2.9 Thermal Overload Protection
227
7UT613/63x Manual
C53000-G1176-C160-2
tables. As the nominal data of the protected object and the current transformer ratios
are known to the device, the K-FACTOR can be set immediately.
When using the method with hot-spot calculation according to IEC 60354, setting K-
FACTOR = 1 is advisable as all remaining parameters refer to the rated current of the
assigned side of the protected object.
Time Constant Ï„ for
Thermal Replica
The thermal time constant Ï„
th
for the thermal replica is set under 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 de-
termining 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
t6-time; this is the time in seconds for which a current of 6 times the rated current of
the protected object may flow
Calculation examples:
Cable with
permissible continuous current 322 A
permissible 1-s current 13.5 kA
Setting value TIME CONSTANT = 29.4 min
Motor with t6–time 12 s
Setting value TIME CONSTANT = 7.2 min
For rotating machines, the thermal time constant set under TIME CONSTANT is valid
for as long as the machine is running. The machine will cool down significantly slower
during stand-still or running down, if it is self-ventilated. This phenomenon is consid-
ered by a higher stand-still time constant KÏ„-FACTOR (address 4207) which is set as
a factor of the normal time constant. This parameter can only be set with DIGSI under
Additional Settings.
If it not necessary to distinguish between different time constants, e.g. with cables,
transformers, reactors, etc., retain the factor KÏ„-FACTOR = 1.0 (default setting).
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