ABB RELION SMU615 Technical Manual page 150

Supervision functions
Figure 99: Connection diagram for current circuit supervision with two sets of
three-phase current transformer cores (protection and measurement)
Example of incorrect connection
The currents must be measured with two independent cores, that is, the phase
currents must be measured with a different core than the reference current. A
connection diagram shows an example of a case where the phase currents and the
reference currents are measured from the same core.
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SMU615
Technical Manual