WHY is Vbus HOT on a USB-C receptacle/cable so dangerous? (or, WHY are 'degenerate A-A' cables bad?)
tl;dr: PELIGRO! You can fry your USB ports, charger, motherboard or device.

Benson has discussed this but never quite explained how it causes physical damage on #USB #TypeC . (It's a topic people who are familiar with using jumper cables would understand.) It is one of his least-commented FAQ entries despite being one of the most important.

FAST PUBLISH: Because of alarming feedback from users during the 'USB-C Charger Shootout' I am rushing this article. Please forgive errors, omissions, or lack of flow. (Corrections, comments, or clarifications are welcomed.)

This article has been split into two for clarity. See the companion article expaining HOW TO CHECK here:
https://plus.google.com/102612254593917101378/posts/dgLwqvACbJm

Vbus HOT on USB-C Receptacle and Degenerate A-A cords - Destroyer of Motherboards

tl;dr: "I was told there would be no math." Just look at the table image.
https://en.wikipedia.org/wiki/USB#Power-use_topology
http://electronics.stackexchange.com/questions/23943/what-would-happen-if-i-connect-two-different-dc-voltage-sources-in-parallel

The issue arises because the "5v" the charger generates isn't quite exactly the same as the "5v" of anything else. When connected, the two try to push voltage backwards across each other/short cicuit. When you do that to a a regulated power supply (like a computer), it tends to "let out the magic smoke".
https://en.wikipedia.org/wiki/Magic_smoke

This is where the math begins. Second image I've included a diagram of something very important to this discussion: a Thevenin Equivalent Circuit. (If your eyes are rolling back in your head, bear with me. I'll explain in a moment.)
https://www.ibiblio.org/kuphaldt/electricCircuits/DC/DC_10.html#xtocid230279
https://www.ibiblio.org/kuphaldt/electricCircuits/DC/DC_10.html#xtocid230272

Basically, it is an approximation of how a charger reacts. The more you load it, the lower the voltage goes. (In reality it doesn't go down in a straight line -- it's more of a knee-shape -- but close enough.) Note how this TEC can roughly simulate USB ports on a motherboard: usually you have one 5v source for a bank of 4 USB ports that share a single controller chip. (In reality each port's power is independently controlled, but from my tests some powered hubs do not.)

In the third image, I've shown what happens electrically when (a) you connect an Vbus HOT device/cable to a computer and (b) what happens with a *compliant* USB-C device/cable. (This can be a C-C cable, an A-to-C, or a captive-cable-charger.)

https://plus.google.com/+BensonLeung/posts/hF6b228zGvy

Normally, as Benson explained, compliant USB-C devices can tell this is happening and simply don't connect the voltage lines. (In the case of captive cable charger, you are relying on the computer's USB-C port. Note: this means a computer's USB-C ports can be unsafe/Vbus HOT too! Dun dun dun....)

-- At the top of the third image I used TEC's to approximate the current that would go through your motherboard if connected. ("Mesh current".)

-- At the bottom of the third image I show what happens when everything is USB-C compliant. *Absolutely nothing!* Great! That's the way things are supposed to work.

I did an example mesh current analysis on the table in the first page and you can see how messy it got. I omit the 3-loop version here with ground offsets for sanity. (Plus there are more unknowns.)

The key takeaway:

Any ONE Vbus HOT device in the chain can opens up a giant can of worms that can potentially cause damage. This is why unlike voltage sag, Benson gives 'A-A degenerate cables' the lowest possible rating. (Short of that one cable that blew up his Chromebook Pixel... I'm pretty sure he had that one stuffed and mounted.)
http://www.amazon.com/review/R2H1CUPXWPC972

Personally, I propose Vbus HOT cables/adapters should be entered in the same category as A-A cables for the exact same reasons.

Likewise, if using USB-C compliant devices, there is (almost!) nothing you can do that will intentionally cause a dangerous failure. (Except maybe plugging in USB-C cables with your hands wet, according to Apple.)
https://goo.gl/kA6vkD

Any manufacturer eschewing these precautions the name of "compatibility" should be met with the greatest scepticism. Safety experts may argue these adapters should not exist in nature at all... I say let them exist, but store them alongside the sharp pointy objects, toxic chemicals, and ETHERkiller cords.
http://www.fiftythree.org/etherkiller/
http://hackaday.com/2015/10/10/the-usb-killer-version-2-0/

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