Abstract: In an electrical power transmission system, transmitter-receiver pairs at stations which communicate with similar pairs at other stations to block tripping of circuit breakers or for other control functions are periodically tested. An electrical circuit periodically activates a transmitter to send a test signal from a first station over a power line to a receiver at another station in the system. The receiver at the other station activates the associated transmitter in response to receipt of the incoming signal to send a signal back to the receiver of the first station. If this cycle is not completed in a specified time, indicating loss of communication between the two transmitter-receiver pairs, an alarm is activated.