Protective Device Coordination Study

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.

Coordination Study Summary

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.

  1. The time current curves provided in the study show that all cables and all transformers are adequately protected by the over current protective devices. The settings chosen for the circuit breakers is an attempt to provide the best possible protection for downstream devices while trying to maintain selective coordination.
  2. Complete coordination of the system may not or is not possible due to fixed curve molded case breakers which have instantaneous trip characteristics extending to the interrupting rating of the OCPD. At higher levels of fault current, the instantaneous region of the OCPD characteristic operates. Fault currents can flow through a multiplicity of OCPDs thus causing multiple OCPDs to open. In such case, the instantaneous regions of the circuit breakers overlap. The value of available fault current will cause the load side OCPD to trip, however in the time needed to clear the fault, the upstream OCPD unlatches and also opens. In some cases multiple devices can trip open. This is true for circuit breaker combinations with other circuit breakers and/or current-limiting fuses, which may be seeing fault currents in their current-limiting range. Some of the branch panel board circuit breakers have fixed characteristics and cannot be adjusted to improve coordination. Where circuit breakers have fixed characteristics, it is noted in the recommended circuit breaker settings table.
  3. Additional coordination problems exist due to a number of several same size protective devices in a single circuit where ampere ratings of OCPDs in series are identical. This is typical to circuit breakers protecting feeders to panel boards, and the panel board having a main circuit breaker of the same ampere rating. These devices cannot coordinate. In this case there is no coordination in both the overload and instantaneous region.
  4. Where electronic trip type circuit breakers are provided, the ability to set short-time delays and pick-up settings improves the coordination. If adjustment to the instantaneous region of the circuit breaker can be made, it is set to consider maximum coordination with other OCPDs. If the OCPD is on the primary side of a transformer the transformer excitation inrush current must be considered. The transformer damage curves and the inrush current are plotted where applicable.
  5. All breaker coordination was analyzed and provided with arc flash incident energy level consideration. The settings suggested provide the best possible coordination while minimizing the corresponding arc flash incident energy level.

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.