34)DOs & Donts

33)DOs & DONTs

b) Step 2

b) Step 2 Calculation of Short ckt forces & short ckt stress.










Calculate Peak electro-magnetic forces between phase conductors



Calculate Peak electro-magnetic forces between sub conductors

20)Step 1

a) Step 1

Collect the system data for which busbar system has to be design


VOLTAGE RATING 415V, 50Hz. 3Ph. - 3Wire

CURRENT RATING 2000Amp.

WELL VENTILATED ROOM

MAX. FAULT CURRENT 50 KA

LENGTH OF BUSBAR = 500mm

AVERAGE AMBIENT TEMP. = 50 deg. Centigrade


Calculate the derating factors for ambient temperature, enclosure, proximity effect.

1) Derating factor for Ambient Temperature



Df1=0.81

Actual average ambient temp. = 50 deg. C
Manufacturer average ambient temp. = 35 deg. C
Final ambient temp. = 85 deg. C


2) Derating factor for enclosure & ventilation


From table we can see that for well ventilated room, where cross-sectional area of busbar is 10% of total cross-sectional area of enclosure derating factor is

Df2=0.65

3) Derating factor for Proximity effect

For 3W phase spacing derating factor for proximity effect is

Df3=0.82

Calculate effective derating factor.

Df = 0.81 * 0.65 * 0.82

= 0.43




Select the busbar size

Minimum rating of busbar required

= 2000/0.43

= 4651 amp.

Busbar selected = 203.2 * 9.53 mm (3 flats)

19) Calculation Example

19) Calculation Example

Up till now we have seen all the theoretical part of bus bar design & all the formulae which are required for calculating short ckt forces & stress.
Now will take one practical example for busbar calculation for better understanding.

The example is divided in three steps

Step 1) In this we are going to collect the system data for which busbar system is to be designed & then calculate derating factors for ambient temperature, ventilation& proximity effect. Then we will calculate effective derating factor & calculate the busbar size.

Step 2) After selecting the busbar calculate the value of “as” this is used to calculate peak electromagnetic short ckt forces between phase & between sub conductors. Then calculate peak electromagnetic short ckt forces between phase & between sub conductors. Then calculate total stress on busbar.

Step3) Adjustment should be carried out to maintain the stress within limit of busbar; accordingly busbar support must be selected.

10)Stiffeners

Stiffeners



Formula for Design of insulating supports


Area of SMC reqd. = Peak force × Safety factor /Strength of SMC


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9)Calculation stresses created by short circuit force

1) Stresses because of s/c forces between Phase conductors





Where, z = section modulus

= 0.167*d2* b for horizontal orientation
=0.167 n* d* b2 for vertical orientation

2) Stresses because of s/c forces between sub-conductors




Where, zs =0.167d2 b


Total stress on busbar conductors

TOTAL = Stresses because of s/c forces between Phase conductors + Stresses because of s/c forces between sub-conductors

8)Table for value of as

Table for "as"



Note:Value "as" used as a input quantity in short ckt calculation

7)Calculation of forces generated because of short circuit

1) Peak electro-magnetic forces between phase conductors




Where, ip = peak value of 3 phase s/c current, in ka
l = distance between supports
a = centre to centre conductor distance


2) Peak electro-magnetic forces between sub-conductors



Where, ip = peak value of short circuit current, ka
ls = distance between supports/stiffeners
n = number of sub-conductors per phase
as = effective centre to centre distance


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6)Short circuit forces which act on Busbar

Short circuit forces which act on Busbar

Depending upon the direction of current carried by adjacent phases there are two kinds of forces produce
1) Attractive force
2) Repulsive force
Following fig. shows different current carrying combination by adjacent busbar conductor.

5)Std. size of Busbar & their current carrying capacities

Std. size of Busbar & there current carrying capacities

4)Derating factor for volume of enclosure & ventilation

Derating factor for volume of enclosure & ventilation





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3) Derating factors for proximity effect

3) Derating factors for proximity effect

Phase Spacing Derating Factors
3W 0.82
4W 0.89
5W 0.95
6W 0.99
>6W 1.00
What is w?

It is distance between busbar of same phases as shown in fig




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2) Derating Factors for Ambient Temp, altitudes

Derating Factors for Ambient Temp, altitudes.


1) Average ambient temperature




Where,

T1 = temp. Rise at the ambient temp. as referred by the BB manufacturer
T2 = temp. Rise at the ambient temp.where the panel will be installed


2) Derating factors for altitudes

Where busbars are intended for use at high altitudes & is tested near sea level, the limits of temperature rise should be reduced by 1% for each 1000 feet in excess of 3000 feet above sea level at which the busbar is to be installed



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1)Busbar Design Temperature rise considerations

Busbar Design Temperature rise considerations

Following are the points which need to be considered while designing the busbar system.

1) Material of busbar (e.g. For Aluminium final temperature is Limited to 85 °c
2) Shape of conductor
3) Arrangement of conductor(vertical or horizontal)
4) Volume of enclosure
5) Ventilation
6) Average ambient temperature
7) Material of insulating supports (e.g. SMC can safely withstand 150 °c).
8) Type of joint (e.g. plated joints can withstand higher temperature)
Kindly note that while claiming the ratings the manufacturer of the busbars made following assumptions

1) Ratings are for 50°c rise over 35 ° c ambient temperatures in still but unconfined air

2) For multiple bar arrangements, the space between bars is equal to the bar thickness

3) A.C. ratings are based on spacing at which proximity effect is negligible

A)Step by step busbar design

Now we will see how busbar system is designed?

1) Busbar Design Temperature rise considerations
2) Derating Factors for Ambient Temp, altitudes
3) Derating Factors for ventilation.
4) Derating factors for proximity effect
5) Std.Busbar size & their current carrying capacities
6) Short circuit forces which act on Busbar
7) Calculation of forces generated because of short circuit
8) Calculation stresses created by short circuit force
9) Stiffeners spacing
10) Insulating support design
11) One example for busbar calculation

Under progress

Yhis post is under progress

Reserved

Work is going on this post

e)Slitting

SLITTING: The busbars should be slit according to the picture for better joint and alignment.







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e) Torque

TORQUE: When terminating busbars at the terminals, they should be tightened with “Conical Spring Washer DIN 6796-X Spring Steel” x = dia of corresponding bolt in mm. These are locally available. The tightening torque of the bolts used for tightening the busbars should be of 8.8
class as per the following table :

Fig:Torque table



Fig Top view & side view of washer

Fig Front view of washer



Fig: different nuts & bolts used for busbars


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d)Bending

D)BENDING: The bending radius for busbar should be as follows


Fig: Bending radius table


Fig:Bending redius diamension


Fig:Bending process on machine


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c) Joints

C) JOINT: When joining busbars, the overlap of the busbars must be equal to the width of the busbar








Fig:Example of joints

b)Disposition

Disposition: The busbars are to be disposed at the terminals in the following manner after which auto minimum gap equal to the thickness of the busbars to be maintained during the subsequent run.


Fig:With two bus bars (Observe the clearence & overlapping area)



Fig:With four bus bars



Fig:Good example of vertical busbars



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a)Cleaning

Cleaning:

-Terminal of the ACB which is plated has to be cleaned with cloth only.
Don’t use Steel brush for cleaning of breaker terminal.


Fig:Cleaning guide lines for breaker terminals



-Busbars before connection to be cleaned to obtain a bright finish with a steel wire brush.

Fig:Cleanig of busbars with steel brush

After removal of the metal particles with auto clean cloth, a thin film of neutral petroleum jelly to be applied immediately and connection made.


Fig:Petrolium jelly for busbar termination


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10)Busbar heating

Heating problem of Busbars


Fig:Heated busbar

• The values of drop measured should be less than 4.0mV at the rated current of breaker. mV volt drop can be calculated by given formula:







Fig: mV drop test for Busbars

Remedy:Tight the busbar bolt with proper torque


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9) Corrosive Atmosphere:

Corrosive atmosphere has auto profound effect on connections. Non-conductive layers are formed on the connecting surface which heat up the joint and deteriorate the performance. Frequent maintenance of the joints / connections is required in such cases. Monitoring the joint temps with non contact thermometers is also useful


Corrosive Atmosphere:


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7)Support

Supports


Fig:Supports

The busbars emanating from the ACB terminals have to be adequately supported as shown in the following figures. Without proper support, damage may be caused to the system




























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