A 50 kVA single-phase 7,200 V transformer sits on a lateral. Its full-load primary current is about 7 A. Someone proposes a 6K link and someone proposes a 15K link. Which protects the transformer without nuisance-blowing? Pick first.
The 6K link. The transformer draws 50,000 / 7,200, which is about 6.94 A. A 6K link is rated right at that full load, and because K and T links carry roughly 150% of their ampere rating continuously, a 6K rides about 9 A of load and inrush without nuisance-blowing while still clearing a real fault on the transformer or its secondary. A 15K link is oversized at more than double the full-load current. It would ride happily through a secondary fault and might never clear it, leaving the transformer to fail instead of the fuse. The job of the link is to protect the transformer without nuisance-blowing on inrush or cold-load pickup, and 6K does that with sensible margin while 15K gives up the protection.
A fuse cutout carries three separate ratings, and they answer three different questions. The voltage class and BIL (for example 15 kV class, 95 kV basic impulse level) say what system it can hang on. The continuous current (for example 100 A) says how much load current it can carry without nuisance operation. The interrupting rating (for example 10 kA) says the largest fault current it can break and quench safely.
Those are not interchangeable. A cutout is adequate to interrupt a fault only when its interrupting rating in kA exceeds the available fault current at that pole. A 10 kA cutout on a system with 6 kA of available fault current is fine; the same cutout where 12 kA is available is not, and it could rupture trying to clear.
K and T are a different axis entirely. K links are fast and T links are slow (time-delay). A 6K and a 6T both carry 6 A continuously, but they melt on different speed-ratio curves: the K clears quicker at a given overload, the T rides through longer before it goes. That is why you coordinate a lateral with all K or all T, not a mix. The letter is about timing; the number is about current; neither one is the interrupting rating.
So when you spec a cutout, read all three ratings, then pick the link size against full-load current with inrush margin, and pick K or T to match how the rest of the lateral coordinates.
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