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John Livingston's

The Nuclear Electrical Engineer

An educational resource for electrical engineers in the nuclear power industry

Here you can find design tips on technical skills like cable and breaker sizing, short-circuit evaluations and more, PLUS deep-dive analysis of nuclear-specific topics like design basis, Tech Specs, UFSARs, and safety-related SSCs — built from primary source documents.

Sizing Breakers · Feature

Coordination curves

John Livingston
December 11, 2014 · 2 min read

Sometimes you’ll need to use more exact methods for sizing your breakers than the rules given by the NEC . . . . In a more complex electrical system, there are hierarchies of breakers, usually all of different characteristics. Breaker coordination is the process of designing your circuit protection elements (breakers in particular) so that they all trip in a specific order. The technical purpose of breaker coordination is to quickly and efficiently isolate a fault and minimize its impact on the rest of the system.

For example, consider appliances in your house. It’s easy to understand that if your refrigerator shorts out then you would want the breaker feeding it to trip before the breaker that supplies your entire house. It would be inconvenient if, every time an appliance shorted out, all power were lost to the entire house. It would also make diagnosing the problem much more difficult because you couldn’t easily narrow down which branch circuit the fault originated on.

The same is true in a power plant, but on a much larger and more complex scale. That’s why coordination curves should be drawn up (using software) for critical electrical loads.

Coordination plots consist of the time-current curves (TCCs) of the cables, breakers, and fuses that make-up a circuit. A coordination plot visually shows you two things:

  1. first, that a cable will remain protected during a fault; and
  2. second, the trip sequence of the circuit’s protective devices. Ideally, the breaker closest to the fault will trip first.

To read more about coordination plots and see some examples, click here.