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The goal of a Protective Device Coordination Study is to determine overcurrent device settings and selections that maximize selectivity and power system reliability. In a well‐coordinated protective device, scheme faults are cleared by the nearest upstream protective device. This minimizes the portion of the electrical distribution system interrupted as a result of a fault or other disturbances. At the main distribution panel level, feeder breakers/fuses should trip before the main. Likewise, panel board branch breakers should trip before the feeder breaker/fuse supplying the panel.

As with Short Circuit Studies, a Protective Device Coordination is usually performed using the same specialized integrated software that is used for the Short Circuit, Load Flow, and Arc‐Flash calculations. Protective device types, ratings, and settings can be incorporated while the model is built or added after initial studies are completed. The use of computerized calculations allows the system protection engineer to evaluate a number of setting options in a short period of time, thereby allowing him (or her) to fine‐tune settings to achieve the best possible coordination.

Using the computer model, a Time Current Curve (TCC) is developed for each circuit fed from Service Switchboards and other critical buses. The curve includes the over current devices for the largest loads in series on the circuit – the worst case from a coordination aspect. All fuses, breakers, electromechanical or electronic relays, and trip devices are entered into the computer model, if not entered previously. Transformer and cable damage curves should be selected for inclusion on the TCC to verify that critical equipment is being protected. Transformer inrush points must be included to verify that the protective device feeding the unit will not operate when the transformer is energized. Device selection and settings from the database are reviewed to determine if changes are required to improve coordination. If so, settings or selection are modified, and the resulting TCC’s are printed for inclusion in the report. Protective device settings and selections are also summarized in tabular form.

Figure 3.3 Sample TCC Curve Analysis

(Source: Courtesy of PMC Group One, LLC)

In many cases, protective device coordination is a matter of compromise. Rather than being a choice of black or white, device coordination requires making selections that result in the best coordination that can be achieved with the devices that are installed: fuse characteristics are not as varied as electronic trip devices; instantaneous elements in series cannot be reliably coordinated; transformer protection must be selected to provide protection from damage from a fault while passing inrush current when the unit is energized.

Luckily, where power system reliability must be maximized, and therefore strict coordination is required, electronic relays and trip devices offer a variety of settings, curve shapes, and other functions that allow the system protection engineer to achieve this goal. Electronic relays come with a variety of trip characteristic curves, which, along with time delay and pick‐up settings, allow a great deal of flexibility when programming the device. The Zone Selective Interlocking feature available in many static trip devices allows the arming of Instantaneous settings on breakers in series without losing coordination. The upstream breaker trip device (such as the main device on a bus) communicates with downstream breakers. If the downstream device sees a fault current event, it sends a signal to the main device to block tripping the main breaker, thus allowing the downstream device to operate, minimizing the extent of the electrical system affected by the fault.

As discussed, state‐of‐the‐art protective devices can make a significant contribution to protective device coordination, minimizing or eliminating unnecessary outages due to compromised coordination. If project requirements demand strict coordination, electrical equipment selection may be affected. It is, therefore, important to consider how these requirements affect equipment selection, specification, and layout as early in the project as possible. The selection of the wrong type of equipment may negate the ability to take advantage of the technological advances discussed above.