That’s an impossible to answer question. But fuses don’t tend to blow for no reason. There is likely a measurable short somewhere on the board, likely a failed cap, but just as possible for it to be any other active component on the PSU.

on the photo you posted, the black rectangle at the bottom of the photo.
a Full Bridge Rectumfrier (rectifier)
It's the first component after the fuse.
If that is ok, then likely one of the two components that are mounted to the bottom heatsink on the photo.
Depending on how old that psu is. It might need a replacement of all electrolytic capacitors, and I'd personally try to verify the big red sorta rectangular capacitor located between the bottom heatsink and big transformer.

In my experience, the sequence of events is: One of the primary transistors overheats and fails short. This creates a dead short over the intermediate 300V DC rail when the second primary transistor turns on, so it also overheats and fails short. This creates a permanent short over the 300V DC rail which will kill the rectifier (again by overheating it). Usually you find two out of four diodes short in the rectifier, the diodes that conduct the half cycle that was active while the failure happened. Starting with the next half cycle, the other pair of diodes conducts, but the shorted diodes don't do their blocking job, so they short out the rectified mains. That's about the moment where the fuse starts to blow, possibly the main circuit breaker before the glass-tube fuse in the power supply.

In this scenario, the replacement fuse will immediate blow again when you turn on the supply, as semiconductors tend to fail short, not open. If your supply is dead now without blowing the fuse again, I guess that the big white box next to the fuse, a ceramic resistor, is broken. These resistors are designed to get quite hot, and could start a fire. That's why there are safety resistors that have an internal overtemperature trip and are guaranteed to fail open before they get hot enough to start a fire. This is the most likely component to fail open.

Unfortunately the nature of failures in case of that kind of PSU is usually catastrophic.
This fuse is meant to prevent fire not to protect the circuit.
You should start checking from the low voltage side (transistors, caps and diodes for shorts, resistors and coils for break) and continue to high voltage side.
Greatz's bridge check should be the last thing to do 😉 Even of it's really broken then quick fixing this is the beginning of the troubles. Often explosive troubles xD

disaster wrote on 2023-08-13, 13:26:

Unfortunately the nature of failures in case of that kind of PSU is usually catastrophic. This fuse is meant to prevent fire not […]
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Unfortunately the nature of failures in case of that kind of PSU is usually catastrophic.
This fuse is meant to prevent fire not to protect the circuit.
You should start checking from the low voltage side (transistors, caps and diodes for shorts, resistors and coils for break) and continue to high voltage side.
Greatz's bridge check should be the last thing to do 😉 Even of it's really broken then quick fixing this is the beginning of the troubles. Often explosive troubles xD

Shorts on the low voltage side usually don't make it to the primary side. They cause the supply to emit squeeky noise, they prevent power-up, but having a secondary-side short blow the fuse is extremely rare.

Nevertheless you are completely correct that the fuse is blowing because something else blew before. Without fixing that something else, a replacement fuse is usually going to blow within a split second. Unless some chinese "cost optimizers" chose to use a bridge rectifier with a maximum reverse voltage of 200V for use in a 230V supply (peak reverse voltage around 320V), the Graetz Bridge will usually only fail if something else failed before. So don't blindly change just the bridge, but take a broken bridge as hint that there is a dead short on the primary side that needs to be fixed first.

Agree with mkarcher ! usually the fuse blows because primary side has the short though it could be on the secondary but more rare. Check the rectifier bridge, primary caps and the primary mosfets first !!

mkarcher wrote on 2023-08-13, 22:12:

Shorts on the low voltage side usually don't make it to the primary side. They cause the supply to emit squeeky noise, they prevent power-up, but having a secondary-side short blow the fuse is extremely rare.

You are correct but the catastrophic nature of PSU failures mean that it's also cascade failure. In worst case scenario there are destroyed elements all across the board.
That's why I suggested starting from the LV side. Better be safe than sorry 😀