Power rail faults are found with Strip Down testing.
First desktops then laptops, never fake Desktops.
You are here for 1 reason, and that is you suspect the power rails (all feeds of power) are overloaded with shorted modules of any kind in the PC.
The PC is dead, or turns on for 1 second and then turns it self off and all fans go dead.
The first thing to do is remove all USB devices, and then unplug the HDD (all of them) an the optical drive, (CD/DVD/BR- d) That is correct all these can short out.
USB is not really much consern here, but some lame old cheap PCs lacked proper USB power feed current limits back in the year 2000. (modern USB has electronic fuses that make USB safe)
Most Desktops are super easy to Diagnose in this ordered list: I will do Desktops then Laptops last, for most tablets have very little anyone can do, but replace the old dead large shorted battery.
Keep in mind there are 4 stuctures that can be shorted, and make a PC dead, power, data, address and controls. (nut just power rails)
Special case #1:
Some PCs with dirt cheap and very old PSU, can stay turned on, but PC is dead , like a dead 3.3vdc rail dead. (and has no under voltage protection on that 1 pin)
DC RAIL TESTING IS EASY on Desktop: (if you own a $20 DMM meter)
A new PSU rated at 500watts or more and v2.3 version up.. $39 at newegg.com "Thermaltake (tm)"
If still dead, the mobo is history.(I've even removed the CPU last step, but I have never seen any CPU short power yet, but can have bent pins due to bad service) Or Lightenting hits sure.
Keep in mind the fans will be dead if BIOS is dead or CPU missing. This is worst case, a full strip down and build up from a minimum configuration.
Warning ( not too mobo made have the same pin out of the below connections, (unless they are axactly the same) there is no ATX spec on this pins, like most the others have.
Before working any mobo and the power switch cable is missing or too far away learn which pin pair is power on. and jump it momentarily.
end desktops (real)
LAPTOP (LT) power structures are complex. (the actual PSU is hidden deep deep inside all LT)
Here is a page I made on Laptop power isolation testing.
The laptop can have many modules to pull first to clear any short. (even useless now, dialup modems tiny card can short)
This is all about power rail (or data/address/control ) structures, shorted, by doing fault isolation testing.
In man cases(no pun) the cases must be totally disassembled to do the work.
The fake Desktops can be diagnosed just like laptops. (skip the huge battery topic)
version 1. 12-12-2018