Loads And Dimensioning - Siemens Sivacon S8 Planning Information
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Also See for Sivacon S8:
- Operating instructions manual (63 pages) ,
- Instructions manual (12 pages) ,
- Manual (11 pages)
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10.2 Loads and dimensioning
Current carrying capacity considering the ambient
temperature
The current carrying capacity can be calculated from the
following relation taking the ambient temperature into
account.
/ I
= DT
/ DT
2
2
I
1
2
1
2
Where the power ratio (of the currents squared) equals the
ratio of temperature differences DT between object and
ambience.
Example of a main busbar:
With a
= 4,000 A and a
rated current I
1
permissible busbar temperature T
there is a rated current I
for an ambient temperature
2
T
= 40 °C
env
∆T
I
= I
x
= I
x
1
∆T
2
1
1
2
90 °C
I
= 4,000 A x
2
95 °C
Rated frequency 60 Hz
According to IEC 61439-1, section 10.10.2.3.1, the rated
current at 60 Hz must be reduced to 95 % of its value at
50 Hz in case of currents greater than 800 A.
Short-circuit current carrying capacity of distribution
busbars and functional units
IEC 61439-1, section 8.6.1 permits a reduction of the
short-circuit strength of the vertical distribution busbar and
its outgoing feeders in relation to the the main busbars "if
these connections are arranged in such a way that a short
circuit between phase and earthed parts needn't be ex-
pected under proper service conditions." The background
for this simplification is the usually higher rated current of
the main busbar compared to the currents of the distribu-
tion busbars, for the contact systems of the withdrawable
units and in the feeder lines to the functional units. Lower
temperature rises can be expected for these lower branch-
ing currents, so that it hardly makes sense to aim at the
same dynamic and thermal short-circuit strength as for the
main busbar.
= 130 °C,
SS
Example:
To attain a required rated short-circuit strength of 100 kA, a
3VL5 MCCB with a switching capacity of 100 A is used as a
short-circuit protective device:
(T
- T
)
SS
env
(T
- 35 °C)
In case of a disconnection on short circuit, merely a peak
SS
current of approximately 50 kA will flow as a let-through
current for a short time, so that a root mean square value
= 3,893 A
(RMS) of 35 kA can be assumed as maximum. It is only this
reduced current which stresses the conductors in this
circuit for the very short disconnect time of the breaker.
Test of dielectric properties
According to IEC 61439-1, section 10.9 the dielectric
properties of the switchboard must be tested in considera-
tion of devices having reduced dielectric properties. This
means: "For this test, all the electrical equipment of the
assembly shall be connected, except those items of appara-
tus which, according to the relevant specifications, are
designed for a lower test voltage; current-consuming
apparatus (e.g. windings, measuring instruments, voltage
surge suppression devices) in which the application of the
test voltage would cause the flow of a current, shall be
disconnected. ... Such apparatus shall be disconnected at
one of their terminals unless they are not designed to
withstand the full test voltage, in which case all terminals
may be disconnected."
SIVACON S8 Planning Principles – Technical annex
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