ABB RELION SMU615 Technical Manual page 142

Supervision functions
Figure 92: Incorrect connection of trip-circuit supervision
A connection of three merging units with a double pole trip circuit is shown in the
following figure. Only the merging unit R3 has an internal TCS circuit. In order to test
the operation of the merging unit R2, but not to trip the circuit breaker, the upper
trip contact of the merging unit R2 is disconnected, as shown in the figure, while the
lower contact is still connected. When the merging unit R2 operates, the coil current
starts to flow through the internal resistor of the merging unit R3 and the resistor
burns immediately. As proven with the previous examples, both trip contacts must
operate together. Attention should also be paid for correct usage of the trip-circuit
supervision while, for example, testing the merging unit.
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SMU615
Technical Manual