7/27/2019 Design Calculation Bus Bar

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Design calculation for Bus bar

1) Fault Level for 3sec - 80 KA

2) Bus bar per phase - 4 Runs of 200 mm X 10 mm

3) Bus bar material - Copper

4) Distance between

Phase to Phase - 175 mm5) Bus bar Support - 14-P-171( R,B & Y Ph.) & 10 P 131( Neutral )

6) Material - SMC

DESIGN CALCULATION

10 -P - 131 Support Used in Neutral ( 2 Run of 200mm X 10 mm )

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a) SHORT CIRCUIT

14 -P - 171 Support Used in R,Y & B Phase(4 Run of 200mm X10 mm)

Minimum cross sectional area of Copper Bus bar required during 80 KA for 3 sec. Short circuit test.

The formula to calculate this is given in the J & P Switchgear Hand Book .

( I/A ) t = 0.122

or A = ( I/0.122) t

or A = ( 80 /0.122 ) 3

or A= 1135mm2

The cross section of Bus bar being used (4 10 200) mm2 equals to 8000 mm2 ( much higher than

required section ) .

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FORCE CALCULATION

Support 14-P-171 C-C(Ph) 175 mm

Dynamic Forces developed per meter ( Fm )between bus bar due to 80 KA for 3 sec. Short Circuit

Test.

16 I2 10-4 K

Fm = --------------------------- N/m

S

Where, I = The R.M.S. Value of the current in amp =80 103 A

K= Space factor for Rectangular BarsS = Spacing between the conductors (mm) = 175 mm

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The value of Space Factor (K) is determined from the graph firstly by determiningthe ratio (s-a)/(a + b) and then from the value on the base scale projecting upwards to the pt. Of

intersection with the curve appropriate to the ratio a/b.( a=total thickness of Bus bar per phase &

b= width of Bus bar ) . The value for the K can be calculated from the Graph given in J&P Handbookby taking the ratio of the following

s-a------- and comparing it with the value of a/b.

a + b

Here value of a = 70 mm & b = 200 mm

Distance between Phase to Phase ( s) = 175 mm

so ( S -a ) / ( a + b ) = 0.4 and ( a/b) = 0.35

From Graph Value of K ( approx. ) = 0.86

16 80 80 106 10-4 0.86

Dynamic forces developed (Fm) = -------------------------------------------- N/m

175

= 50322 N/m ( approx.)

50322 350

Forces at a Span of 350 mm(Support to Support Distance ) = ----------------------1000

= 17612 N

17612= ------------- Kg f ( as 1 NEWTON = 9.8 Kg f )

9.8

= 1797Kg f

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Breaking Force of the Bus bar support

Section side - 42 mm 35.4 mm

Area of Cross Section( which is likely to break during short circuit ) support

= Surface area of the support

= ( 4.2 cm 3.54 cm )

= 14.87 cm2Breaking force of the bus bar support = Area of Cross Section ( which is likely to break,during Short

Circuit ) of the support Tensile Force of the material ( 700 Kg f )

= 14.87 cm2 700 Kg f/cm2

= 10407 Kg f

Breaking force of the Bus bar Supports > cal. Value (1797 kg f ) which is quite suitable to

withstand force.

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Factor of Safety

Breaking Strength

Factor of safety = ---------------------------

Calculated Load

10407= -------------

1797

= 5.8

Calculated value of the Dynamic Force from the data given is lesser than the Breaking Strength

of the Support used. So Bus bar Support can easily withstand on the Fault Level of 80 KA for

3 sec.

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VALUE OF K ON Y -AXIX

VALUE OF (s-a ) / ( a+b ) ON X- AXIX .

Graph Taken from J&P Switchgear Handbook