Picture a TCC with a substation relay curve sitting only 0.1 s above a lateral fuse at the maximum fault current. Is this coordinated? Judge it before the rule is stated.
No. A 0.1 s gap is too tight. You want about a 0.3 s coordination time interval, the CTI, between the two curves at the worst point. At only 0.1 s of separation, the relay and the fuse can trip together or in the wrong order, and the substation breaker may drop the whole feeder for a fault a single lateral fuse should have cleared on its own.
Coordination means stacking every device’s curve on one plot and proving they operate in the right order across the full range of fault current. Relay, recloser, and lateral fuse all land on the same log-log time-current plot. Lower on the plot is faster. The device closest to the fault must be the fastest, so it clears the fault before anything upstream of it moves. That keeps the outage as small as the fault itself.
You do not check coordination at every current. You check it at the maximum through-fault current, the worst case. That is where the curves draw closest together, so it is the hardest place to hold the margin. If the gap is wide enough there, it is wide enough everywhere lower on the plot.
The margin you are proving is the CTI. The downstream device’s curve must sit below, faster than, the upstream device’s by the CTI so the downstream device always trips first. The classic relay-to-fuse CTI is about 0.3 s on an electromechanical relay, about 0.2 s on a digital relay. That number is not arbitrary: it covers the breaker or fuse clearing time, relay overtravel (the relay disk keeps coasting for a moment after current drops), and a safety cushion on top.
If the gap between two curves shrinks below the CTI at high fault current, the two devices can trip together or in the wrong order, and that is a miscoordination. That is the trap in the opening scenario: a relay only 0.1 s above the fuse, well under the 0.3 s it needs, risks tripping the substation breaker alongside the fuse on a hard lateral fault. The figure above shows the fix. Relay 51, the recloser, and the fuse total-clearing curve are stacked with a full 0.3 s CTI held at the 3,000 A maximum fault, so the downstream device clears first and the rest of the feeder stays lit.
This is the whole job of a coordination study: lay the curves on one plot, find the maximum through-fault current, and confirm every downstream-to-upstream pair holds at least its CTI there.
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.