[NEC Guide] Importance of Achieving Selective Coordination for Critical Power Systems!

power systems facilities field unit

One of the most important parts of any facility is the electrical distribution system.

Nothing will stop all activity, halt production, cause inconvenience and possibly panic, more than a major power failure.

Selective coordination is critical for the reliability of the electrical distribution system.

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What is Selective Coordination?

According to the NEC, selective coordination means that the fuse or circuit breaker closest to the fault opens up without opening up the fuse or the circuit breaker that feeds it (from the upstream side). So you won’t have black out conditions that can occur if there is a fault somewhere downstream.

What would happen if you do not have Selective Coordination?

The figure below shows the difference between a system without selective coordination and a system with selective coordination.

Selective coordination for power systems protection
Selective coordination for power systems protection
(SOURCE: EEP-Electrical Engineering Portal)

What happens if you have a Selectively Coordinated system?

There are several other benefit of Selective Coordination. A Selectively Coordinated system:

  • Aids in continuous operation of critical circuits
  • Reduces or eliminates blackouts
  • Increases safety and reliability of the system
  • Minimizes damage and downtime
  • Increases productivity
  • Speeds restoration pf faulted of faulted circuits
  • Reduces arc-flash hazards

Methods of Performing a Selective Coordination Study

There are a few methods used for performing a selective coordination study. Learn all about them here in detail!

Regardless of which method is used, a thorough understanding of time-current characteristic curves of overcurrent protective devices is essential to provide a selectively coordinated system.

Fuse Coordination Table

For fuse systems, verification of selective coordination is quick and easy, merely adhere to fuse ampere rating ratios as indicated by the manufacturer.

line side fuse load side fuses settings
line side fuse load side fuses settings

Circuit Breaker Coordination Study

circuit breakers plugged in rail in panel boards
circuit breakers plugged in rail in panel boards

The ability of circuit breakers to achieve coordination depends upon the maximum available fault current at the overcurrent protective devices where selective coordination is required. This fault current can be calculated using several programs.

Selective coordination with circuit breakers is often an iterative process as selection of upstream circuit breakers may need to be altered to achieve selective coordination.

Let us know if you have any queries regarding this topic and do provide us with your feedback in the comments.

About The Author

Abdur Rehman is a professional electrical engineer with more than eight years of experience working with equipment from 208V to 115kV in both the Utility and Industrial & Commercial space. He has a particular focus on Power Systems Protection & Engineering Studies.

Abdur Rehman is the CEO and co-founder of allumiax.com and creator of GeneralPAC by AllumiaX. He has been actively involved in various roles in the IEEE Seattle Section, IEEE PES Seattle, IEEE Region 6, and IEEE MGA.

Leaders in Industrial & Commercial Power Systems Engineering