of these dimensions and a copper bar of these dimensions.

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