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My understanding of power surges is that they can simultaneously damage anything that's plugged in to AC power.

My question is whether a drive that's connected to the computer via USB (only) is better protected than a drive that has its own power supply. My thinking is that there are so many components between the external drive and the computer's power that the chain may provide a degree of surge protection. But, that's only a guess--I don't actually know.

Does anyone actually know how the electrical process of surge works and whether there have been any laboratory experiments?

(PLEASE no answers of the form "you need all the protection you can get" or "you need to protect anyway". I'm not asking for advice about protection from surge.)

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    Your thinking is flawed: there are no components (unless you include circuit board traces and wires as components) between the PSU and the external drive power connections. – Andrew Morton Jul 25 '16 at 17:31
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    @AndrewMorton right, You compare device own PSU and PC PSU. But it depends. For example if PC is notebook running on battery then it definitely better protected. – Mikhail Moskalev Jul 25 '16 at 17:36
  • Downvote is unfair but I think this question is too opinion based – Dave Jul 25 '16 at 18:15
  • Close voters: This is a computer hardware question that requires a little relevant electrical knowledge. That doesn't make it off-topic or opinion-based. This is factually answerable by people with the education, and plenty of users on this site have it. If people without that education post opinion-based answers, those answers can be downvoted. Please don't vote to close questions based on speculation about the answers it might receive because you aren't sure of the answer. For any question on the site, some users can answer it and some can't. You might fill a gap in your knowledge. – fixer1234 Jul 25 '16 at 20:56
  • @AndrewMorton, the power contacts of a USB port are not direct traces back to the PSU. – fixer1234 Jul 25 '16 at 20:58
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These are actually a very good question(s). Yes, there is a branch of engineering that deals with protection of computing equipment from external electrostatic discharges and power surges/transients. This is not a matter of any opinion or discussion. Please take a look here link. Any PC box that comes from a reputable manufacturer goes through a set of comprehensive tests that use very expensive facilities and test equipment.

Regarding the main question, is a wall plug less protected than a PC when supplying USB power through PC box, the answer is "it depends". The PC power supply has much bigger AC transient filters, and the supply chain from AC outlet to USB port has few more intermediate inductors, active switches, and capacitors on the way, which will act as filters. More, bus-powered USB devices have a common ground reference inside the PC box, so it is less likely to have a huge ground bounce surge. So your PC might die, but USB drive might survive.

Wall adapters (good ones) also use surge protection filters and are supposed to be "isolated" from AC, but there is parasitic coupling. Due to smaller size of isolating transformer, it likely has smaller voltage breakdown threshold.

So, unless you put your actual configuration into a proper cage and run all IEC 61000-4-4 tests on it, it is hard to tell.

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