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dkf295 t1_jaeotpp wrote

A few things.

  1. The default voltage for USB-C is a measly 5V. Think half (okay 5/9) that of a 9V battery if you've ever licked one. It can go as high as 20V but the receiving device specifically needs to request that - even then, not necessarily fun to be shocked with but not particularly dangerous.

  2. The USB-C connector delivers power through pins on the INSIDE of the connector, making it extremely difficult to make contact

  3. A USB connector serves DC power, your wall outlet is AC.

So in order to implement it on wall plugs you'd need all wall outlets and devices (from power strips to computers to stoves to vibrators) to be swapped out with new versions that negotiate the proper voltage, convert to DC. And new connectors - which is harder to make in a non-exposed package if you need to support up to 120V.

Edit: mixed up AC/DC


wackshot55 t1_jaepd09 wrote

Spot on, except you mixed up the AC/DC. AC from the wall, DC through the USB


dkf295 t1_jaepmca wrote

D’oh! I never was into hair metal after all.


Graega t1_jaeq0el wrote

My take away here is someone licking batteries.


KeyStomach0 OP t1_jaeqkj9 wrote

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?


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.


KeyStomach0 OP t1_jaeslx3 wrote

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


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


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.


GalFisk t1_jaf1o0t wrote

EV charger connectors are a bit like that. They don't energize until signaling pins have been connected, and the car has told the charger what it needs (or vice versa). Those pins connect last and break connection first, so the big contacts are always without power when they disengage, preventing arcing.


TehWildMan_ t1_jaeobo2 wrote

The outside shell of the connector is electrically neutral, the only points in the connector with a different voltage are inside the connector, and no properly designed device should ever supply power along a USB+c connector without verifying it can receive power

Wall plugs have two different exposed conductors an inch or so apart with a over a hundred volts AC between them


Jason_Peterson t1_jaeoca2 wrote

In a USB plug the pins are tucked inside surrounded by ground, so they are not likely to get shorted by the plug falling onto a metal object. It would definitely damage the computer if they were. USB is low voltage, so it would never be dangerous to a person.


KeyStomach0 OP t1_jaeq5y0 wrote

Thanks, So there's no feature verifying the wire is plugged or not like I suspected, the metal part is just neutral and the real hot contact is protected by the metal.


wackshot55 t1_jaep8il wrote

Because it’s low voltage DC. Like ~5-24V DC. That “block” that inserts into the receptacle and the usb plugs into is actually a mini transformer. Which converts the 120V AC into whatever DC voltage it states on the side of the “block”


XsNR t1_jaesnua wrote

USB as a whole is both female and male. They have interconnected shapes that make use of both types, with the power being supplied from the internal connection. Most AC connectors utilise the same principal where the pins are shielded from prying fingers, the direct wall plugs being the exception, but they're also fairly poorly designed as a whole, and from a time before our current understanding of proper power safety mechanics. Some are better than others, the US' standard is quite poor by comparison to the other standard's.

If you've ever had a laptop or games console with a barrel plug (looks similar to USB-C, but circular), they are the reverse of USB-C, with the live outside, and ground inside. They're still not terribly dangerous, as they also don't carry a very high voltage. Although you can actually feel the power flowing through those, where as USB standards, if you touch the metal are such low voltage that its almost negligible. Both work on the same principal though, that DC is not terribly dangerous in low voltages, where as AC is consistently quite dangerous.

The lightning cable (Apple's phone standard) is effectively the reverse of USB-C, with the male cable plugging into the female device. Although these are live on the outside of the connector, Apple doesn't utilise quick charge technology on it's lightning connector, so the power delivery is lower than most of the previously mentioned barrel plugs.


frustrated_staff t1_jaet516 wrote

Because male-male USB plugs still have their power-providing contacts out of reach of your grubby little fingers AND only deliver 5V DC.

A male-male wall plug has the power-providing contacts exposed (while "hot"), delivers 120V A/C, and it would require redesigning nearly every single appliance that uses 120V A/C in order to make it worthwhile. Not to mention that 1) they're explicitly illegal in the U.S. and 2) they violate nearly all existing electrical codes.

Are you trying to electrocute yourself?