Zone selective interlocking and circuit protection device monitoring in a power distribution system

1. A zone selective interlocking (ZSI) power distribution system comprising:

a first circuit protection device configured to interrupt a current flowing through said first circuit protection device;

a second circuit protection device downstream from said first circuit protection device, said second circuit protection device comprising:

a trip mechanism configured to interrupt current flowing through said second circuit protection device; and

a trip unit operatively coupled to said trip mechanism, said trip unit configured to:

monitor current flowing through said second circuit protection device;

output a first signal when the monitored current is less than a threshold value; and

output a second signal when the monitored current is greater than or equal to the threshold value, wherein the first and second signals are different from each other and different than an absence of a signal; and

a controller configured to:

control said first circuit protection device in a first mode in response to the first signal;

control said first circuit protection device in a second mode in response to the second signal; and

control the first circuit protection device in a third mode in response to the absence of a signal from said second circuit protection device.

2. A ZSI power distribution system in accordance with claim 1 , wherein said controller is disposed within said first circuit protection device.

3. A ZSI power distribution system in accordance with claim 1 , wherein the first mode is an unrestrained operation mode and the second mode is a restrained operation mode.

4. A ZSI power distribution system in accordance with claim 3 , wherein the third mode is selectable between the restrained operation mode and the unrestrained operation mode.

5. A ZSI power distribution system in accordance with claim 4 , wherein said controller is further configured to provide an alert in response to the absence of a signal from said second circuit protection device.

6. A ZSI power distribution system in accordance with claim 1 , wherein the threshold value is a ZSI blocking threshold less than a tripping threshold for said second circuit protection device.

7. A ZSI power distribution system in accordance with claim 1 , wherein said controller is configured to determine an absence of a signal from said second circuit protection device when a determined period of time passes without receiving at least one of the first signal and the second signal.

8. A ZSI power distribution system in accordance with claim 1 , further comprising:

a third circuit protection device downstream from said first circuit protection device, said third circuit protection device configured to output the first signal when a monitored current through said third circuit protection device is less than the threshold value, and output the second signal when the monitored current through said third circuit protection device is greater than or equal to the threshold value; and

an aggregator communicatively coupled to said first, second, and third circuit protection devices, said aggregator configured to receive the first and second signals from said second and third circuit protection devices and provide an aggregated output to said controller.

9. A ZSI power distribution system in accordance with claim 8 , wherein said aggregator is configured to provide an aggregated output indicating whether the first signal, second signal, or an absence of the first and second signals has been detected.

10. A ZSI power distribution system in accordance with claim 9 , wherein said aggregator is configured to:

output the first signal to said controller when the first signal is received from both of the first and second circuit protection devices;

output the second signal to said controller when the second signal is received from either of the first and second circuit protection devices; and

output the third signal to said controller when the first and second signals are not received from one of the first and second circuit protection devices and the second signal is not received from the other of the first and second circuit protection devices.

11. A circuit protection device for use in a zone selective interlocking (ZSI) power distribution system, said circuit protection device comprising:

a trip mechanism configured to interrupt current flowing through said circuit protection device; and

a trip unit operatively coupled to said trip mechanism, said trip unit configured to:

monitor current flowing through said circuit protection device;

output a first signal when the monitored current is less than a threshold value; and

output a second signal when the monitored current is greater than or equal to the threshold value, wherein the first and second signals are different from each other and different than an absence of a signal; wherein the threshold value is a ZSI blocking threshold less than a tripping threshold for said circuit protection device.

12. A circuit protection device in accordance with claim 11 , wherein the first signal comprises an intermittent signal repeated at a substantially constant interval while the monitored current is less than the threshold value.

13. A circuit protection device in accordance with claim 12 , wherein the intermittent signal has a duration that is less than the substantially constant interval.

14. A circuit protection device in accordance with claim 11 , wherein the first signal has substantially the same magnitude as the second signal, and the first signal has a different duration than the second signal.

15. A controller for use in a zone selective interlocking (ZSI) power distribution system, said controller comprising:

an output configured to be communicatively coupled to a first circuit protection device;

an input configured to be communicatively coupled to a second circuit protection device downstream from the first circuit protection device;

a memory device; and

a processor coupled to said memory device, said controller programmed to:

control the first circuit protection device in a first mode in response to detecting a first signal at said input;

control the first circuit protection device in a second mode in response to detecting a second signal at said input; and

control the first circuit protection device in a third mode in response to failing to detect the first signal or the second signal at said input within a determined period of time.

16. A controller in accordance with claim 15 , wherein the first mode is an unrestrained operation mode and said second mode is a restrained operation mode.

17. A controller in accordance with claim 16 , wherein the third mode is selectable between the restrained operation mode and the unrestrained operation mode.

18. A controller in accordance with claim 15 , wherein said controller is further configured to provide an alert in response to failing to detect the first signal or the second signal at said input within the determined period of time.

19. A zone selective interlocking (ZSI) power distribution system comprising:

a first circuit protection device configured to interrupt a current flowing through said first circuit protection device;

a second circuit protection device downstream from said first circuit protection device, said second circuit protection device comprising:

a trip mechanism configured to interrupt current flowing through said second circuit protection device; and

a trip unit operatively coupled to said trip mechanism, said trip unit configured to:

monitor current flowing through said second circuit protection device;

output a first signal when the monitored current is less than a ZSI blocking threshold value; and

a controller communicatively coupled to said second circuit protection device, said controller configured to:

control said first circuit protection device in an unrestrained operation mode in response to detecting the first signal; and

control said first circuit protection device in a restrained operation mode in response to failing to detect the first signal.