Page 941 - Industrial Power Engineering and Applications Handbook

Carrying power through metal-enclosed bus systems 28/891
28.9 Sample calculations for Continuous current rating 2500 A
designing a 2500 A non-isolated Ambient temperature 50°C
85°C
Maximum permissible operating temperature
phase aluminium busbar system Permissible final temperature at the end of the
fault 185°C
Example 28.12
Design parameters (A) Rectangular sections
Supply system three-phase four-wire 415 V ? IO%,
50 HZ ? 3% (i) Minimum size of busbars for short-circuit conditions
Fault level 45 kA The minimum size of busbars for an operating temperature
Duration of fault 1 second of 85°C and a final temperature of 185°C can be ascertained
from the curves of Figure 28.5, suggesting
870 r. 4 = 0.0799
A
f or A=-. 45 4i
0.0799
1
= 563.2 sq. mm
Maximum temperature rise of the busbars at the rated current
= 85 - 50
= 35°C
Assume the temperature of the busbars at the time of fault =
85°C and rectangular flats of electrolytic grade E-91 E or its
Insulator supporting equivalent. Busbars chosen for each phase - four (152.4
mm x 6.35 mm) - which are more than the minimum size
MS or aluminium required to account for the thermal effects during a short
enclosure circuit condition
(All dimensions in mm )
for neutral -two (52.4 mm x 6.35 mm)
Figure 28.33(a) Busbar arrangement and enclosure size for
bus duct of ExamDle 28 12 Size of busbar enclosure - 870 mm x 380 mm (Figure
28.33(b))
Ground terminal

Side view

Top view (without cover)
(All dimensions in mm)
Legend
1. M.S. anqle 4. Bottom and top covers 2 mm M.S. or 3 mm AI. 7. Gasket
2. AI. bus i 152.4 x 6.35 mm 5. Ground bus 50 x 6 mm 8. Louvres
x
3. Side frames 2 mm M.S. or 3 mm AI. 6. Insulators 9. Metallic spacers
Figure 28.33(b) General arrangement of a typical running section of the busduct of Figure 28.33(a)