VAMP 259 - Page 100

2.13 Thermal overload protection T>
(49)
2 Protection functions
Technical description
100
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VM259.EN007
Heat capacitance, service factor and ambient temperature
The trip level is determined by the maximum allowed
continuous current I
max
corresponding to the 100 %
temperature rise 
TRIP
i.e. the heat capacitance of the cable.
I
max
depends of the given service factor k and ambient
temperature 
AMB
and settings I
max40
and I
max70
according the
following equation.
n
IkkI 
max
The value of ambient temperature compensation factor k
depends on the ambient temperature 
AMB
and settings I
max40
and I
max70
. See Figure 2.13-1. Ambient temperature is not in
use when k = 1. This is true when
 I
max40
is 1.0
 Samb is ―n/a‖ (no ambient temperature sensor)
 TAMB is +40 °C.
Figure 2.13-1 Ambient temperature correction of the overload stage T>.
Example of a behaviour of the thermal model
Figure 2.13-2 shows an example of the thermal model
behaviour. In this example = 30 minutes, k = 1.06 and k = 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 0.85xIn and the temperature rise starts to
approach value (0.85/1.06)
2
= 64 % 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.