Reading a time-current curve

A time-current curve answers one question: for a given current, how long does this device take to operate? Everything else about a coordination study is built on top of that.

Why both axes are logarithmic

Fault current spans several orders of magnitude, from a few amperes of overload to tens of kiloamperes of bolted fault. Operating time spans just as wide a range, from hundredths of a second to minutes. A linear axis would compress almost every interesting region into the corner of the page.

On a log-log plot the inverse-time relationship of most protective devices becomes close to a straight line, which is what makes two curves easy to compare by eye.

Coordination, read off the plot

Two devices in series coordinate when the downstream curve sits below and to the left of the upstream one across the whole range of fault current they will both see. Below means faster; to the left means it starts operating at a lower current.

Where the curves cross, or come closer than the margin your practice requires, the upstream device may operate first. That is a miscoordination, and it costs you more of the plant than the fault should have.

Curves that merely do not touch are not the same as curves that coordinate. The margin has to cover relay tolerance, breaker clearing time and CT error, not just the plotted lines.

What the plot does not show

A TCC plot is a steady-state view. It says nothing about inrush, about whether the device is rated to interrupt the fault current available at its terminals, or about arc-flash energy. Those are separate checks, and passing the plot does not substitute for any of them.