2.7 Thermal Overload Protection
95
7ST6 Manual
E50417-G1176-C251-A3
tripping temperature, is reached, the protected object is disconnected from the net-
work. Reclosing is prevented until the temperature has dropped again below
TEMP
CLOSE
. The figure below shows the pickup and trip logic of the overload protection.
Figure 2-23
Pick-up and tripping logic of the overload protection
2.7.2
Ambient Temperature Sensing
Figure 2-24 shows a schematic representation of the ambient temperature sensing.
The ambient temperature is taken only once per substation with a separate device
(7SW40) that has its own, galvanically separated power supply and its own housing.
A Pt100 temperature sensor is used for this purpose. This device delivers at its output
a load-independent current (4-20 mA TD input) which can be looped through up to 20
connected 7ST6 devices via an isolating amplifier. The ambient temperature sensing
feature is available for the temperature ranges between -30
°
C and +55
°
C and
between -55
°
and +55
°
C.
A failure of the ambient temperature sensing, e.g. because of a wire break of the Pt100
sensor, a defective device or a voltage failure, is detected by the main protection
equipment by comparison with defined current values in the current loop.
After each update of the ambient temperature sensing, the device checks whether the
current is in the permissible operating range. If this is not the case, a malfunction of
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