Schweitzer Engineering Laboratories SEL-701, Инструкция по эксплуатации

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Date Code 20010719
701 Motor Protection Relay
Settings Calculation
RTD-Based Protection
4.35
As ambient temperature rises, the motor’s ability to shed heat to the surroundings
is reduced and internal temperatures rise. To preserve insulation life, NEMA standards
suggest a 1°C reduction in RTD trip temperature for each 1°C rise in ambient
temperature over 40°C. When you enable RTD biasing, the SEL-701 Relay
automatically reduces the RTD trip temperatures for all winding RTDs when ambient
temperature is above 40°C. The relay reduces the trip temperatures by 1°C for each
degree rise in ambient temperature over 40°C.
Finally, when you enable RTD biasing, the relay provides an RTD Bias Alarm
when the RTD % Thermal Capacity exceeds the thermal element % Thermal Capacity
by more than 10 percentage points while the winding temperature rise is higher than
60°C over ambient. This alarm can be a useful indicator that the motor has lost coolant
flow or that the winding RTD trip temperature is conservatively low.
For all the RTD thermal capacity and bias calculations described above, the relay
uses the winding RTD whose measured temperature is closest to its trip value.
Table 4.26 lists the RTD resistance versus temperature for the four supported RTD
types.