POTT scheme relies on receiving a trip signal from the remote end relay and the local relay to initiate breaker trip. The signal is keyed using a Frequency-Shift Keying (FSK) transmitter/receiver.
Under normal conditions, the relay transmits a “guard” frequency. This frequency disengages the relay from operating.
Under abnormal conditions, the relay switches to a higher or a lower frequency to transmit a “trip” signal. When relays on both ends of the line shift to the “trip” frequency, only then are the breakers tripped. The guard/trip frequencies are maintained and specified by the utility such that no two lines in the system use the same frequency.
Usually POTT schemes are not implemented using the power line carrier. A fault on the T-line may short out the trip signal and therefore inhibit relay operation. In this scenario, multi-phase coupling of the carrier signal provides the necessary redundancy for the secure operation of this scheme.
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