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4.4
Ultimate Trip Current
Functional Description
The Ultimate Trip Current (UTC) sets the current level at which a trip eventually occurs
when no RTD stator temperature data is available.
The current level 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 (without 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.
CAUTION!
Setting the UTC above 100% times the service factor can result in motor damage.
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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EMR-3MP0
4 Protective Elements
4.4 Ultimate Trip Current