Medium Voltage Application Guide

14011.A
SWITCHGEAR
Exercise
A busbar system has two end-mounted busbars per phase,
made of ¼ hard copper (maximum allowable stress of
1200 daN/cm 2 ). Each bar is 8 cm high and 1 cm wide, with
a gap of 1 cm between bars of the same phase. The phase
centres are 15 cm and each phase has 6 insulator stand-offs
at 80 cm spacing. The insulator stand-offs are 15 cm high,
with a bending resistance of 1000 daN.
Check that the busbars and insulator stand-offs are suitable
for the installation.
1 cm
4 cm
15 cm 15 cm
1 cm
8 cm
15 cm
Step 1: Calculate the forces between the parallel busbars of different phases.
Assume a short-time withstand current rating, I k , of 31.5 kA at 50 Hz.
p
k
2 78.75 2 10 2
2 5.33 6201.5625 0.01
The force between busbars is 661 daN.
Step 2: Calculate the forces absorbed at the head of each insulator stand-off.
The force to be absorbed by each individual standoff:
The force imparted on each stand-off is 954 daN.
The imparted force is less than the bending resistance of the insulator stand-off: 954 < 1000 daN.
The insulators are suitable for the application.
Step 3: Calculate the maximum stress exerted on the busbars.
According to the selection table, the modulus of inertia, l / V, for 8 cm x 1 cm end-mounted copper busbar pairs
is 11.55 cm 3 .
F 1 l
V 12 I
66180 1
12 11.55
1
4406.67
11.55
381.53 daN/cm 2
The stress imparted on the busbars is 381 daN/cm 2 .
The imparted stress is less than the maximum allowable stress for ¼ hard copper busbars: 381 < 1200 daN/cm 2 .
The busbar dimensions and material are suitable for the application.
710-12280-00A Medium Voltage Application Guide Page 155