Directional zone select interlock method

1. A method of operating a circuit breaker in an electric circuit, said method comprising:

identifying a first circuit breaker as a reference circuit breaker;

determining when said identified reference circuit breaker is in reverse current pick-up; and when not, determining when a second circuit breaker is in short-time pick-up, indicative of a fault on the electric circuit; and when not;

comparing a third direction of current flow through a third circuit breaker with a first direction of current flow through said identified reference circuit breaker;

determining if said third circuit breaker is carrying current in a forward direction with respect to said identified reference circuit breaker and when said third circuit breaker is not in short-time pick-up, executing the steps of:

comparing a first direction of current flow through a first circuit breaker other than said identified reference circuit breaker with said first direction of current flow through said identified reference circuit breaker;

determining when said first main circuit breaker other than said identified reference circuit breaker is in reverse current pick-up; and

when said first main circuit breaker other than said identified reference circuit breaker is in reverse current pick-up,

tripping said first main circuit breaker determined to be in reverse current pickup;

when the first circuit breaker other than said identified reference circuit breaker is not in reverse current pick-up executing the steps of:

calculating a partial differential current for each of a plurality of zones in said electrical system; and

identifying one of said zones in said electrical system as carrying a maximum partial differential current; and

tripping said circuit breaker in said identified one zone identified as carrying said maximum partial differential current; and

increasing by a predetermined amount a trip time delay of said first and said second circuit breaker upstream of said third circuit breaker.

2. The method of claim 1 further comprising, when said second circuit breaker is determined to be in short-time pick-up, indicative of a fault on the circuit, the step of:

comparing a second direction of current flow through said second circuit breaker with said first direction of current flow through said identified reference circuit breaker;

identifying when said second circuit breaker is carrying current in a forward direction with respect to the identified reference circuit breaker; and if not, comparing said third direction of current flow through said third circuit breaker with said first direction of current flow through said identified reference circuit breaker;

determining when said third circuit breaker is carrying current in a forward direction with respect to the identified reference circuit breaker; and is in short-time pick-up, and when so;

increasing by a predetermined amount the trip time delay of said first and said second circuit breaker upstream of said identified third circuit breaker.

3. The method of claim 2 further comprising, when said second circuit breaker is carrying current in a forward direction with respect to said identified reference circuit breaker, the steps of:

determining when said third circuit breaker is carrying current in a forward direction with respect to said identified reference circuit breaker; and is in short-time pick-up, and when so;

increasing by a predetermined amount the trip time delay of said first and said second circuit breaker upstream of said identified third circuit breaker.

4. The method of claim 3 further comprising, when said third circuit breaker is not carrying current in a forward direction with respect to said identified reference circuit breaker, the step of:

increasing by a predetermined amount the trip time delay of said first circuit breaker upstream of said identified second circuit breaker.

5. The method of claim 1 , wherein said first circuit breaker is a main circuit breaker.

6. The method of claim 5 , wherein said second circuit breaker is a sub-main circuit breaker.

7. The method of claim 5 wherein said third circuit breaker is a feeder circuit breaker.

8. The method of claim 1 wherein said second or said third circuit breaker carrying more than 10-times its rated current is considered to be carrying current in the forward direction.

9. The method of claim 1 wherein said predetermined amount of trip time delay of said second circuit breaker upstream of said third circuit breaker is increased by about 100 ms, and said predetermined amount of trip time delay of said first circuit breaker upstream of said third circuit breaker is increased by about 200 ms.

10. The method of claim 2 wherein said predetermined amount of trip time delay of said second circuit breaker upstream of said third circuit breaker is increased by about 100 ms, and said predetermined amount of trip time delay of said first circuit breaker upstream of said at least one third circuit breaker is increased by about 200 ms.

11. The method of claim 3 wherein said predetermined amount of trip time delay of said second circuit breaker upstream of said third circuit breaker is increased by about 100 ms, and said predetermined amount of trip time delay of said first circuit breaker upstream of said third circuit breaker is increased by about 200 ms.

12. The method of claim 4 wherein said predetermined amount of trip time delay of said at least one first circuit breaker upstream of said identified second circuit breaker is increased by about 100 ms.

13. The method of claim 1 wherein said first circuit breaker is tripped without intentional delay.

14. The method of claim 1 wherein said circuit breakers in said identified one zone identified as carrying said maximum partial differential current is tripped without intentional delay.