When a power supply experiences an overvoltage condition, a protective circuit rapidly places a low-resistance path across the output terminals. This action immediately drops the voltage to near zero and blows the upstream fuse, saving sensitive downstream components from destruction. A circuit in this state requires a power cycle or fuse replacement to function again.
Heard on the job
The 5V rail crowbarred as soon as I powered up the rack, so we definitely have a short in the new card.
Insider tip
If a supply keeps crowbarring on startup, check for faulty bypass capacitors on the main rail before blaming the power supply itself.