Coordination curves

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.