Siemens 6MD85, Manual

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Operate Curve
If the ambient temperature is not measured and set to 40°C, you can get the operate curve as following:
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t Operate time
τ
th
Time constant
I Measured load current
I
preload
Preload current
k Setting factor according to VDE 0435 part 3011 or IEC 60255-8 (K factor)
I
rated, obj
Rated current of the protected object
Ambient Temperature
This function can take the ambient temperature into consideration. The reference temperature of the thermal
model is 40°C. If the ambient temperature drops below the reference temperature, the thermal limit increases.
The equipment can be stressed more. If the ambient temperature is higher, the conditions change.
The
Default temperature
parameter can be used to fix or measure the ambient temperature. The
Minimal temperature
parameter limits the coupled ambient temperature. If the measured ambient
temperature is lower than the minimum temperature, the minimum temperature is processed in the thermal
model.
The ambient temperature refers to the overtemperature of the protected object, which sets itself at the rated
current (parameter
Temperature rise at Irated
).
The measured ambient temperature is measured by an external RTD unit (RTD = Resistance Temperature
Detector) and provided by the functions RTD unit Ether. or RTD unit serial of the function group Analog
transformer. When using the
Temperature sensor
parameter, the respective temperature sensor can be
selected.
If the temperature measurement is faulty, for example, due to an open circuit between the device and the RTD
unit, the device reports this. In this case, the process continues with either the temperature measured last or
the value set under the
Default temperature
parameter, depending on which value is the highest.
Current Influence
The thermal replica based on the single-body model applies with limitations to high overcurrent situations
(short circuits, motor startup currents). To avoid an overfunction of the overload protection, the thermal
replica must be influenced for overcurrents (exceeding
l
threshold
). You can select between 2 strategies for this:
•
Freezing of the thermal memory
•
Limitation of the input current for the thermal replica to the set current. The temperature rise is thereby
retarded at high currents.
Warning Thresholds
The thermal warning threshold issues a warning indication before the tripping threshold (tripping tempera-
ture) is reached. In this way, for example, a load can be reduced in sufficient time and a disconnection
avoided. At a normal
K-factor
of
1.1
, a thermal memory value of 83 % sets in at continuously flowing
rated current.
Apart from the thermal warning threshold, the overload protection also has a current-warning threshold. This
current-warning threshold can signal an overload current in sufficient time before the overtemperature value
has reached the warning or tripping threshold.
Protection and Automation Functions
7.26 Thermal Overload Protection, 3-Phase - Advanced
884 SIPROTEC 5, High-Voltage Bay Controller, Manual
C53000-G5040-C015-9, Edition 11.2017