What is selective coordination?
The term selective coordination refers to the selection and setting of protective devices in an electric power system in such a manner as to cause the smallest possible portion of the system to be de-energized due to an abnormal condition. This ensures any over-current event is first tripped by the smallest breaker in the system before allowing a larger main breaker to intercept the fault. This limits the service interruption to only the circuit experiencing the problem and does not shut down a larger portion of the facility.
How is selective coordination achieved?
In most cases selective coordination is achieved via the timing characteristics of the devices to be coordinated. For example, each of the circuit breakers in system has its own time-current characteristic; by coordinating these, selective coordination may be achieved. This is usually accomplished by comparing the device time-current characteristics to ensure there is no overlap in protection between the upstream and downstream circuits.
How can I optimize my design for selective coordination?
- Likely the biggest benefit is to perform a study to determine the short circuit current available at various points in the system. This may allow the selection of circuit breakers with a lower interrupting rating and a higher instantaneous trip level.
- Work from the bottom up, starting from bottom of the system and coordinate the branch lighting panels first, then the power distribution panels, then the switchboard or switchgear.
- Use the short circuit selective coordination tables. They may be found in the Short Circuit Selective Coordination for Low Voltage Circuit Breakers data bulletin.
- You may also increase the frame size of main or feeder breakers which increases the instantaneous trip level of these breakers.
- Incorporating ground fault protection into feeder and branch circuits so that low level ground faults will be cleared before they escalated into high level phase-to-phase faults.
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