Figure 3.1-3 Application Of Thermal Overload Time Constants - Siemens 7SR17 Rho Manual

Gn 49 Hot/Cold Ratio
The hot/cold ratio setting determines the percentage of thermal capacity available for a motor running at full load
current compared to that available when the motor is cold. It modifies the IEC255-8 hot curve as below:
2
I (1
t . ln
2
I I
Where;
H/C = hot spot weighting factor = 49 Hot/Cold Ratio setting
A setting is available to switch this feature out of service, however this Hot/Cold ratio setting will normally be used
on all motors as it will assist with the accuracy of the thermal modelling. The hot curve is only operational when
the equivalent thermal current calculated by the relay is less than the thermal pickup setting. Once the equivalent
thermal current exceeds the thermal pickup setting the relay operates on the Cold thermal curve.
The purpose of the H/C ratio is to allow for the fact most motors are designed thermally to withstand the onerous
starting conditions rather than the running conditions. This leads to the fact the motors will tend to run at a much
lower temperature than their insulation class allows when thermal equilibrium is reached
The thermal algorithm uses 'hot' and 'cold' curves, it also uses alternate time constants during different phases of
motor operation, to summarise:
Gn 49 Capacity Alarm
An alarm can be given if the thermal state of the system exceeds a specified percentage of the protected
equipment's thermal capacity setting. This can be used to warn the operator that a relay thermal trip will occur if
this level of motor current continues.
Gn 49 Load Alarm
An alarm is available to provide warning of a sudden increase in load. The Load Increase Alarm setting is set as
a multiple of the thermal overload setting I .
Gn 49 Overload Alarm
An instantaneous alarm output is given if the equivalent thermal current I
whilst the motor is in it running state.
©2014 Siemens Protection Devices Limited
H
2
)I
P
C
2

Figure 3.1-3 Application of Thermal Overload Time Constants

Chapter 1) 7SR17 Rho Description Of Operation
exceeds the thermal overload setting I
e
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