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Coordination Studies

Electrical Protection Coordination Studies

The objective of a protection scheme in a power system is to minimize hazards to personnel and equipment while allowing the least disruption of power service. Coordination studies are required to select or verify the clearing characteristics of devices such as fuses, circuit breakers, and relays used in the protection scheme. These studies are also needed to determine the protective device settings that will provide selective fault isolation. In a properly coordinated system, a fault results in interruption of only the minimum amount of equipment necessary to isolate the faulted portion of the system. The power supply to loads in the remainder of the system is maintained. The goal is to achieve an optimum balance between equipment protection and selective fault isolation that is consistent with the operating requirements of the overall power system.

Short-circuit calculations are a prerequisite for a coordination study. Short-circuit results establish minimum and maximum current levels at which coordination must be achieved and which aid in setting or selecting the devices for adequate protection. When facilities are changed or upgraded, it is necessary to revisit the existing protection scheme to determine if change is needed to be made to ensure that devices are coordinated properly. A change in load or equipment could change the timing and coordination of the protective devices.

Compliance: The coordination will be performed in accordance with the recommended practices and procedures set forth in ANSI/IEEE 399 and ANSI/IEEE 242. Protective device selection and settings shall comply with requirements of NFPA 70 National Electrical Code/OESC, following the recommendations of NETA.

Study Report: Following NETA recommendations, the results of the coordination study will be summarized in a final report containing the following items:

  • Basis, description, purpose, and scope of the study and a corresponding one-line diagram.
  • Time-current curves demonstrating the coordination of time-overcurrent protective devices. Coordination curves graph
  • Tabulations of protective devices identifying circuit location, manufacturer, type, range of adjustment, IEEE device number, current transformer ratios, recommended settings or device size, and referenced time-current curve.
  • Conclusions and recommendations.

What benefits would I receive from having a Study performed?
A Protective Device Coordination Study will show you the probable and possible values of fault currents within the system and show you what impact short circuits and failures would have on your fuses, circuit breakers, and your facility's operation. Furthermore, the study will determine proper frames, sensor sizes, and settings of the protective devices in order to minimize equipment and personnel damage while preventing nuisance trips and unnecessary outages due to inrushes and transients, as well as lack of selectivity pertaining to the protective devices.  More specifically:

  • Increased facility reliability
  • Increased equipment protection
  • Increased operating efficiency
  • Will aid in current operations and help prevent downtime
  • Prevent damage by identifying underrated equipment
  • Prevent damage by identifying overloaded equipment
  • Minimize nuisance and false trips of protective devices due to transients, inrush amperage, and starting currents
  • Avoid unnecessary blackouts and brownouts to the electrical power system
  • Minimize damage to electrical equipment such as transformers, cables, motors, generators and panelboards due to faults
  • Minimize damage to electrical apparatus due to overloads