Technical note: 021 Dated: 28 th
Oct 15
Subject: Few Design Aspects On Bus bars in Switchboards
Heat generation has always been the nemesis of electrical engineers from a cell phone to a transformer; from a motor to switchboard. Let us look at certain aspects to ensure that the temperature rise of bus bars in a switchboard is within limits.
A. Current density:
Due to economic consideration, in India we use aluminum bus bars and not the better suited copper bars especially in Low Tension Boards, where high
D. Sizing of bus bars:
It is advisable to run wider and thinner bus bars as the cooling surface increases. There is a considerable reduction in skin effect of thinner bus currents are encountered. The easiest way to control temperature rise is to reduce heat generated. This can be achieved by lowering the current density.
However beyond a limit this is neither practical nor economical. Then what? Instead of controlling heat generated let us explore the means of removal of generated heat. bars as compared to thicker bus. Having said this thinner bus have lower mechanical strength hence there is a reduction in short circuit withstand ability.
E. Location of horizontal bus chamber:
Due to space restriction and disregard to safety aspects the switch boards get located in least accessible spaces. The cable entry is from top. The
B. Number of bus bars per phase:
To lower the current density and in turn lower the heat generated there is a tendency to run number of bus bars in parallel. This number should be restricted to 2 in rare cases 3. It has been found that if 4 bus bars are run in parallel the fourth bus bar hardly carries any current due to skin effect further the fourth bus bar impedes the flow of air and restrict the heat from being carried away.
C. What is Skin Effect?:
Skin effect is the tendency of an alternating electric current to become distributed within a conductor such that the current density is largest near the surface of the conductor, and decreases with greater depths in the conductor. The electric current flows mainly at the
"skin" of the conductor, between the outer surface and a level called the skin depth. The skin effect causes the effective resistance of the conductor to increase at higher frequencies where the skin depth is smaller, thus reducing the effective cross-section of the conductor. The skin effect is due to opposing eddy currents induced by the changing magnetic field resulting from the alternating current. switchboard manufacturer shifts the bus bar chamber from top of the board to bottom. Thus the heat generation section goes to bottom. Heat rises through the board heating all the elements in its path. This is detrimental to the operation. Hence it is always advisable to have the bus bar chamber on top of the board.
F. Black Body Radiation:
An increase of around 10% extra capacity will be available if the bus bars are painted in ‘Black” bus bar paint with matt finish. Black body has the best heat radiation capacity. Phase identification bands can be provided at intervals for identification.
G. Spacing between the runs of bars:
It is an industry norm that the space between the bus bars of the same phase, is equal to the thickness of the bus. However if this gap is kept at twice the thickness better results are obtained.
H. Interconnection between sections:
The switchboards are generally dispatched from the manufacturer to the site in convenient shipping section. After erection the bus bars are connected with fish plates with width same as the bus. If spliced fish plates are used for connecting, misalignment and skew if present will be taken care. Thus reducing the mechanical stress.
I. Interleaved bus bars:
For rating above 3200 amps it is advisable if the bus bars are interleaved. Good results are also obtained if the runs of bus bars of the same phase are in vertical configuration.
J. Conclusions:
The above points are simple and obvious, but it is the obvious that is most difficult to perceive. The steps given above will result in reduction of temperature rise around 4 to 5 degrees.