Three fault regions of a time-current plot

In a previous article I discussed the importance of coordination curves. Here I provide some home-made examples. Yep, you got it. Fashioned in the fires of Microsoft Powerpoint . . . .

The time-current curves of protective devices show how long it takes each device to interrupt the circuit at a given current flow. The larger the current, the quicker the protective device operates. A device’s time-current characteristics can be broken up into three different regions:

  • Instantaneous – the initial period of an electrical fault, from zero up to several milliseconds. This portion of the device response is adjustable on more complex breakers.
  • Short time – the period of time in the fault ranging from tenths of a second to several seconds. It’s the middle-region between the instantaneous region and the long-time region. Usually low-voltage molded-case circuit breakers don’t have the means to adjust this response band. Larger-current feeder breakers usually omit an instantaneous trip function and allow adjustments to be made to the short-time and long-time characteristics to allow for better coordination with downstream breakers.
  • Long time – the period of time during a fault ranging from minutes to hours. The long-time response of a protective device defends against unintentional overloads that are greater than the design loads but are extended in duration because they escape detection by the short-time or instantaneous-trip mechanisms.

It’s much easier to understand how coordination works by looking at a well-illustrated plot. First, the three fault regions are shown here:

time-current curve fault regions

Example plots follow. The first shows a cable that is protected by a breaker and one that is not.

time-current curve protected cable

The second plot shows a cable whose protection is uncertain because its damage curve overlaps the breaker’s uncertainty band.

time-current curve uncertain protection