Patent ID: 7542256

Claim:
A method of operating a relay device, the method comprising the steps of: a) providing a an electrical circuit comprising first and second main circuit breakers and a bus tie breaker connecting the first and second main circuit breakers; b) defining a partial differential zone comprising a first circuit breaker and the bus tie breaker; c) determining if at least one of the circuit breakers is in short-time pick-up indicative of at least one fault on the circuit; d) determining a direction of fault currents in the circuit, the direction determined to be flowing into or flowing out of the partial differential zone for each of the fault currents if the short-time pick-up of the at least one of the circuit breakers exceeds a predefined threshold for the at least one of the circuit breakers; e) assigning a first value to the direction of the fault current flowing into the partial differential zone; f) assigning a second value to the direction of the fault currents flowing out of the partial differential zone, wherein the first value is not equal to the second value; g) comparing the values assigned to the fault currents; h) determining if the each of the fault currents is flowing into the partial differential zone; and i) determining if at least two of the each of the fault currents is flowing in a different direction with respect to the partial differential zone wherein one of the each of the fault currents is flowing into the partial differential zone and one of the each of the fault currents is flowing out of the partial differential zone.