Properly engineered systems will allow only the protective device nearest the fault to open, leaving the remainder of the system undisturbed and preserving continuity of service. A protective device coordination study is required to properly select and set the power systems protective devices to achieve this goal. This is accomplished by performing a time current evaluation among the protective devices.
Coordination is generally a compromise between the mutually desirable but somewhat inconsistent goals of maximum protection and maximum service continuity. With the use of molded circuit breakers it is usually impossible to coordinate the system in the instantaneous region due to the overlap of curves unless a zone interlock is utilized. For this and other reasons, such as established system design, many combinations of device settings may be classified as acceptable. The settings suggested in the arc flash hazard calculation study result from an exercise of judgment as to the best balance between competing objectives. Read more at http://arcadvisor.com/faq/protective-device-coordination-study