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The PSU has a a glass-housed fuse rated at 4 Amperes, which will, of course, blow if too much current is drawn. Under normal operation, the +5 Volt section draws a little over 3 Amperes, so, it's a good fuse value for protection. However, one of the common ways in which a 780x voltage regulator fails is to deliver an elevated output (voltage). In some scenarios, the current could still be under 4 Amps, even if the regulator output swings high. So, in order to protect the logic chips, in the card cage and in the control panel (e.g., the 7-segment LED drivers for the digital LPI readout), a good thing to add is a crowbar protection circuit. This circuit constantly measures the voltage and, as soon as it goes above the safe operating level for the IC components, determined by a (reversed) Zener diode, it turns on a silicon-controlled rectifier (SCR) that suddenly offers the output of the fuse a path of very low resistance to Earth, parallel to the logic circuit, causing the 4 Amp fuse to blow, since the 1 Amp 7805 regulator is being boosted with a parallel-connected, 7 Amp, PNP transistor (2N 5871).
For now, I've selected a Zener diode with a reverse breakdown voltage of 5.6, so, it will have a good chance of protecting the chips in case the 7805 swings its output high, but there is a little buffer between its turn-on point and the highest sustained voltage a civilian chips's Vcc pin can handle to prevent the unnecessary blowing of fuses in the event of any mild, and brief, surges at power-on.
The schematic, below, was made by slightly modifying another one, which was found online, that was drawn, originally, by someone named, Ravi.
Here's the surge tolerance of the selected Z5.6 diode.