How exactly does a PSU's short circuit protection work?

[itsTyrion]

So I connected a fan to the floppy power connector (originally an old PSU's fan)
  

 

One of the red wire's inner... wires? stood out and touched GND. My be Quiet! PurePower 11 400W instantly reacted by cutting the power, the wire even survived.

 

My question: How can the short-circuit protection be THAT sensitive? And how does it detect the short? Current? I doubt that 0.0x mm wire can transport more than a few mA.

[Mling]

It will be the current that triggers it. If I remember high school physics correctly a short will cause the PSU output voltage to drop to almost zero and the current to skyrocket. When I was in high school I had a big old transformer that could melt thin copper wires. I think you're wondering how a thin wire can carry a lot of current. Well it can for a short time. Until it gets so hot it melts.

[mariushm]

It can. The whole length of wire between the power supply and the connector acts like a heatsink for the pins in the connector, and as a resistor. 

 

The wires could handle 10+ Amps of current for a few seconds, as they'll heat up from regular ambient 20-25c all the way to 80-100c where the insulation on the wires could start to melt.

 

The beQuiet site says that model has 2 rails ... one with 24A current limit, the other with 20A... I suppose the wires could handle 20A of current.

[itsTyrion]

6 minutes ago, mariushm said:

It can. The whole length of wire between the power supply and the connector acts like a heatsink for the pins in the connector, and as a resistor. 

 

The wires could handle 10+ Amps of current for a few seconds, as they'll heat up from regular ambient 20-25c all the way to 80-100c where the insulation on the wires could start to melt.

 

The beQuiet site says that model has 2 rails ... one with 24A current limit, the other with 20A... I suppose the wires could handle 20A of current.

Not the entire wire, just one of the hundreds of super thin internal copper.. wires (is there a better term for them? English is not my native language).
It even survived.

Edit:
So that one tiny thing had a 20A short and the protection just kicked within the time of thermal interia from room temp to the melting point of copper?

[Juular]

SCP works by monitoring current and resistance to the ground, if the current is high but resistance is low - that's a short, trip it. Thus, if the short is not to the ground or it's a high-resistance short, SCP wouldn't necessarily work, that's more of OCP thing. @jonnyGURUcan provide some details maybe.

[itsTyrion]

59 minutes ago, Juular said:

SCP works by monitoring current and resistance to the ground, if the current is high but resistance is low - that's a short, trip it. Thus, if the short is not to the ground or it's a high-resistance short, SCP wouldn't necessarily work, that's more of OCP thing. @jonnyGURUcan provide some details maybe.

So a mix of resistance and current? Makes sense, I guess I could've thought of that. But didn't

[Guest]

3 hours ago, Juular said:

SCP works by monitoring current and resistance to the ground, if the current is high but resistance is low - that's a short, trip it. Thus, if the short is not to the ground or it's a high-resistance short, SCP wouldn't necessarily work, that's more of OCP thing. @jonnyGURUcan provide some details maybe.

Nope.  You nailed it. 

 

3 hours ago, Mling said:

It will be the current that triggers it. If I remember high school physics correctly a short will cause the PSU output voltage to drop to almost zero and the current to skyrocket. When I was in high school I had a big old transformer that could melt thin copper wires. I think you're wondering how a thin wire can carry a lot of current. Well it can for a short time. Until it gets so hot it melts.

What you're defining is OCP, which works when SCP cannot.

 

[Glenwing]

 

21 hours ago, itsTyrion said:

Not the entire wire, just one of the hundreds of super thin internal copper.. wires (is there a better term for them? English is not my native language).
It even survived.

Strands

21 hours ago, itsTyrion said:

So that one tiny thing had a 20A short and the protection just kicked within the time of thermal interia from room temp to the melting point of copper?

Well let's say it's 18 AWG wire made up of 100 strands. A little 1 cm section of that strand, 1/100th the area of 18 AWG wire, still has a resistance of only around 0.02 Ω, and will conduct around 600 A of current if you apply 12 V across it, assuming you have some kind of superman power supply; real power supplies wouldn't be able to put out 600 A so they would drop voltage instead. Well actually, their protections would trip to prevent exactly that from happening. And the power dissipation would be... I²R = (600 A)² (0.02 Ω) ≈ 7 kW, so yes it would definitely melt if the protections didn't kick in fast enough. Anyway, don't think that tiny wires can't carry massive currents. They can, hundreds or thousands of amps will flow through tiny hairline wires, but it will just make them rise to impossible temperatures and destroy themselves, which means it can't be used in practice. But short circuits don't care about being useful.

[itsTyrion]

Thanks everyone, topic can be closed. Now I learned not only that the SCP in my power supply works, but how it does as well