Abstract: There is provided an apparatus for producing a trip signal to activate a circuit breaker in an alternating current power line, said apparatus comprising: a sensing circuit operable to sense values representing: a phase difference between current and voltage in said power line; and current amplitude in said power line; a comparison circuit operable to compare said sensed values of phase difference and current amplitude with a trip characteristic defining fault conditions and non-fault conditions and to generate a fault indication signal when a fault condition arises; a fault discriminator operable in response to said fault indication signal to generate a fault-identifying signal discriminating between: a first class of fault in which a mean current value after said fault indication increases relative to a mean current value before said fault indication; and a second class of fault in which a mean current value after said fault indication does not increase relative to a mean current value before said fault indi

Abstract: There is provided an apparatus for producing a trip signal to activate a circuit breaker in an alternating current power line, said apparatus comprising: a sensing circuit operable to sense values representing: a phase difference between current and voltage in said power line; and current amplitude in said power line; a comparison circuit operable to compare said sensed values of phase difference and current amplitude with a trip characteristic defining fault conditions and non-fault conditions and to generate a fault indication signal when a fault condition arises; a fault discriminator operable in response to said fault indication signal to generate a fault-identifying signal discriminating between: a first class of fault in which a mean current value after said fault indication increases relative to a mean current value before said fault indication; and a second class of fault in which a mean current value after said fault indication does not increase relative to a mean current value before said fault indi

Abstract: A rotary lawn mower blade assembly having replaceable cutting edge members which may be replaced without replacing an entire blade or sharpening the used blade. The cutting edge members slide into place on corresponding “T”-shaped tongues of the blade body until encountering an integral stop. The cutting blade member is securely held in place by a blade retainer having a rectilinear portion which mates with a retainer stud fastened in the blade body and an extension plate portion with mounting cap screws reaching through the blade retainer extension plate into the cutting blade member. In another embodiment the cutting edge members are secured by set screw extending downward from the upper surface of the cutting edge member and bearing against the upper side of the “T”-shaped tongue of the blade body.