GENERAL
INFORMATION ABOUT
BUSBAR SYSTEMS
Nowadays, only two metals are used for busbar:
Copper and Aluminium
All the parameters are in favour of use of copper, except for weight. When we compare
copper and aluminium, electrical conductivity of copper allows the use of bars of a smaller
cross sections, and this is an important consideration in the design of a low voltage Busbar
set which is reducing the space occupied by the busbar system.
PROPERTIESCu
Al
Weight per unit length for
the same conductivity
Conductivity (per cross section)
Electrical Thermal
1
0,30
1
1
0,61
0,50
Tensile strength 1
0,40
Hardness 1
0,44
Modulus of elasticity
1
0,55
Coefficient of expansion
1
1,40
Melting point
1
0,61
General Parameters Needed To Take Care During Busbar Design
For a detail of busbar set, the designer must consider several parameters:
Electrical
The current to be carried, in a bar or set of bars, the cross section of the conductor for a
given current, will determine the voltage drop.
You can check the “Continuos Load Current of Copper Busbar” table.
Also, two contributory factors limit the current passes throught a conductor:
+ The working temperature of the conductor
+ The energy losses due to heat generation
Mechanical
The dimensions and the mechanical strength of the bars is dependent on their mass and
conditions of use.
Skin Effect
Skin effect is a phenomenon which has the effect of concentrating the current on the
perimeter of the bars. Its importance depends on the frequency, resistance of the meterial of
busbar, the bar thickness and the ratio of width to thickness.
The coefficient of skin effect for rectangular copper bars emphasizes the advantages of using
busbar where the ratio of width/thickness is greater.
For the same cross sectional area of busbar, this type of bar has a greater surface area for
cooling in addition to having a more uniform current density.
Schematic
representation
of alternating
current
Heat Dissipation By Convection
The heat dissipation by convection depends on:
+ Shape of the conductor
+ Dimension of the conductor
+ Temperature increase limit of the conductor
+ Air flow rate round the conductors (this is important for cooling)
The cooling better for a rectangular bar standing on its edge.
For optimum cooling, the bars should be arranged in such a way that they effect air circulation
minumum. And also, the arrangement should present maximum surface area for cooling.
It is preferrable to use several thin bars on edge, rather than a single thick bar.
Clearance and Creepage Distance
C
B
A
D
A
A) Horizantal copper bars
B) Screen (Barrier)
C) Clearence distance is the shortest distance in free air between two live parts
D) Creepage distance is the shortest surface distance between two live parts
The minimum clearance distance is 14 mm, according to IEC 60439-1, air pollution degree 3.
How To Achive A Good Connection Between Bars
1) Contact Surface:
The surface of the bars must be flat and not polished. They must be oxide and grease free.
2) The overlap (H) should not be less than 5 times the thickness of the thinnest of the
conductors.
S=HxL
3) Necessary Clamping Force (F)
It is a direct function of the contact area (S) and of an optimised value k = 2,5 dN/mm2
F=KxS
70
60
50
40
30
20
10
0
F=kxs
0
15
2
3
46
7
4) Clamping Torque Calculation:
Use a class 8,8 zinc coated bichromated bolt, with “contact” washers tightened using a torque
wrench without lubrication
• Zinc coating is protecting the ferrous metals against corrosion effect of oxigen and damp.
When there is a fracture appers on ferrous metal, zinc coating will contact with oxigen and
damp before than ferrous metal because zinc is more electropositive than iron.
• Chromate coatig, is a method of protection against corrosion. Also it is a pre process in
a paint shop, it creates a film on the metal surface, this film improves the powder coating
quality.
5)What Is Class?
When we call a class 8,8 it means
Class = 8,0 + 0,8
CLAMPING TORQUE CALCULATION
EXPLANATION OF CLASS
class
8,8
resistance to breaking
8,0x100 = 800daN/mm2
M6 M8 M10 M12 M14 M16
Ø
800 1450 2300 3700 4400 6000
F (daN)
Clamping Turque Nm 13 30 60 110 174 274
Elastic resistance
for 0,2% elongation
800x0,8 = 640 daN/mm2