BusBar Short Circuit Calculation

01.03.2007 (Rev.)

CALCULATIONS IN SUPPORT OF FAULT LEVEL OF 65 kA FOR ONE SECOND

4000 A TP ALUMINIUM BUS DUCT DRAWING NO.G 7643 R0

(1)

i)

Rating

4000 Amps

ii)

Bus Bars Per Phase

3 x 200 mm x 12 mm

iii)

Fault Level

65 kA for 1 Second

iv)

Enclosure

In a ventilated room

v)

Ambient Temperature

50OC

vi)

Working Temperature

85OC

vii)

Rating Chart

INDAL Electrical Grade Aluminium

THERMAL RATING a)

Rating as per Table 2 (INDAL) For 3 x 200 mm x 12 mm Aluminium Bus Bars

b)

Derating Factor

c)

5300 Amps

•

Temperature Rise of 35OC over an ambient of 50OC.

0.817

•

Magnetic Enclosure

0.850

•

Proximity Factor

1.000

•

Enclosure Factor

0.920

•

Paint / Sleeving Factor

1.250

•

Combined Derating Factor

0.817 x 0.850 x 1.000 x 0.920 x 1.250 0.799 Say 0.80

Actual Current Rating of above Bus Bars

Conclusion

0.80 x 5300 4240 Amps

Safe as the rating works out to be 4240 Amps as against 4000 Amps required.

….Contd..2

(2)

(2)

SHORT TIME RATING I

Short Circuit Current

=

65 kA

T

Short Circuit Time

=

1 Second

Q1

Initial Temperature

=

85 C

Q2 =

Final Temperature

=

200OC

A

Cross Sectional Area in Sq. inch

=

A =

Ix

O

t

14 x 10

4

Q2 + 258 log 10 Q1 + 258

X)

O

Cross sectional Area in case of initial temperature of 85 C and final temperature of 200OC.

=

3

65 x 10 x 14 x 10

1

4

458 log 10 343 = =

1.309 Sq. inch 845 Sq.mm

Conclusion

(3)

Area considered is 7200 Sq.mm – Hence SAFE

MECHANICAL STRESS & INSULATOR SPAN

FM

Mechanical Force Developed at the time of Short Circuit.

I

Short Circuit Current.

K

Space Factor For Rectangular Bus Bar.

S

Spacing between the Phases = 190 mm.

FM

=

16 x I

2

x 10

-4

xK N/M

S ….Contd..3

(3)

The value of K as calculated from the graph given in J&P Hand Book page 212 by taking the ratio (S-a) / (a+b) on comparing it with a/b will be 0.50.

Hence FM =

6

16 x 65 x 70 x 10 x 10

-4

x 0.50

190 = =

19158 N/M 1954.88 Kg Per Meter

Force Developed at a span of 350 mm

= =

1954.88 x 0.350 684.20 Kg.

Since there are two supports – one at on top and one at bottom, force per support will be 342.10 Kg.

(4)

a)

Powermat supports (SMC) type 312-K having capacity to withstand force of 2000 kg will be provided meaning thereby that there will be a safety factor of 5.85 times.

b)

Distance : The support to support distance of max 350 mm will be maintained.

c)

Above calculations / recommendations are OK for a phase to phase gap of 130 mm.

FINAL DESIGN OFFERED Chamber Size

700 mm (minimum)

Bus Bars

3 x 200 mm x 12 mm per phase

Phase to Phase Gap

130 mm

Support to Support Distance

350 mm