You are handed a finished coordination study and a fault report: a lateral fuse blew when the recloser should have fast-tripped first. On the composite TCC, where is the miscoordination? Spot it before reading on.
Look where the lateral fuse minimum-melt curve crosses the recloser fast curve. Below that current the fuse sits above the fast curve, so the recloser fast-trips first and the fuse survives, exactly what fuse-saving wants. But the fuse minimum-melt dips below the recloser fast curve above 2,400 A, and on a TCC lower means faster. Above 2,400 A the fuse melts before the recloser can act, so fuse-saving fails and the fuse blows. That crossover is the maximum-coordinating-current, the upper limit of fuse-saving, and a real study marks it.
A coordination study stacks every protective device on one composite log-log plot. It carries the relay 51 and ground 51N, the recloser fast and slow curves, and the lateral fuse minimum-melt and total-clearing curves, all on the same current axis. The study proves these devices trip in the right order with adequate margin across the full range of fault current, from a weak far-end fault to the maximum bolted fault at the bus.
The phase elements are only part of the picture. The 51N (or 51G) element detects ground, or residual, overcurrent: the sum of the three phase currents, which is near zero under balanced load. A single-line-to-ground fault is the most common overhead fault, but it can draw less current than a phase fault, so 51N is set more sensitively than the phase 51 to catch it. The composite plot must coordinate the ground elements too, not just the phase elements, or a low-current ground fault clears in the wrong order.
A few more ANSI elements round out a real study. Device 27 is undervoltage, 59 is overvoltage, 79 is reclosing, and 87T is transformer differential, the primary protection on the substation transformer that trips on the current difference across it. You will see these device numbers on the one-line and in the relay settings, even though only the overcurrent curves appear on the TCC.
Reading a study critically means checking the margins, not trusting the legend. Walk every adjacent pair of curves and confirm the downstream device stays below the upstream one with a clear time interval at the maximum fault current through both. Confirm the fuse total-clearing sits below the recloser slow curve, and the fuse minimum-melt sits above the recloser fast curve, all the way out to the maximum-coordinating-current. Where a curve crosses, coordination is lost, and the study should call it out.
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.