A fuse TCC shows a BAND made of two curves, not one line. Which curve would you use to be SURE the fuse has fully cleared a fault, and which one just marks where it starts to melt? Decide first.
The total-clearing curve is the one that guarantees the fault is fully interrupted. The minimum-melt curve only marks where the element just starts to melt. Every fuse link has two curves because melting starts before the arc is fully out. The metal element reaches its melting point at one instant, but the arc that follows takes more time to extinguish, so there is a band of time between starting to melt and fully interrupting the fault. The lower curve is minimum-melt (MM), the start of melting. The upper curve is total-clearing (TC), the moment the arc is gone and the current is zero.
Total-clearing always sits above minimum-melt (longer time) at any given current, because clearing happens after melting begins. The vertical gap between the two is the arcing time the link needs to actually break the circuit. Reading a fuse curve, you are never reading one line; you are reading the band those two curves bound.
This pairing drives coordination. You coordinate an upstream device against the downstream fuse total-clearing curve, because the upstream device must wait until the fuse has completely finished. If the upstream recloser or relay acted at minimum-melt, it would trip while the fuse was still arcing, and both devices would operate for one fault. Going the other way, you coordinate the downstream fuse minimum-melt against the upstream slowest operate, checked at the maximum fault current through both. Compare the wrong pair and the study is wrong even though the math looks clean.
The K versus T distinction from the equipment track rides on top of all this. K links are fast, with a speed ratio of about 6 to 8; T links are slow, with a speed ratio of about 10 to 13. Both letters still have their own MM and TC curves; the letter just sets where that whole band sits in time for a given multiple of the rating. A K link melts and clears sooner at a given overload; a T link rides through longer before it goes.
The practical trap is mixing them. If a lateral was coordinated with K links and someone replaces one with a same-ampere T link, the band shifts and the careful margin you built between the fuse and the device upstream is gone. Same number, different speed, broken coordination. Read both curves, pick the right pair for each comparison, and keep the K-or-T letter consistent down the lateral.
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.