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.
Recommendation of protective device settings are included in the coordination study section of the arc flash hazard calculation study. Plots of the time current characteristics for these devices are included to show how these recommendations were derived.
Over-current protective device coordination is reviewed for each one-line diagram. The Time-Current Characteristic (TCC) selection represents those conditions which potentially pose the worst coordination conditions. In general, coordination can be obtained in the overload regions of the Over-Current Protection Devices (OCPDs), however only limited coordination is usually achieved in the instantaneous region. As a result of the coordination study, the following would be noted.
The settings of the adjustable trip circuit breakers in the system are summarized in the study. After the report is reviewed the settings recommended in the study should be made on the respective devices. For further discussion of system coordination, reference would be made to the time current curves and respective comments in the study.
Protective device coordination as established in the report requires that the individual protective device operating characteristics do not depart appreciably from those shown on the time current plots. The specified settings will provide operation of the protective devices essentially as shown. For low voltage direct acting trips and fuses the tolerance bands permit a deviation in operating characteristics. However, the protective relay tolerance and the difficulty in exact field settings may result in deviations from the specified operating times. Therefore, it is recommended that the relay settings be calibrated by field test to obtain the desired relay response and be calibrated and checked at regular future intervals.
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