Siemens SIPROTEC 4 7UT6 Series Manual page 206

Functions
2.9 Thermal Overload Protection
[thermueberl-einstellfaktor-k-021026-rei, 1, en_GB]
The permissible continuous current is at the same time the current at which the e-function of the overtemper-
ature has its asymptote.
When using the method with a thermal replica, it is not necessary to evaluate any absolute temperature nor
the trip temperature since the trip temperature rise is equal to the final temperature rise at k · Ι
turers of electrical machines usually state the permissible continuous current. If no data are available, the K-
FACTOR is set to 1.1 times the rated current of the assigned side of the protected object. For cables, the
permissible continuous current depends on the cross-section, the insulation material, the design and the
method of installation, and can be derived from the relevant 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.
Unlike the devices 7UT613/63x and 7UT612 V4.6, which operate with the nominal current of the protected
object, device 7UT612 V4.0 uses the nominal current of the protection device. Here, the setting value of the k
factor shall take the mismatching into account:
[einstellwert-kfaktor-allg-230902-oz, 1, en_GB]
with:
I
max prim
I
N Motor
I
N Wdl prim
For the method with hotspot calculation according to IEC 60354, K-FACTOR = 1 makes sense as the other
parameters refer to the rated current of the assigned side of the protected object.
Time Constant τ for Thermal Replica
The thermal time constant τ
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
[thermueberl-zeitkonstante-1sstrom-021026-rei, 1, en_GB]
permissible current for application time other than 1 s, e.g. for 0.5 s
[thermueberl-zeitkonstante-andere-021026-rei, 1, en_GB]
t6-time; this is the time in seconds for which a current of 6 times the rated current of the protected object may
flow
[thermueberl-zeitkonstante-t6zeit-021026-rei, 1, en_GB]
206
thermally continuously permissible primary current of the object
Nominal Current of the Object
Nominal primary CT current
for the thermal replica is set under address 4203 TIME CONSTANT. This is also
th
. Manufac-
N Obj
SIPROTEC 4, 7UT6x, Manual
C53000-G1176-C230-5, Edition 09.2016