Can some confirm this ?Desktop power supply

many PC power supplys are 'fused' (internally).

Switching mode power supplys can store a pretty healthy jolt.
1) Disconnect the output side (all the cables that go from the power supply TO your motherboard and drives, cards...etc............make sure power supply is unplugged from the IEC connector (AC plug) then wait about 10-20 minutes for charges to bleed off. It isn't a bad idea to discharge the capacitors so that when you are handling the board you don't get a 'discharge' ZAP.
Generally a very GOOD idea to avoid touching the traces of the printed circuit board at all (since it is hard to tell from the copper traces which one may be directly/or indirectly be connected to a charged up capacitor. Charged up capacitors like those in power supplys (and photo-flash capacitors charged upto 200-300Vdc) can arc/melt the tip of a screwdriver if given a direct path to ground. And you certainly don't want to accidentally pass a discharge of those to any of the devices attatched to the OUTPUT side of the power supply (i.e. your Mobo, harddrives etc...this is why it is a good idea to COMPLETELY disconnect it from everything OUTPUT side and the AC side).

We are all familiar with the typical BUSSMAN type glass fuses......PC power supplys rather use ceramic BUSSMAN fuses with pigtail leads soldered right into the PC board (it is rare to find a power supply with fuse holders).
CERAMIC fuses are approximately the same shapoe as the BUSSMAN glass fuses but are 'white' (ceramic).
They however can be tested the same way standard fuses are checked with a DVM or Multimeter.
Doing a continuity reading across the terminals will give you either an 'OPEN' reading (infinite ohms = blown fuse) or a finite reading of afew ohms or less (indicating the fuse is 'good').
They have voltage AND current ratings stamped on the metal end caps just like the glass fuses.
After determining if the fuse is 'good' or 'bad' ...after checking it with your DVM....if it is BAD ('open') you will need to remove the PC board from the power supply case carefully and unsolder and replace the blown fuse. You will need to replace it with a fuse of EXACTLY the same rating and type. There are both quick blow and slo-blow ceramic type fuses, so match what is in the power supply exactly if you need to replace it.

Switching mode power supplys are pretty tightly regulated. They offer over voltage and under voltage protection....
over current protection and automatic shutdown if voltages on the OUTPUT side power supply rails are out of tolerance (+3.3, +5, +12, -12). The +5 (or the +3.3 rail if your Mobo and CPU are the 3.3volt type) power most of the logic/CPU memory...while the 12V spin most of the motors in your drives (HD / CD/ DVD motors & printer ports, RS232C ports).
anything 'bad (shorting out or fried) on any of the power supply rails can drag a power supply down enough to put it into internal shutdown mode (OVER CURRENT / UNDER VOLTAGE fault condition)....and it senses and sees that in thousandths of a second....(couple of spins of the fan and off it goes).
SO disconnectings/removing things one device at a time is one way to diagnose a power supply shutdown situation.
Removing ALL drives is a good start (to seeing if the power supply will stay up)......device failures can cause the fuse to blow. A harddrive with 'frozen' bearing may cause the !2 supply regulator to go into high gear and crank out excess current (trying to keep the 12v rail at 12v) and help exceed the maximium amp rating of the fuse on the !20 input side.


on the INPUT SIDE (120V) there are components that can go bad and cause the fuse to blow.....Capacitors, MOV's, Rectifier (or bridge diode assembly).
...bad cords....dead wall outlets.....bad surge protector strips....tripped breakers have 'fooled' more than afew people into thinking their pwoer supply died.
fuses do sometimes weaken from heat and blow prematurely.....shock vibration can also damage them (break internally). External surges can Pop (or weaken them)....(i.e. partial indirect lightening strike).
Componenet failure (inside the power supply itself) on the voltage regulators can cause excessive current draw and Pop the fuse. (Number ONE cause of electronic device failure is "thermal runaway" from overheating.) (hint: adequate ventilation is VERY important - don't block cooling vents, make sure fans are working properly).
Sometimes people just DON't read......
if your power supply is rated at 250W max output......DON'T add that new video card that says NEEDS 350W power supply WITHOUT upgrading the power supply FIRST. It may work briefly,...but your power supply will be running at maximium capacity just to try to keep up with things (I see OVERHEATING and THERMAL RUNAWAY in your near future if you believe you can do this kinda stuff).