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Importance of proper busbar sizing and termination.

 
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4 Jul 2011

DESIGN CONSIDERATIONS

Given a set of electrical, mechanical and environmental bus bar requirements, the focus of the design process is to find the optimum combination of materials, configurations and manufacturing processes that will yield a finished product that meets the requirements of the application.

Factors influencing the sizing of the busbar:

a) The Voltage and Current rating of the system
b) The enclosure IP rating
c) Forced ventilation or natural ventilation
d) Working and ambient temperatures
e) Heat loss by convection and radiation
f) Heat generated in the busbar

Importance of proper busbar sizing and termination.

Electrical - Delivering Voltage and Current The starting point for bus bar design is identification of the voltage(s) and current(s) that the bus bar will be required to distribute. Then, a candidate cross sectional area can be selected and an initial conductor layout can be designed. The electrical properties of the conductor(s) must then be evaluated to determine if the voltage and current requirements will be met.

The most important electrical property is resistance, which applies to all types of voltages and currents. If the bus bar is carrying AC current two additional electrical properties need to be considered: capacitance and inductance. Each of these causes its own type of reactance (opposition to electrical change). Those reactances contribute to the impedance of the bus bar, which is resistance (to steady current) plus the total reactance.

Resistance-Conductor resistance is calculated from the resistivity of the conductor material and the cross-sectional area of the conductor:

Current through the conductor will generate heat, and the resistance of the conductor will then increase proportionally to the heat. This sounds like an out-of-control spiral, but the system will eventually come to an equilibrium determined by the amount of heat dissipated by the surroundings of the bus bar. An allowable temperature rise will need to be determined, then the resistance recalculated at that temperature to check the impact on bus bar performance.

For the sake of simplicity we shall consider that the current rating of the main panel would depend on the transformer or the submain panels and MCCs on the load to which it is connected.

Also we assume that the LV switchgear installed in indoor, hence panels with an ingress protection of IP41 or IP 42 would be acceptable.

Forced ventilation is also normally accepted for capacitors banks and VFD panels only, hence normal panels are to be designed based on natural ventilation. Thus in order to keep the temperature within the specified limits, the switchgear needs to be designed to dissipate the heat generated to the surrounding atmosphere by radiation and by conduction through connected busbars/cables. Hence in addition to the general understanding that busbars are meant only for carrying current, it should be understood that they are also functional in carrying away the heat generated in the switchgear. Terminations by virtue of their contact resistance are also sources of heat. But in an electric system, the heat generated at the terminations is less than that generated within the switchgear. The importance of proper selection of busbar ensures that in any switchgear the busbars do not act as sources of heat but as heat sinks instead.

Factors influencing emissivity of heat Through Busbars

i. The conductivity of the Copper links/busbars should be high. Preferably electrolytic grade Copper of 99.99% purity to be used. Use of lower conductivity material leads to increased overheating and instead of working as a heat sink the bar behaves as a heat source.

ii. The copper used in the GCC is tinned copper which has an emissivity of 0.5. Tinning of busbars also ensures inhibition of oxide formation.

iii. When busbars/links are connected in parallel, some gap should be maintained between parallel busbars/links to improve heat dissipation. This gap should be equal to the thickness of the busbars/links or 10 mm whichever is higher.

iv.The conducting links/busbars should have only the required number of holes for joints. The size of holes should be slightly larger than that of the connecting stud/bolt. The fairly common situation of the hole being significantly larger that the stud/bolt, resulting in the bolt head pushing in the washer through the hole, must be avoided. Again while joining, the number of contact surfaces in series should be kept to the minimum.

v. While installing busbars/links, care should be taken to ensure accurate positioning of fixing holes. Terminations that forcibly take care of misalignments/offsets lead to undue pressure being exerted on the terminals and the insulated terminal plates, resulting in their breakage. Also, even if there is no offset, the busbars/links should be adequately supported along their length so that they do not exert any undue pressure on switchgear terminals under normal or short ciucuit conditions.

vi. For proper terminations between links and switchgear terminals, the contact area must be adequate.

Having sized the bus bar further 3 factors to be considered ie Voltage drop, skin effect and the short circuit current the busbar and the support needs to withstand.

Terminations of Cables Onto Switchgear :

Cables can be terminated onto the terminals of a switchgear either directly or through cable lugs. Precautions to be taken while terminating cables or through cable lugs onto switchgear Terminals. Cables should be well-supported and it should be ensured that they do not exert any undue pressure on switchgear terminals.

Correct mechanical pressure during termination should be applied on the conductor.
Over tlghtening of clamping screw may sever the conductor while under tightening can cause overheating at the termination point. For proper tightening of terminals, the user should adhere as closely as possible to the manufacturer's recommendations on tightening torques.

Long term advantages:

Energy losses in the relatively short conductors of distribution equipment are generally only taken into account as far as their physical effects are concerned: heating and emission of calories. However the economic aspects of the energetic efficiency of a conductor are far from negligible at low voltages; it is easy to show that an assembly of conductors carrying 2500 A, loses, in a year of continuous use, an amount of energy whose cost is about the same as that of the copper they contain. With proper busbar sizing its possible to drastically bring down the energy loses and in turn make the panel energy efficient.

It is of course the role of Faisal Jassim, a manufacturer of LV switchgear and distribution panels, to have complete command of the technical problems involved in the design of that equipment. However the job of a manufacturer is not limited merely to supplying products; he owes it to his clients to make available his technical expertise to enable them to install and use the equipment in the best possible way.

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