In the case of cables, the permissible continuous current depends on the cross-section, insulation material,
design type, and the manner in which the cables have been laid. In the case of overhead lines, an overload of
10 % is permissible.
EXAMPLE
For the Permissible Continuous Current
Cross-linked polyethylene cables (N2XS2Y): 10 kV 150 mm
2
(Cu)
Current-carrying capacity (underground laying): I
max, perm
= 406 A
Selected K factor of 1.1
This yields a rated current of I
rated, obj
= 369 A
Parameter: Thermal time constant
•
Default setting (_:110) Thermal time constant = 900 s
The Thermal time constant parameter is used to define the tripping characteristics of the stage. If no
data on the Thermal time constant is available, you can determine this from the short-time current-
rating capacity of the cable, for example, from the 1-s current. The 1-s current is the maximum current permis-
sible for 1 s application time. The 1-s current can be found in the cable specifications. Calculate the Thermal
time constant according to the following formula:
[fodauers-170309-07.tif, 2, en_US]
If the short-term current-rating capacity is specified for an application time other than 1 s, use the short-time
current instead of the 1-s current. Multiply the result by the specified application time.
For a given short-term current-carrying capacity of 0.5 s, use the following formula:
[fodauers-170309-01.tif, 2, en_US]
EXAMPLE for a Cable
For a cable
Permissible continuous current: I
max, perm
= 406 A
Maximum short-circuit current for 1 s: I
1s
= 21.4 kA
Therefore, for the Thermal time constant, it follows
[fokonsta-310510-01.tif, 2, en_US]
with 46.29 min = 2777 s
Parameter: Cooling time constant
•
Default setting (_:111) Cooling time constant = 3600 s
The Cooling time constant parameter is used to define the dropout behavior of the stage. Cables and
overhead lines have the same time constant for both heating and cooling. Therefore, set the same value for
the Cooling time constant as for the parameter Thermal time constant.
Protection and Automation Functions
7.26 Thermal Overload Protection, 3-Phase - Advanced
SIPROTEC 5, High-Voltage Bay Controller, Manual 887
C53000-G5040-C015-9, Edition 11.2017
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