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Protective Device Coordination

Protective Device Coordination

Coordination studies evaluate breakers, relays, fuses, and other protective devices to determine if the device closest to the fault clears first. When practical, settings are adjusted to improve coordination while still protecting conductors, transformers, generators, motors, and other equipment.

Coordination

What is it?

Coordination is the time-current relationship between protective devices that determines which device operates first for a fault or abnormal condition. Proper coordination allows the protective device nearest the fault to clear the condition while upstream devices remain closed where practical, limiting the outage to the affected portion of the system.

A coordination study evaluates protective device characteristics and selects settings intended to achieve that behavior while balancing fault isolation, equipment protection, and continuity of service.

Settings should not be so high that equipment is exposed to damaging currents, or so low that normal transients, transformer inrush, motor starting, or acceptable operating conditions cause nuisance trips.

Selective coordination fault isolation diagram from the source Word document
Example Coordination Illustration.
TCC Curves

Time-current curves (TCCs)

A time-current characteristic curve (TCC) plots current on the horizontal axis and time on the vertical axis, typically using logarithmic scales. By overlaying downstream and upstream protective devices, the study can evaluate whether the downstream device clears first and whether adequate separation exists between device curves.

TCCs are also used to compare protective device settings against transformer inrush, motor starting, cable and transformer damage curves, and generator decrement curves. The curves are evaluated up to the available fault current calculated in the short circuit study.

Example time-current characteristic curve showing protective device curves and transformer inrush
Example time-current characteristic curve.
Practical Review

Timing and coordination considerations

Relay-based devices

Protective relays often show trip time only. Breaker clearing time, relay output time, auxiliary devices, and CT accuracy must be considered separately.

Low-voltage breakers

Published low-voltage breaker TCCs commonly show total clearing time, so the curve already includes the breaker clearing behavior represented by the manufacturer.

Coordination time interval

Legacy IEEE 242 Buff Book guidance includes reasonable CTI allowances between combinations such as breakers, relays, and fuses, including tolerances and breaker operating time.

Instantaneous regions

Coordination in the instantaneous region can be difficult to achieve. Selectivity may depend on device type, manufacturer data, , , or tested combinations.

Protection Balance

Balancing coordination and protection

The study checks whether protective devices clear faults fast enough to protect equipment damage curves while still allowing expected transient events to ride through. Transformer inrush, motor acceleration, generator decrement, cable withstand, and transformer through-fault limits can all shape the final settings.

Where code-required selective coordination applies, such as emergency systems, legally required standby systems, critical operations power systems, fire pumps, and other specific systems, the study should clearly document any cases where the required level of coordination could not be achieved.

If equipment is identified that may not comply with protective device related NEC requirements, the condition should be communicated to site personnel. Examples include missing or inadequate ground-fault protection of equipment for qualifying 1000 A or larger disconnects on solidly grounded wye systems, or missing arc-energy reduction provisions for circuit breakers with a highest continuous current trip setting of 1200 A or higher.

Typical Coordination Objectives

  • Downstream devices operate first for downstream faults where practical
  • Conductors and equipment remain protected by applicable damage curves
  • Transformer inrush and motor starting do not cause nuisance tripping
  • Settings are clearly documented in support of field commissioning and testing
  • Arc flash mitigation is an add-on service performed after initial arc flash runs are performed
Study Process

What goes into a coordination study

Collect Data

Gather protective device manufacturer, model number, frame, trip unit, sensor, fuse class, relay model, CT ratios, and existing settings where applicable.

Build unique paths

Create TCCs for each meaningful source-to-load path, including utility, generator, tie, bypass, and maintenance configurations where applicable.

Optimize Settings

Adjust long-time, short-time, instantaneous, and ground-fault, or time overcurrent/definite time responses including time delays, to balance coordination and protection.

Documentation and Review

Provide setting tables so commissioning agents and testing technicians can set devices exactly as intended. Review as-left relay settings files to ensure relay settings match the recommended settings.

Next Step

Study sequence

Incident energy depends heavily on protective device clearing time at the calculated arcing fault current. For this reason, the coordination study should be completed before final arc flash calculations and labels are developed.

The coordination study develops protective device settings with arc flash performance in mind. However, arc flash mitigation through reduced or modified settings requires an additional review of the settings. This add-on service, if requested by the Client, is performed after the initial coordination settings are established and the preliminary arc flash results are available. The evaluation identifies locations where incident energy may be reduced through settings adjustments while maintaining appropriate coordination and equipment protection.

Supporting Deliverables

  • TCC plots for unique electrical paths
  • Breaker and relay settings tables
  • Summary of noteworthy coordination or protection findings in report write-up
Request a coordination study
References

Standards and code sections commonly considered

  • NEC 700.32, 701.32, 708.54, and 695.4(C)(3) - selective coordination requirements for specific emergency, standby, critical operations, and fire pump systems, where applicable and as adopted by the authority having jurisdiction.
  • NEC 230.95, 215.10, 240.13, 240.87, and 240.67 - protective device requirements that may affect coordination studies (requirements depend on the adopted NEC edition, system grounding, voltage, equipment type, and AHJ interpretation).
  • IEEE 242 Buff Book and successor IEEE color-book/3000-series guidance - protective device coordination principles, CTI considerations, and industrial/commercial power system protection practices.
  • IEEE Houston technical presentation - public educational discussion of CTI elements and Buff Book references.