Is there any effect on the rated current due to holes present in the BlokSTt D/Mf/Ms/Dc Vertical bus bars as it reduces the effective cross section of the copper?

The main function of the holes is to ensure flexible / adaptable connection for the corresponding feeders, irrespective of the feeder position in the feeder column.

The current carrying capacities of the vertical bus bars are decided based on certain parameters as mentioned below:

• Cross section of the vertical bus bars
• Length of the vertical bus bars
• Amount of heat generation & dissipation
• Orientation of the bus bars
• Power loss of the devices connected to the vertical bus bars
• IP & Ambient conditions and so on

Similarly the heat dissipation of the vertical bus bars mainly depends up on the conduction phenomenon (more than 70% heat dissipation occurs by conduction) and to a very small extent by convection & radiation

Addition of too many holes will certainly result in loss of cross sectional & surface area but it doesn’t drastically affect the current carrying capacity / dissipation of the vertical bus bars. This is because; as the vertical bus bars are quite long (approximately more than 1.8m) it helps in dissipating the heat more effectively.

Moreover, the complete length of the vertical bus bar is never going to carry its rated current because of the load drawn by the respective feeders in the cubicle as explained in the below figure.

So depending on direction of current flow (either from top to bottom or bottom to top depending up on horizontal bus bar position), a major portion of the vertical bus bar is going to act as heat sink which will further aid in dissipation of heat more effectively.

Considering all these parameters, the series of holes of 25mm pitch along one edge of the vertical bus bars are added. It is also been verified and proved by testing that the performance is satisfactory with the addition of holes at a pitch of 25mm.

Released for:Schneider Electric New Zealand