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
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