EMR-5000
IM02602012E
Ultimate Trip Current
Elements:
Ultimate Trip Current
Functional Description
The Ultimate Trip Current (UTC) sets the current level at which a trip eventually occurs and is settable to a value as a
percentage of Full Load Amps (FLA). This value represents the vertical line on the upper portion of the non-RTD as
shown in the protection trip curve labeled Motor Protection Curve Example 2 ( with out RTD). The ultimate trip current
setting in this example is at 1 times the FLA.
Note that in systems where an RTD is used the UTC pick-up point is biased by the measured temperature. This is
shown in the example trip curve labeled Motor Protection Curve Example 3 ( with RTD) were you will see a shift in the
UTC value to 2 times the FLA
For normal use, set UTC to the
service factor
times 100%. The available range is 85% to 150%. The service factor is
found on the motor nameplate or in the manufacturer’s data. Note that the relay does not trip at the moment the current
goes above UTC during motor running. Instead, it models the gradual stator heating for currents above UTC,and trips
only after some time has passed. The trip time depends on a variety of setting and operating factors, including the
motor nameplate data contained in other setting values.
Use a conservative value. In this case, a lower value of UTC than that dictated by the service factor if the motor
ambient temperatures may rise above 40ºC (104ºF) and the optional URTD Module is not used. Also, consider
lowering the UTC value if the motor is suitably rated, yet additional safety is critical for the application.
If UTC is set above 100% times the service factor, motor damage could
result.
If stator RTDs are not used and there is the possibility that the ambient may rise above 40°C (104°F), the ultimate trip
current should be set below the value indicated by the nameplate service factor to avoid stator insulation damage or
loss of motor life.
If stator temperature measurements are available, the algorithm may keep from tripping, even if the effective current is
above the ultimate trip current setting, depending on stator temperature reports. It is still important to set a correct
ultimate trip current so that the motor is well protected. If the RTDs, the module, or its communications to the relay fail,
the algorithm falls back to use of UTC. Also, note that if all RTD channels are set to OFF, the algorithm reverts to the
non- RTD calculation, which is based strictly on UTC.
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