Abstract: The invention relates to protective apparatus used on AC power lines. An embodiment of an apparatus for putting the invention into practice comprises impedance sensing means having an impedance characteristic defining at least three distinguishable zones in its RX diagram, and being arranged to produce respective different signals, one for each zone, indicating the zone in which the sensed impedance lies. The movement of the point representative of the apparent impedance of the line across these zones is different according to whether the movement is due to loss of synchronism between the generators supplying the line or due to power oscillations. A logic circuit is provided to detect this difference in movement.

Abstract: Directional relay and method for monitoring at least one A.C. power transmission line to determine the position of a fault on the line with respect to a measurement location thereon.According to a representative embodiment characteristic parameters, typically the respective phases, of a first signal representing the difference of complex voltages after and before the occurrence of the fault and of a second signal representing the line current after the occurrence of the fault, are compared, the result of the comparison providing the desired indication.