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3-48
Protection Functions
857-UM001A-EN-P – July 2009
k
Q
1.2
I
MAX40
1.0
I
0.8
MAX70
0.6
10
20
30
40
50
Figure 3.24 – Ambient Temperature Correction of the Overload Stage T>
Example of a Behavior of the Thermal Model
Figure 3.25 shows an example of the thermal model behavior. In
this example τ = 30 minutes, k = 1.06 and kτ = 1 and the current has
been zero for a long time and thus the initial temperature rise is 0 %.
At time = 50 minutes the current changes to 0.85xI
temperature rise starts to approach value (0.85/1.06)
according the time constant. At time=300 min, the temperature is
about stable, and the current increases to 5 % over the maximum
defined by the rated current and the service factor k. The
temperature rise starts to approach value 110 %. At about 340
minutes the temperature rise is 100 % and a trip follows.
Initial Temperature Rise After Restart
When the relay is switched on, an initial temperature rise of
70 % is used. Depending of the actual current, the calculated
temperature rise then starts to approach the final value.
Alarm Function
The thermal overload stage is provided with a separately settable
alarm function. When the alarm limit is reached the stage activates
its start signal.
AmbientTemperatureCompensation
60
70
80
Q
(°C)
AMB
and the
MODE
2
= 64 %
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