

Not Shocking
October 6, 2011 9:40 AM   Subscribe

posted by parallax7d to Computers & Internet (17 answers total)
From the CDC :
When (60hz AC) current greater than the 16 mA “let go current” passes through the forearm, it stimulates involuntary contraction of both flexor and extensor muscles. When the stronger flexors dominate, victims may be unable to release the energized object they have grasped as long as the current flows. If current exceeding 20 mA continues to pass through the chest for an extended time, death could occur from respiratory paralysis. Currents of 100 mA or more, up to 2 Amps, may cause ventricular fibrillation, probably the most common cause of death from electric shock.

Ventricular fibrillation is the uneven pumping of the heart due to the uncoordinated, asynchronous contraction of the ven-tricular muscle fibers of the heart that leads quickly to death from lack of oxygen to the brain. Ven-tricular fibrillation is terminated by the use of a defibrillator, which provides a pulse shock to the chest to restore the heart rhythm. Cardiopulmonary resuscitation (CPR) is used as a temporary care measure to provide the circulation of some oxygenated blood to the brain until a defibrillator can be used.
I can't find any recorded instances, however, there was this :
Video store owner electrocuted when he contacted an energized circuit while repairing an air conditioning thermostat.
So as with any appliance, the potential is there. As a general rule, as long as you're not poking about the power supply, and there is no ungrounded short, the risk is low. Everything after the P/S is DC and fairly low potential. That said, it doesn't take much current at all to pose a risk - most household circuits are rated for between 10-20 amps.
posted by Pogo_Fuzzybutt at 9:58 AM on October 6, 2011 [1 favorite]

Will a laptop power connector kill you if you put the end in your mouth? And a Meta Discussion.
posted by R. Mutt at 10:10 AM on October 6, 2011

I think your biggest danger with computer components would be to mess with the PSI, as the capacitors certainly hold a significant charge.

/I've been electrocuted when working on my bass amp. No fun.
posted by handbanana at 10:18 AM on October 6, 2011

Power supply unit not psi, damn autocorrect
posted by handbanana at 10:19 AM on October 6, 2011

The internals of a modern computer operate at a maximum of 12V. You're going to have a really hard time killing yourself with a 12V electrical system.

The power supply is another story of course, but that's why those are boxed off and isolated from the rest of the machine.
posted by pharm at 10:23 AM on October 6, 2011

There was this story from last year:

Toddler Burned By USB iPod Cable 16-Month-Old Hospitalized For Third-Degree Burns
"Rhianna Anderson said Trinity stopped breathing and had to be shocked to bring her heart back to its normal rhythm."
posted by bottlebrushtree at 10:58 AM on October 6, 2011

Probably never happened. Ancedata- While back I worked at a repair shop that repaired PSU's and tube monitors. I think every tech got bit once or twice working on something. I even saw a guy get thrown across the room once. Never saw anyone go to the hospital, or heard about anyone being killed.

Thinking on horror stories from actual electricians, I can't recall any that ended in death and they work with much more dangerous currents.
posted by anti social order at 11:22 AM on October 6, 2011


[Don't try any of this at home]

Firstly, computer power supplies usually have a +12V and a -12V rail, which equates to 24V of potential between those two points within the power supply.

If you get shocked by a DC current, though, you'll presumably be from one of the rails, and into the ground, and therefore no more than 12V, as you'd need to be touching both a -12V and +12V voltage source to get a 24V shock, and at that, the shock will only really be harmful if you're touching each source on the opposite sides of your body. Electricity follows the path of least resistance, so if you're touching a +12V terminal with your thumb, and -12V with your pinky on the same hand, the current will only travel through your hand, and your vital organs should be unaffected.

If you touch a +12V terminal and have no other path to ground or another non-12V voltage source, nothing at all will happen. This is more of a problem at higher voltages, because the charges will have enough potential to jump through your shoes/the air to the ground, or another voltage source. Electricity only "works" when your two endpoints have different voltages (and as such, voltage is usually calculated as the difference between those two points; the technical term for voltage is the potential difference). If you grab a +12V terminal in one hand, and a +10V terminal in the other, your body will only experience a 2V shock.

Sidenote: Since I said that voltages are measured in relation to each other, how do we get measurements like +10V and +12V, given that this relationship could be expressed as +1V and -1V or +2V and 0V? Both of those interpretations are technically correct if you're only deailing with those two terminals. However, in the real world, we treat the ground (literally, the ground. the soil beneath your feet) as 0V.

Secondly, you can (easily) kill yourself with 9V, provided that the current is high enough, and even more easily if you break the surface resistance of your skin. If you've got an unusually high concentration of sodium in your body, your body will conduct electricity like a copper wire once you penetrate the skin (this is significant, because voltages normally attenuate as they travel through your body toward your heart and lungs, making them less harmful especially at lower voltages). There's an urban legend about this, although its specific veracity is in doubt... a 9V battery might not quite have enough juice to deliver a lethal current.

Remember that electrical power needs to be thought of in terms of both voltage (strength) and current (quantity). Voltage can most easily be thought of as an electrical charge's "potential" to jump across a substance that normally doesn't conduct electricity (ie. the air, or your skin). Static electricity can be measured as several thousand volts, which explains why it's so damn easy for you to get zapped. However, the charge quantities are so small that there's virtually no current.

If I throw a pebble at you, it's still made out of a fairly dense, heavy material. However, because a pebble is quite small, it's more or less harmless. However, if I throw a big f--ing rock at you with the same force/speed that I threw the pebble, you'll probably get injured (and I'll probably throw out my back), because there's a whole lot more mass in the big rock.

These equations always involve several variables, and the simple fact is that "12 volts" doesn't provide enough useful information, nor does "10,000 volts." Depending on the circumstances, both could either be perfectly safe, or extremely deadly, just like "Throw a rock with all my might" really depends on the size of that rock (and where I hit you).

Oh, and the physics of alternating currents are a lot different, and don't quite apply to some of what I said above. However, the odds of getting an AC shock through computer equipment are quite low, as the conversion from AC to DC happens fairly early on.* In fact, the odds of getting any sort of hazardous current between a keyboard/mouse/monitor/etc into your body *before* melting and shorting out said peripheral are fairly slim.

*One exception here. Old (circa 1998 and earlier) AT-style PCs had mechanical power switches that were wired up like a normal light switch (ie. the full 120V household mains voltage traveled through the switch). If you physically damaged the switch, it was possible to get an AC shock. These days, all power supplies are controlled by a low-voltage relay so that virtually no power goes through the switch. (The voltages and currents through an ATX power switch are so low that they won't even travel through your fingertip.)
posted by schmod at 11:24 AM on October 6, 2011 [5 favorites]

Perspiring man electrocuted by his PC
posted by Knappster at 11:29 AM on October 6, 2011

anti social order: "Thinking on horror stories from actual electricians, I can't recall any that ended in death and they work with much more dangerous currents."

A few things here:
Electricians are trained professionals, and the safety procedures aren't terribly complicated once you know and understand them. As you mentioned, electricity can be lethal, but TV Repairmen do not risk life and limb during the course of their jobs.

Electricity won't kill you instantly, and if you're surrounded by trained professionals and have access to emergency medical services, your odds of surviving a big shock are pretty good.

If you get shocked, the three things that can kill you are:
  1. Ventricular fibrillation. Basically, your heart's pacemaker goes out of whack, and stops pumping blood efficiently. If you've got a defibrillator on hand, your odds of surviving v-fib are very high if you get attention ASAP. CPR can tide you over for a few minutes.
  2. High DC voltages cause your muscles to clench. With an extremely-high voltage, this will kill you on the spot. With a less extraordinary voltage, you'll be paralyzed and unable to let go of whatever's shocking you. More importantly, the muscles that let you breathe won't work. Again, if somebody is nearby to cut the power and give you CPR ASAP, this is an injury that you can recover from.
  3. Burns. If you're being electrocuted for long enough, you'll cook. Again, this is time-sensitive. If somebody cuts the power, you might live.
So, basically, if you're working with high voltages, best do it around other people; preferably ones who have first aid training. Also, more sets of eyes help prevent accidents from occurring in the first place.
posted by schmod at 11:33 AM on October 6, 2011 [1 favorite]

Great info schmod, thanks.
posted by parallax7d at 1:41 PM on October 6, 2011

Secondly, you can (easily) kill yourself with 9V, provided that the current is high enough...

But the current won't be high enough. Skin resistance is significant, and there won't be significant current flow. You can touch and hold live 12V all day without any harm.
posted by Chocolate Pickle at 4:24 PM on October 6, 2011

bottlebrushtree: if you follow that story further (e.g. here), you'll see the problem was actually a faulty lamp and not the USB cable.
posted by Pinback at 5:16 PM on October 6, 2011

I'd be surprised if it has happened. I've worked with ELV (12V) lighting circuits that can deliver many amps of current and I often worked live with no particular worries about skin contact etc. The greater concern was making sure the equipment would survive whatever I was doing.

It is possible a computer power supply has killed someone, however people building or modifying computer systems generally don't open that box.

I've also worked with voltages up around 500VDC and 3 phase 415VAC systems, with the latter sometimes rated for 400A per phase. The procedures for dealing with that stuff are entirely different and the risk of death is very real if you don't know what you are doing.
posted by deadwax at 6:29 PM on October 6, 2011

FWIW, when I did more technician work, everyone warned me about the CRT monitors. Apparently, they can store up a very large charge that takes some time to dissipate, even after the monitor has been powered off and unplugged. If you touch the wrong connectors, you CAN get a pretty powerful capacitive discharge.
posted by Multicellular Exothermic at 8:45 PM on October 6, 2011

BTW, all that UL/CSA stuff that you see is about this. I think it's VDE in Europe.

An Underwriter's Lab (UL) or Canadian Standards Association (CSA) approved product is assurance that a device meets safety requirements, including leakage currents. These are verified with hi potential tests (HiPot) that apply a high voltage for a specified interval to a product while monitoring for leakage currents. Designs and manufacturing processes are reviewed and audited periodically to verify that they have not changed. Certification is withdrawn if they are not. It's a pain in the ass as a designer and manufacturer, but consumer electronics with UL/CSA certs mean something.

FWIW, I think the cutoff voltage for UL stuff is 28 volts or more. Below that, no big deal.

My historical way of circumventing such things in designs is to use UL power bricks external to the thing I am powering. That meets the letter and spirit and doesn't compromise safety. Incorporating/including higher voltages inside a device raises the complexity for safety issues exponentially.
posted by FauxScot at 4:39 AM on October 7, 2011


Right, but the point is that if you break the skin, things get a whole lot more dangerous, even at surprisingly low currents. Also, if you're unusually salty, your bodily fluids will conduct electricity with significantly less resistance (contrary to popular belief, pure water actually doesn't conduct electricity well at all until you add (even tiny amounts of) salt. Even your tap water has enough salt to be reasonably conductive)
posted by schmod at 9:10 AM on October 7, 2011

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