A Ubiquiti USW-Pro-Max-24-PoE, an $800 rackmount switch with 24 2.5 Gbit RJ-45 ports, two 10 Gbit SFP+ ports, and 400 watts of PoE++ budget, came in with its entire PoE subsystem dead. The BusyBox console confirmed it: every port returned bad for PwrGood status, and a PoE splitter test left no doubt. The switch itself booted fine, touched screen and all.
A multimeter and thermal camera found a short on the PCB where the PoE power board connects. The TVS diode was intact. The culprits were two Trisil thyristor surge protection devices, marked as fuses, wired as crowbar circuits. They are designed to short to ground only during a surge event, absorbing the hit so upstream components survive. Both measured shorted. They had done their job and burned out doing it.
Replacing the two Trisils restored full PoE operation. The fix itself was not complex, but the diagnostic path is what makes the original repair video by The Parallel Port worth watching in full. Understanding crowbar circuit behavior is what separated a fast component swap from an expensive misdiagnosis or a needless board replacement.
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