Simply put this over wire going to load like motor or pump and when the pump energizes the contacts open or close or NC contacts ) in CT or current transformer..
DDC controller is wired to this CT and the wires terminate at CT and back at the controller on input terminal.
DDC input terminal can monitor the switch to tell if the switch is open or closed and depending on the programming DDC controller processes the input and uses it in manner for response depending on the program..
The downside to this type of status is that the motor or load can still be running however, the fan or pump can have broken belt or coupling and one would continue to read status unaware of the real problem..
This type of CT also reacts to the current of the wire going to the load except for this type of CT also reads the actual amperage in the wire.
The program can be set to accept certain tolerance ranges so if the load goes too far out of tolerance for the expected amperage range and if the load goes out of tolerance alarm is raised to let the operators or maintenance staff know there is problem.
DP type of status can be found on pumps with tap on the suction side of the pump and one on the discharge side of the pump..
By having the contacts close at certain pressure across the filter bank, DP type of status can also be found on fans and used for filter statuses to tell if the filter needs changing.
The predecessor to DP switch in these applications is the sail switch where sail switch was placed in the ductwork and when there was airflow the sail was pushed down and closed the switch..
When the relay or contactor is closed the aux contacts can tell the input at DDC whether the load is energized or not.
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