TABLE OF CONTENTS It is not enough to select protective devices based solely on their ability to carry the system load current and interrupt the maximum fault current at their respective levels. A properly engineered system will allow only the protective device nearest the fault to open, leaving the remainder of the system undisturbed and preserving continuity of service.
We may then define selective coordination as the act of isolating a faulted circuit from the remainder of the electrical system, thereby eliminating unnecessary power outages. The faulted circuit is isolated by the selective operation of only that overcurrent-protective device closest to the overcurrent condition.
Currently, two methods are most often used to perform a coordination study:
Overlays of time-current curves, which use a light table and manufacturers' published data, then hand-plot on log-log paper.
Computer programs, which use a PC and allow the designer to select time-current curves published by the manufacturers and transfer to a plotter or printer, following proper selections.
This text will apply to both methods.
The following steps are recommended when conducting a selective coordination study.
One-line diagram: Obtain or develop the electrical system oneline diagram that identifies important system components, as given hereafter.
Transformers: Obtain the following data for protection and coordination information of transformers:
kVA rating
Inrush points
Primary and secondary connections
Impedance
Damage curves
Primary and...
