You are handed a TCC plot with two device curves and a vertical line drawn at 1,200 A. At 1,200 A, which device operates first, and roughly in how many seconds? Read it off before reading on.
The lower curve operates first. On this figure that is the recloser fast curve, and at 1,200 A it crosses the time axis at about 0.05 s, roughly a twentieth of a second. The curve sitting above it operates later at the same current. That single comparison is the whole skill: when two curves share a current, the one lower on the page trips first.
A time-current characteristic (TCC) plot has current in amps on the X axis and operate time in seconds on the Y axis. Both axes are log scales. Each device, a relay, a recloser, or a fuse, draws a curve showing how long it waits before it operates at any given current. Higher current is farther right; longer operate time is higher up.
The one reading rule that does the most work: a curve that sits lower on the page operates faster, because the Y axis is time and lower means less time to trip. This is why coordination is something you can see by eye. The device you want to clear first sits below the device that should hold back, with a clear time gap between them at the fault current they share.
To pull an operate time off the plot, go up the vertical line at your fault current until it crosses the device curve, then read straight left to the time axis. At 1,200 A, the fast curve crosses near 0.05 s and the slower curve crosses higher up at a longer time. The vertical line picks the current; the crossing point gives the time.
The axes are log-log for a plain reason. Fault currents run from a few hundred amps far down a feeder to tens of thousands of amps close to the bus, and operate times run from hundredths of a second to tens of seconds. Each range covers several orders of magnitude. A linear axis would crush the small values into a single line at the bottom and push the large ones off the page. Log scales give each decade the same width, so a 100 A reading and a 10,000 A reading are both legible on one sheet, and so is a 0.05 s trip next to a 10 s one.
A coordination study is only as good as the device data behind it. DistroForge Insider covers protection equipment sourcing and the OEMs actually shipping relays, reclosers, and fuses on current lead times.