The first installment of a three-part series about time current curves (TCCs) provides a quick overview of item identification and how to read TCC plots.
A time current curve (TCC) plots the interrupting time of an overcurrent device based on a given current level. These curves are provided by the manufacturers of electrical overcurrent interrupting devices, such as fuses and circuit breakers. These curves are part of the product acceptance testing required by Underwriters Laboratories (UL) and other rating agencies. The shape of the curves is dictated by both the physical construction of the device as well as the settings selected in the case of adjustable circuit breakers. The time current curves of a device are important for engineers to understand because they graphically show the response of the device to various levels overcurrent. The curves allow the power systems engineer to graphically represent the selective coordination of overcurrent devices in an electrical system. Modern power system design software packages, such as EasyPower, SKM Power Tools, and Etap, contain graphical libraries of curves to allow the power system engineer the ability to plot, analyze, and print the curves with minimal effort compared to the previous methods used when coordinating a power system.