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KeyStomach0 OP t1_jaeqkj9 wrote

Reply to comment by dkf295 in ELI5: why are male to male USB-C cables not dangerous like male to male wall plugs are? by KeyStomach0

Got it, thanks. Never really thought that it would be as simple as shielding the contacts like that.

If we can shield contacts in USB-C, why can't do the same with wall plugs. Is there any practical reason why the contact wires need to be exposed on wall plugs?

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mmmmmmBacon12345 t1_jaer9rk wrote

The contact wires aren't exposed on the wall plug, they're protected inside the outlet

The plug that you're putting in isn't supposed to be energized until its mostly inserted and difficult to touch so the fact that its exposed metal normally is fine. Basically the reason male-male AC plugs are bad is because the violate the rule of keeping energized surfaces out of reach

Small interior pins that lay on pads struggle to carry any significant amount of current without overheating. For anything that you need to pump >8A through you really want a very firm connection which is generally a pin or equivalent with springy metal bits pushing on it from each side to ensure good contact

We do this with wall plugs, the prong slides between two bits of springy metal that ensure contact. High power test equipment uses similar pins that slide into spring contacts.

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KeyStomach0 OP t1_jaeslx3 wrote

So it's like that to reduce heating, thanks!

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dkf295 t1_jaerbnr wrote

When delivering more current a given piece of metal heats up more. This is why you see warnings about not using flimsy extension cords for high current appliances like space heaters. A USB-C connector only needs to deliver up to 240 watts. A standard 120/15a outlet will go up to 1800 watts - so to keep the same reliable and safe temperature on your connector, those power-delivering terminals would need to be 7.5 times larger.

But yes, you could make a new connector that would be harder to make contact with accidentally. But then you’d need to convince people to swap out potentially billions of power outlets

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Pocok5 t1_jaeuygn wrote

> why can't do the same with wall plugs.

There is absolutely no scenario when a properly used wall plug is exposed when energized. Is is only possible when utter morons use "suicide cords":

  • for extensible christmas lights they installed backwards so the socket end is near their wall socket. In that case, the user needs to suck it up and take it down then put it up the right way. It's an educational experience.

  • for plugging in generators to a wall socket when the power goes out. This is illegal as fuck and dangerous to linemen working on the pole outside. An excellent way to bumble your way into manslaughter. When generators are used for a home, a generator inlet socket and transfer switch is used The house side is male with exposed pins, but due to the transfer switch it is physically impossible to have it connected to the house wiring while the house is fed from the mains, so it is safe.

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