BUSBAR SIZING CALCULATION
Published on Published onDecember 21, 2015
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Generally there are 'THREE' Factors are involved for the Fire Causes. They are" I) Accidental. II) Natural. III) Incendiary. Comparing 'THREE', the most dangerous one is 'ACCIDENTAL'. Here Electricity plays major role for the Fire Accident most of the Times. There are 'THREE' Elements must be present, for a 'FIRE' to initiate. These are Oxygen, Fuel and Heat. Electricity Plays major role to provide 'HEAT' most of the Times. SOURCES for the 'HEAT' in the 'ELECTRICAL SYSTEMS' : Most of the Time, Fire happens in the Electrical Panels, Which we observed from the Existing Cases. So it is better to mention that, Sources for the 'HEAT' in the Electrical Panels. a) SHORT CIRCUIT.
b) OVER LOAD. c) POOR QUALITY IN EARTHING DESIGN. d) ABSENCE OF VENTILATION IN THE PANELS. e) POOR QUALITY IN DESIGNING. [Ex. Under Sized Cables]. Anything else apart from the Above,...........................................Yes. That Source should be Top in the above list. What is that 'Important Factor'??... It is none other than 'UNDER SIZED BUSBAR IN THE PANELS'... This Article Gives Clear Picture of 'BUSBAR SIZING CALCULATION' and What Care We should have to take 'Sizing of Busbar'. Here I will like to describe Busbar Sizing of LT & HT Panel System. While This Methodology doesn't have any relation with 'CALCULATION OF CONDUCTOR BUS SIZING [ACSR]' for the Switch yard Projects. Both Calculations having different Steps & Procedure to find Final Results. The 'Electrical Design Engineer' Should have in-depth knowledge of 'Sizing of BUSBAR for the Electrical Panels [Both HT & LT]. I have seen many times in my past experience, Most of the Engineers doing Busbar Sizing Calculation by 'Thumb Rule' Method, Even though Some of the Senior Engineers in the Consultant offices & Leading Panel Fabricator will Practice the Same 'THUMB RULE' method. There are methods are exist in the world, To determine Proper sizing of Busbar for the Electrical Systems. People not have that much Patience for the Practicing Calculation. Now Software available to determine Proper Size of Busbar for the panels. I would like to add more inputs in this article, which I had experienced in my past years.
THUMB Rule for Busbar : For Aluminium For Copper
: 0.7 Amps / 1 Sq.mm of Bar. : 1.2 Amps / 1 Sq.mm of Copper.
Here the above mentioned rules are not standard. Because some case people will add Tolerance in the Positive End. i.e They used 0.75 Amps / 1 Sq.mm of Bar & 1.4 Amps / 1 Sq.mm for Aluminium & Copper Respectively. The Cost of the Aluminium bar in the Market based on the Quality. While Practicing 'THUMB' Rule people should aware the Quality Standards of Aluminium & Copper Bar. Sometimes we didn't aware the bar Quality which is used by the fabricator in the Panel. Panel Fabricators should not take responsible in case Panel gets fire due to the Busbar Under Sized. It will directly hit EPC Contractor Market Value in the Industry.Also it leads to Black Mark in their Industry. He should have to answer Client, Electrical Inspectorate & Consultant. So People should take More Care for Busbar Sizing for both LT & HT Panels. I would like to share one of the Example I experienced in my past. Consultant given Busbar Size for the Main MV Panel. [That Panel should cater 10 MW of Power of the IT Building, Panel gets power from 3 Transformers & 6 Generators]. Panel should capable to Carry 6300A. Busbar Size Should be Equal or More than of 6300A. Consultant Recommended Busbar Size based on the 'THUMB Rule' We noticed, Busbar Size is Low to Carry 6300A. Consultant Challenged Contractor that, His recommended Size of Busbar will carry full load Current, then Contractor approached 'CPRI Testing for their internal verification. Finally Busbar gets melted when it carried full load Current [6300A] for the interval 1 Minute. Here it is not the case for the carrying Short Circuit Current. If Busbar doesn't able to carry the Full Load Current, then how it will carry Short Circuit Current for the Second.???
After that we will fabricate the Panel with Proper Sizing of busbar which we derived from the Proper Calculation. In some cases people only practicing Calculation with only consider the 'K' Factors. There is formula exist apart from the 'K' Factors. The Design Engineer should consider the following points while doing 'BUSBAR SIZING CALCULATION':
Adequate minimum required clearance between Phases and Phase to Earth.
Selection of Adequate Busbar Insulator Standoffs.
Bolting Arrangements for Continuous Busbar Connections.
Thermal Effects on Busbar and Insulator Standoffs under normal and Fault conditions.
Electrodynamic Forces applied to Busbars and Insulator Standoffs under Fault Conditions.
Avoidance of mechanical resonance under normal operating and Fault Conditions.
TEMPERATURE RISE: Maximum Permissible Temperature Rise for bolt - connected devices, including busbars
Source Reference : IEC 62271 - 1
Co-Efficient Factors 'K' : The Total 'K' Factor derived from Six 'K' factors, which is listed below : K = K1 x K2 x K3 x K4 x K5 x K6 K1 is a Function of the Number of Bars per Phase and their space. K2 is Corresponds to the Surface Finish of the Busbars. K3 is a Function of the Mounting Arrangement. K4 is a Function of the Installed Location. K5 is a Function of any Artificial Ventilation. K6 is a Function of the Type of Current.
Busbar Clearances as per IS : 4237 - 1967
Example : Transformer Rating : 11 / 0.433 kv, 800 kVA Transformer. What is the recommendable size of Copper Busbar in the LT Panel ??
If we consider 80 x 10 mm of Copper Bar, How many bars required to carry full load current of Transformer Secondary End ? Transformer Secondary Full load Current = 1066.72A, Breaker Size Needed = 1250A. [Either ACB or MCCB] According to Thumb Rule, For Copper
: 1.2 Amps / 1 Sq.mm of Copper.
for the Area 800 Sq.mm [80 x 10 mm], Current Carrying Capacity will be 960A.., Then additional one Run of Busbar needed to carry the Current of 1067A. According to the Formula as per IEC 62271 - I K = K1 x K2 x K3 x K4 x K5 x K6 K = 1 x 1 x 1 x 0.8 x 1 x 1 = 0.8
According to the Main Formula, Derived Value will be = 1547A Rated Current = 0.8 x 1547 A = 1238 A Conclusion: To Carry the Current of 1067 A, One Run of 80 x 10 mm of Copper Bar is enough. Cheers !!!
