Siemens siprotec 7SD5, Руководство пользователя, Страница: 375 / 698

2.21 Thermal Overload Protection
375
7SD5 Manual
C53000-G1176-C169-1
sulation material, the design and the way they are laid, and can be derived from the
relevant tables.
Please note that the overload capability of electrical equipment relates to its primary
current. This has to be considered if the primary current differs from the nominal
current of the current transformers.
Example:
Belted cable 10 kV 150 mm
2
Permissible continuous current I
max
= 322 A
Current transformers 400 A / 5 A
Setting value
K-FACTOR
= 0.80
Time Constant
τ
The thermal time constant
τ
th
is set at address
4203 TIME CONSTANT
. This is also
provided by the manufacturer. Please note that the time constant is set in minutes.
Quite often other values for determining the time constant are stated which can be
converted into the time constant as follows:
1-s current
Permissible current for application time other than 1 s, e.g. for 0.5 s
t
6
-time; this is the time in seconds for which a current of 6 times the nominal current of
the protected object may flow
Example:
Cable as above with
Permissible 1-s current 13.5 kA
Setting value
TIME CONSTANT
= 29.4 min
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