Medium Voltage Application Guide

SWITCHGEAR
Resonant frequency
The busbar system must be checked for potential resonance under normal operating conditions and fault conditions.
This is done by calculating the natural resonant frequency of the system, which must meet the following criteria:
50 Hz supply: not within the ranges 48 Hz to 52 Hz and 96 Hz to 104 Hz
60 Hz supply: not within the ranges 58 Hz to 62 Hz and 116 Hz to 124 Hz
Calculation requirements
Busbar systems are subjected to thermal and electrodynamic stresses under normal operating conditions, but more
so under short circuit fault conditions. It is important to ensure the busbar system will function safely under all
known conditions. When checking the design, the most important considerations are the nominal operating
current, expected fault current at the point of installation, average ambient temperature and the altitude of the
installation.
To check the safety of a busbar system:
Check that the current rating of the busbar system (I r ) exceeds the expected nominal current. Main
factors affecting the busbar rating are busbar material and configuration, ambient temperature and
maximum permissible temperature rise.
Check the maximum expected temperature rise of the busbar during a short circuit fault. In the event of
short circuit current flow (I th ), the surface temperature of a busbar must not exceed the thermal limits of
any material coming in contact with it (ie insulator standoffs).
Check the maximum expected electrodynamic forces imparted on the busbars and insulator standoffs, due
to the peak short circuit fault current (I dyn ). Do not exceed the mechanical limitations of the material.
Check that the busbar system will not resonate under normal operating and fault conditions.
Refer to Busbar Calculations on page 149 for calculation details and examples.
Busbar bolting arrangements
Typical busbar bolting details for single overlap copper bar
Bar width
(mm)
Joint
overlap
(mm)
Joint area
(mm 2 )
Number of Metric bolt size
bolts 1 (coarse thread)
Bolt
torque
(Nm)
Hole size
(mm)
Washer
diameter
(mm)
Washer
thickness
(mm)
16 32 512 2 M6 7.2 7 14 1.8
20 40 800 2 M6 7.2 7 14 1.8
25 60 1500 2 M8 17 10 21 2
30 60 1800 2 M8 17 10 21 2
40 70 2800 2 M10 28 11.5 24 2.2
50 70 3500 2 M12 45 14 28 2.7
60 60 3600 4 M10 28 11.5 24 2.2
80 80 6400 4 M12 45 14 28 2.7
100 100 10000 5 M12 45 15 28 2.7
120 120 14400 5 M12 45 15 28 2.7
160 160 25600 6 M16 91 20 28 2.7
200 200 40000 8 M16 91 20 28 2.7
Source: Copper for Busbars http://www.copperinfo.co.uk/busbars/pub22-copper-for-busbars/homepage.shtml
1
Number of bolts based on using high-tensile steel or bronze (CW307G, formerly C104)
Page 82 Medium Voltage Application Guide 710-12280-00A