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Some Googling revealed that a faulty USB device may fry the entire motherboard+processor of a computer if it is not fitted with appropriate fuses (if it is known though, I wonder why they wouldn't!), however I wonder if other devices on the same hub could be fried as well?

Edit after Olin's answer: I don't mind killing the hub, but I do mind that the PC or the USB devices are at risk. Does that mean that it is risky to have USB devices which are powered from mains because they could theoretically fail and apply mains to the hub, take it out, and in turn apply mains to the other devices' USB receivers? But isn't that theoretically possible as well that a 2.1 sound system fail and apply mains to a 3.5mm jack as well?

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  • \$\begingroup\$ You are wise to seek protection for certain USB devices. I blew up my computer, keyboard, and mouse when 24V found its way onto the 5V rail. \$\endgroup\$ – CurtisHx Mar 25 '15 at 16:33
  • \$\begingroup\$ How did that happen? I'm very interested to know what sort of failure led to this. \$\endgroup\$ – user42875 Mar 25 '15 at 17:26
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    \$\begingroup\$ I was using a USB to I2C adapter, which provided a 5V pin off of the USB 5V rail. It was essentially a direct connection onto the USB rail on the motherboard. The device I was working with operated on 24V. There was a loose 24V supply wire, and it made contact with the USB to I2C module's 5V pin. I heard a pop and looked over at my computer. The monitors were black and there was smoke pouring out of my keyboard. I replaced the mobo, keyboard, and mouse, and everything came back. I got lucky it didn't take out more. \$\endgroup\$ – CurtisHx Mar 25 '15 at 17:38
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    \$\begingroup\$ Somewhat related thread: What measures should I take to protect the USB ports of my PC during development of a USB device? \$\endgroup\$ – Nick Alexeev Mar 25 '15 at 17:44
  • \$\begingroup\$ Nice anecdote CurtisHx. Thanks for the link Nick Alexeev, I commented there, not sure if it was a wise move though :/ \$\endgroup\$ – user42875 Mar 25 '15 at 18:21
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It is possible, particularly if the device is self-powered. Attempting to draw too much current or even a dead short will not hurt a proper USB host port. However, accidentally applying high voltage to either of the data lines could be bad. At least that could blow out the drivers for that port, but it's no stretch to imagine it could take out the whole hub chip.

This is not a place where fuses are reasonable and would do anything useful anyway. By the time a normal thermal fuse trips, the hub chip will be long dead. Putting anything else in series with these lines is a problem since they have to carry high speed signals. The hub chip will have protection diodes or similar to power and ground. That will protect against the occasional static discharge. Protecting against anything more than that would be expensive and not worth the extra money for the very rare case it saves something. It makes no sense to waste even a few pennies protecting a $50 motherboard when this protection will only matter once in every 10,000 cases.

You can't apply line power to the audio jack, the video output, and most other external connection points of a PC. I don't see why USB should be singled out for extra and burdensome protection.

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  • \$\begingroup\$ Thanks for the clear answer. I don't mind killing the hub, but I do mind that the PC or the USB devices are at risk. Do you mean that it's risky to have USB devices which are powered from mains because they could theoretically fail and apply mains to the hub, take it out, and in turn apply mains to the other devices' USB receivers? But isn't that theoretically possible as well that a 2.1 sound system fail and apply mains to a 3.5mm jack as well? \$\endgroup\$ – user42875 Mar 25 '15 at 13:12
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    \$\begingroup\$ @user: Yes, a powered device could in theory fail and apply high voltage someplace it's not intended. However, this is a very unlikely failure mode. The chance is so remote that it's not something worth worrying about in normal usage cases. And yes, the same issue applies to other devices like audio, your monitor, etc. You can't protect against all possible failures. You have to decide what level of protection is worth the cost. Protecting against the failures you describe isn't worth the cost unless you are doing something very unusual. \$\endgroup\$ – Olin Lathrop Mar 25 '15 at 13:23
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Olin has answered the mains issue, so I'll answer the hub issue.

Basically, you get what you pay for, sometimes. USB hubs come in a variety of shapes, some closer to specifications than others. High-end USB hubs have all the nifty USB features, per-port current control, shutdown mode, static discharge protection diodes, filter capacitors, isolated power, etc.

Cheaper hubs, not so much. There are single-chip hubs that provide zero safety features, and tie the device power directly to the upstream USB port, also known as your computer. These bare-bones USB hubs are obviously more dangerous than the former type.

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  • \$\begingroup\$ Very good complement, thank you. What about industrial USB->RS232 that work based on an internal USB hub? Do you expect them to have all those nifty features? E.G. moxa.com/product/UPort_1610-8.htm \$\endgroup\$ – user42875 Mar 25 '15 at 14:31
  • \$\begingroup\$ Not having seen the inside of one, I can't even speculate. \$\endgroup\$ – Passerby Mar 25 '15 at 15:15
  • \$\begingroup\$ much of the good quality industrial gear will be optically isolated (and proud of it) \$\endgroup\$ – hildred Mar 25 '15 at 15:37
  • \$\begingroup\$ @hildred: Moxa is an excellent brand with high MTBF products as I understand it, and yet they have a separate product that claims to be "isolated" (though they don't say if it's channel-to-channel!), only up to 4 ports. Do you have others in mind? \$\endgroup\$ – user42875 Mar 25 '15 at 16:03
  • \$\begingroup\$ bb-elec.com/Products/USB-Connectivity/USB-2-0-Hubs/… and they also have dedicated isolation products as well. \$\endgroup\$ – hildred Mar 27 '15 at 0:49
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Regarding

Does that mean that it is risky to have USB devices which are powered from mains because they could theoretically fail and apply mains to the hub, take it out, and in turn apply mains to the other devices' USB receivers?

Yes, of course. Which is why powered USB hubs are almost always supplied from a separate power supply unit, which is manufactured by a specialist company, meeting strict electrical standards for safe operation and isolation from the mains. And for the same reason desktop computers are almost always built with the PSU unit as a physically isolated component.

The risk of the separate power supply unit failing and taking out the USB hub and everything attached to it is quite low, because those power supply units are designed, manufactured, tested and certified to not do so even under normal failure conditions.

The same care is taken when mains power supply is integrated into any device: the risk exists, so the device must be carefully designed to contain that risk. A computer where a simple component failure could connect mains power to an audio jack would never get type approval for sale to the retail/home market.

But this is not true for things not directly connected to the mains. Speakers and ordinary 5V USB devices aren't directly connected to the mains, and don't have that kind of protection. If one goes, it may take out anything it is connected to.

It used to be all too common for an audio amplifier to fail and destroy your speakers, or for a speaker to fail and destroy your audio amplifier. It probably still happens, but if your phone fails you just throw it away without even looking inside.

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