PCB burns completely through, parts fall off.......fuse blackened, but intact!

[IanJ]

Hi all,

Not my video, but I thought it worth sharing................

Power supply PCB burns completely through, fire & smoke, parts fall off, solder drips off PCB, outside of case starts to melt.......the internal fuse is blackened.............but intact!



Ian.

[Psi]

Common misconception that a fuse should have blown if there was so much current flowing that the circuit caught on fire.
Fuses protect from massive over-currents, like short circuits, they don't know where all the energy is going.
Is 10W going into a 10W heatsinked IC or into one tiny spot on the PCB.
 
The typical current draw of a circuit is usually plenty large enough to start a fire if focused in one spot instead of going where it's supposed to go,

Example 10 ohm 1/4W resistor.
5V into 10R = 0.5A
5V * 0.5A = 2.5W 

2.5W into a 0.25W resistor and you're going to get fire, but 0.5A is not much current at all when considering fuse values. 
It's made worse when the thing that catches on fire is made of something oil filled or flammable.