I'm trying to track down a short circuit in my PSU. but the protection circuits are preventing me from doing so (clicking fast when switch on, regardless of PS_ON state, and stays off even when PS_ON is grounded). How do I bypass the protection circuits so I can track down a short?

 

To track down a short turn it off and use a continuity measurement or resistance measurement from a DMM. No point risking damaging something further. The fact it is shutting down is preventing damage. If you determine what the shutdown is monitoring you should be able to determine whats going wrong to trigger it.

 

To track down a short turn it off and use a continuity measurement or resistance measurement from a DMM. No point risking damaging something further. The fact it is shutting down is preventing damage. If you determine what the shutdown is monitoring you should be able to determine whats going wrong to trigger it.

Could a potentially charged cap damage the DMM?

 

Could a potentially charged cap damage the DMM?

In the wrong setting yes. Check volts first and ensure things are safe, then proceed in beeping it out safely.

 

First, look for the short with a magnifying glass. That can't possibly do any damage!
Put a resistor (a filament lamp is often useful) in series with the supply to the board with the short, to limit the current to a few tens of milliamps.
Then measure the supply voltage on the board.
If it is around half a volt to 1 Volt, you have a logic IC in back-to-front.
If it is a genuine short, then keep measuring the supply with the meter on the millivolt range at as many points on the board as you can. Where you get the lowest measurement, you are closest to the short.
Trying to measure the resistance on the board using the ohms range will usually achieve no more than measuring the resistance of your meter probes.
If your board is at logic levels (3.3V or 5V) and your meter is a newer auto-ranging type, then you are very unlikely to damage it with a charged capacitor.

 

The damage that can be done by disabling the protective part of a supply can be quite impressive. A whole lot of things can be destroyed in milliseconds.
You give us no hint a to what sort of power supply for what sort of application, and that makes any detailed advice a guess, at best.
Many computer power supplies would do an almost instant shut down if there was not a sufficient load current on the 5 volt output. So if it is a switching type computer power supply that may be the issue.
And if you do succeed in bypassing the shutdown without a load on one of those supplies the damage will be very destructive indeed.