Electric shock from USB cable

[Fred Nerd]

Has anyone else got an electric shock from a USB cable?

Anyone died?

I have my laptop setup on my desk, and since I have always been too busy/lazy to get a USB hub, I am constantly unplugging things, these unplugged cables often hang over the edge of the desk at knee height. A couple of times I've bumped then and thought 'sharp edge on that' but today I took a closer look at the offending plug and it wasn't sharp, it was giving my nice little blue sparks.....
It's the cord that goes to my old printer, and I figure since the printer is plugged into the wall, it must be leaking a bit of power back into the cable.
It still works fine, and I haven't died yet, but I will be wary in case it gets worse (will try to refrain from sucking it.....).


Has anyone else had similar experiences?

[IainB]

Sounds like it's an AC shock. You might have a bad Earth, a Live leak ("dirty" Neutral voltage) or a short somewhere - maybe in your printer if it only happens when that is connected.(?)
Many domestic appliances do not have an Earth wire, and are Earthed to the chassis, expecting Neutral to provide an Earth. The sort of problem you describe can often happen if you don't have a positive multiple earth in your electrical mains (or ringmain circuit, if you have one), and, assuming no problems in the appliance, generally occurs where the power wiring you are plugged into is ancient or incorrectly installed/faulty or just has a bad Earth.
When I have travelled around in SE Asia it was a common problem, and I used to do a kludge fix by connecting the Neutral to the Earth wire in the socket that fed my laptop. If that didn't blow the mains fuse, it would usually fix the problem, but only if the Earth was a true Earth.
Definitely NOT recommended though, if you are not very familiar with electrical circuits. You really first need a multimeter and then go around checking for polarity, DC/AC, current and voltages. Doing it blindly is potentially dangerous to your health/life and could damage the equipment. I always carry a multimeter with me on my travels, as part of my standard toolset. Can come in very useful.

[AndyM]

If that didn't blow the mains fuse,

-IainB (January 11, 2013, 04:48 AM)

[wraith808]

It still works fine, and I haven't died yet, but I will be wary in case it gets worse (will try to refrain from sucking it.....).

-Fred Nerd (January 11, 2013, 04:13 AM)

I'd... replace it before then.  That's just me.  I don't do sparks.  And I don't take a chance with electricity.  For me or my devices.

[40hz]

It still works fine, and I haven't died yet, but I will be wary in case it gets worse (will try to refrain from sucking it.....).

-Fred Nerd (January 11, 2013, 04:13 AM)

I'd... replace it before then.  That's just me.  I don't do sparks.  And I don't take a chance with electricity.  For me or my devices.

-wraith808 (January 11, 2013, 08:44 AM)

Agree. Edison Roulette is not a game you want to play.

[SeraphimLabs]

That sounds like a little more than a USB problem.

USB devices operate at 5V and no more than 1A per port. If you knock two bare wires together you might see a brief spark before the port shuts down due to overload, but it should not be anywhere near enough juice to be noticed- the ports by default only provide a few milliamps and devices have to negotiate with the controller if they want more than that.

If it is making a visible arc or actually shocking you for real, you definitely have some type of electrical problem.

[40hz]

 FYI

This chart is based on an AC line in the USA.
 

So if I=V/R and we estimate the resistance of your  finger is about 2k Ohms and guess that one of the 5V rails is supplying the voltage then: 5/2000 = 0.0025 or 2.5 milliamps. More than enough to feel a tingle. And potentially more dangerous as the voltage goes up if it's not being driven by a 5 volt line. (Considering it's visibly sparking I'm guessing it's more likely to be closer to the 19 or so volts actually coming off the power brick since you usually need around 10 volts minimum to see visible sparking with DC. However, if it's somehow bleeding directly from the AC mains it's already at lethal potential. And the fact that it has a bite at such low voltages makes me think there is some AC bleeding in there. Not good!)

 

[f0dder]

Watt, you don't need more milliamps than that? O_o

Iirc USB is limited to 100 milliamps before negotiation... that's not at a very comfortable spot in that graph.

[Shades]

Yep, according to my old electrical engineering teacher, the Nazi´s were responsible for these electrical insights by their experiments (read: torture). If I remeber correctly, there is also documentation about the exposure times to these various amounts of milli-amps and when these become fatal.

[SeraphimLabs]

Yep, according to my old electrical engineering teacher, the Nazi´s were responsible for these electrical insights by their experiments (read: torture). If I remeber correctly, there is also documentation about the exposure times to these various amounts of milli-amps and when these become fatal.

-Shades (January 11, 2013, 02:00 PM)

It also depends on the total energy discharged. 50 joules in a brief period of time is the threshold of lethality. The body is resistant enough to electric shock that most people have to go out of their way to cause it or be caught in a freak accident to get anything more than a reminder out of a system running below 12V.

But the higher the voltage gets, the faster it can become dangerous. 50 joule discharges are not at all hard to create.

[40hz]

^True. The current is a function of voltage and resistance and is continuously variable. Besides. the human body's resistance can run from 1K ohms to around 120K ohms depending on how much body mass is in the circuit path. So it's not easy to estimate an individual's true electrical resistance in an actual situation with any degree of accuracy. The critical factor is if the circuit path crosses the heart area.

Interestingly each type of current (DC and AC) presents its own dangers. DC makes you more likely to "stick" to a load since it paralyzes muscles including the heart. AC's back and forth keeps you from locking up as hard, so (theoretically) it's easier to pull free. But the alternating current makes your heart to go haywire since it confuses the electrical signals and causes fibrillation, whereas with DC there's a good chance the heart may 'auto-reset' after the DC jolt and resume beating normally. This is why medical defibrillators use DC current.

Although both forms of electrical current are dangerous, serious AC shocks are considered (by many medical people) to pose a greater threat since the human heart can remain confused even after the person is disconnected from the circuit. Often with fatal results.

[f0dder]

I was always under the impression that you needed relatively high amounts of energy to be toast - that you needed either voltage or amperage to be pretty nasty. Like, getting main-currents (what do you have in the states, 110v? We've got 220/230v in .dk). But still got the idea that the amps was the worst part. Never really grokked what ratio between the two you needed before it was dangerous, so I've always kept cautious and switched off the mains when I needed to do any kind of electric works.

Guess I got a scare early on, though. Sometime before third grade, some of the older boys at the school I attended back then teased a younger kid to jump into a transformer station to fetch a DKK20 (or summat) coin - 20+ years ago, adjust coin currency etc. (though I guess you don't time-adjust a human life in the same way). So the 1 minute mourning and the story of the tiny little exit holes in his toe and his temple kept me somewhat fearful of electrical installations ever since.

[Carol Haynes]

Guess I got a scare early on, though. Sometime before third grade, some of the older boys at the school I attended back then teased a younger kid to jump into a transformer station to fetch a DKK20 (or summat) coin - 20+ years ago, adjust coin currency etc. (though I guess you don't time-adjust a human life in the same way). So the 1 minute mourning and the story of the tiny little exit holes in his toe and his temple kept me somewhat fearful of electrical installations ever since.

-f0dder (January 11, 2013, 05:16 PM)

Yikes! Nasty story.

Worrying too as I have a workshop with a power station transformer in a fenced enclosure immediately outside the window. I asked the electric board who was going to paint and repair my window since there are big signs saying "High Voltage - Danger of Death" all round the fence. The electric board told me to climb over the fence and paint the window myself!

[40hz]

Never really grokked what ratio between the two you needed before it was dangerous,

-f0dder (January 11, 2013, 05:16 PM)

The formula is Ohm's Law:

Current in amps = Voltage in volts/Resistance in ohms

Clearest explanation I ever saw is here. Check out the VIR triangle at the link. That's the easiest way to remember the formula variations.

General rule of thumb: once you hit 1/10 of an amp (i.e. 100 milliamps) you're at risk with house current levels and frequencies.

[barney]

No comments on the various formulations & guides - some old tech is still valid  , just a cautionary observation/experience.

When I worked at Hughes Aircraft in Tucson, AZ (USA), we had a lot of DC-powered test stations.  One night a tech was called in to work on a test station.  Unfortunately, his testing had to be done on a hot station, i.e., with the power on.  (It didn't have to be done hot, but the difference was several hours as opposed to a few minutes.)  He got across 400 volts DC.  It literally threw him halfway across the room - perhaps I should say his muscle spasm when he got across the contacts threw him halfway across the room.  I helped to recover him, get him on a gurney the shop nurse brought.  Significant, if not major, burns on this hands and arms,  and a partially dislocated shoulder from jumping away while trying to hold on.

Up until that time, in my ignorance and supposed invulnerability, I was wont to work on 120V AC circuits hot - home wiring and the lot - comfortable in my presumed immunity.  I don't do that any more.  (I've also gotten burned fingers working with 9V lantern batteries, but that's a different matter.)

Anytime you're working with current, you can be in harm's way, even though the amount of current seems minuscule.

[SeraphimLabs]

Yeah. If it is higher voltage than a car's battery, I won't touch it live unless I absolutely have to- and only with proper safety procedures. Although a car battery is also theoretically dangerous, in practice I would have to grab a terminal in each hand while soaking wet to actually get enough juice from it to do damage- and such a thing I know not to do for obvious reasons.

If I am not mistaken the body has a fair bit of capacitance as well. It is for this reason that AC has a far easier time shocking someone to significant currents than DC will do, as it relies on that capacitance to cheat past the resistance and reach damaging current levels. AC voltages above 240V also introduce additional hazards, usually equipment at those levels is dealing with voltages and currents so high that arc faults will explosively destroy anything around them causing severe burns with shrapnel, and above 440V even a frayed end on a cable can and will cause arc faults with explosively destructive results. Extreme care must be taken on such large equipment to leave no sharp edges or loose fibers, and everything must be clean, tied down, and covered before energizing.

But a USB at 5V 100mA max should not be arcing like that for any reason, and even if it did shock you the port should sense the fault and turn off before it causes serious injury. You should check for more serious issues like a ground loop or excessive feedback.

[TaoPhoenix]

Just sayin' this thread is mystifiying 'cause I never got shocked from a cable.

Sloppily trying to plug things into sockets in the dark, that's a different matter!    

[4wd]

I'm going along with IainB on this, it does sound like a dodgy earth, (or earth<->neutral connection).

If they've tied the USB shield to the printer frame along with earth/neutral then it's probably picking up a slight leakage current through the AC wiring.

Antenna sockets on the backs of TVs are a really good place to get hit with this 

Working in Telstra, getting zapped was pretty much par for the course - working on the Main Distribution Frame in close proximity to 25-50 subscriber lines, any one of which might get an incoming call....90VAC doesn't exactly tickle.  The worst part is it would cause your hand to jerk back...into the block of connections behind 

Then you were always wondering if some faulty piece of mains connected subscriber equipment was feeding 240VAC back into the line.

@Fred: Just as a matter of interest, do you have an RCD, (safety switch), installed ?

[IainB]

...Antenna sockets on the backs of TVs are a really good place to get hit with this    ...
...in close proximity to 25-50 subscriber lines, any one of which might get an incoming call....90VAC doesn't exactly tickle.

@Fred: Just as a matter of interest, do you have an RCD, (safety switch), installed ?

-4wd (January 12, 2013, 01:10 AM)

Maybe a bit off-topic but following on from what you said:
(a) TV antenna feeds: Yes, I have been zapped by domestic TV antenna feeds - the shock came off the outer (Earth) shield of the 75ohm (?) coax cable. If it happens, I always earth them (in the junction box) to mains Earth, because the shock could harm a toddler, even if not an adult. It's enough to hurt and make the spark jump a gap (same as the USB spark in the opening post).

(b) Telephone lines: Yes, you need to take care with those. Playing around with modems can give you a healthy respect for the voltages/currents involved. When a ringing signal is being sent there is an AC voltage pulse superimposed on top of the normal DC voltage. This AC "ringing voltage" pulse would nominally be around 90vAC at at a freq of 20Hz, but could peak at around 130vAC at different frequencies.
The potential difference (voltage) across the tip and ring wires is usually around 50vDC when the telephone is not being used (i.e., is "on hook"), and this drops to drops to around 6vDC when it is in use (i.e., is "off hook").

These voltages could be quite handy! I recall seeing one early example of a nifty and compact digital phone with several memories (presumably a hot new feature at the time it was designed) that seemed to have no independent power supply of its own, and was completely parasitic off the phoneline's DC supply. It worked very well too. I think it's illegal to attach such parasitic phones to the PSTN now though.

[4wd]

The potential difference (voltage) across the tip and ring wires is usually around 50vDC when the telephone is not being used (i.e., is "on hook"), and this drops to drops to around 6vDC when it is in use (i.e., is "off hook").

These voltages could be quite handy! I recall seeing one early example of a nifty and compact digital phone with several memories (presumably a hot new feature at the time it was designed) that seemed to have no independent power supply of its own, and was completely parasitic off the phoneline's DC supply. It worked very well too. I think it's illegal to attach such parasitic phones to the PSTN now though.

-IainB (January 12, 2013, 02:30 AM)

Telstra' standard rental phones, (earlier T200 & T400), used the DC in the phone line to keep their memory backup charged, (capacitor if IIRC), thousands of these phones are still in use in Australia - I've got two.
AFAIK, they're still legal - Telstra only replace them if they're faulty or if you want the latest and greatest T1000 phone, (for $20 which I'm too cheap to pay for ).

[Stoic Joker]

(b) Telephone lines:[/b] Yes, you need to take care with those. Playing around with modems can give you a healthy respect for the voltages/currents involved. When a ringing signal is being sent...

-IainB (January 12, 2013, 02:30 AM)

All things in IT tending to be a panic... I once found my self running out of hands while under/behind some office cabinetry while tracing out a large ball of (evil elf macramé) wiring and (not thinking) stuck one of the wires in my mouth so I could better address a rather nasty tangle. And damned if the phone didn't decide to pick just then to ring.

That's a 4 sec pause for you to try swearing in, then your eyes light up for 2 sec, repeat...

[40hz]

tracing out a large ball of (evil elf macramé) wiring

-Stoic Joker (January 12, 2013, 09:42 AM)

That is one of the worst things I have to routinely deal with.

Fortunately, there's something you can get that lessens the chance of you bumping into AC current when it's buried in with a mass of cable. It's an inexpensive device that goes by a variety of names such as VoltAlert - but its generic term is a non-contact AC Voltage probe or detector.

They come in a variety of shapes and colors, but the most common type is a pen-like probe that clips in your pocket for safekeeping. When it's placed in close proximity to  an AC voltage source, it blinks and emits a chirpy beep warning you there's AC voltage present.

I have a model made by Fluke which looks something like this.



Fluke charges a premium (About $25) because of their name. You can get less expensive ones ($8-$15 USD) that work equally well (it's a pretty simple circuit inside these devices) from other makers. GBC and Klein are good choices. And they can be found in most large home improvement stores like HomeDepot.

Any time I'm poking around in a mass of wires or sticking my head up in a drop ceiling to do something, I poke one of these little guys around a bit before I touch anything just to be sure I don't connect with something that can kill me.

[SeraphimLabs]

(b) Telephone lines:[/b] Yes, you need to take care with those. Playing around with modems can give you a healthy respect for the voltages/currents involved. When a ringing signal is being sent...

-IainB (January 12, 2013, 02:30 AM)

All things in IT tending to be a panic... I once found my self running out of hands while under/behind some office cabinetry while tracing out a large ball of (evil elf macramé) wiring and (not thinking) stuck one of the wires in my mouth so I could better address a rather nasty tangle. And damned if the phone didn't decide to pick just then to ring.

That's a 4 sec pause for you to try swearing in, then your eyes light up for 2 sec, repeat...

-Stoic Joker (January 12, 2013, 09:42 AM)

There was a day I was fixing some phone wiring and couldn't find my tools. So I simply bit the end of the wire to use the notch in my teeth to strip the insulation.

Go figure it would be right then that the phone rings, full blast 90V.

Couldn't talk for about an hour, my tongue was numb.

[Shades]

Fluke does indeed charge through the nose for their products. But I am still of the opinion that you do get what you pay for. These things can take a serious beating before they give up and last forever when used normally.

Their multi-meters (Fluke 75) I used in school were touched by at least 1000 electronics students over a 4-year period and they were just as good as they were new. To me that was a proper investment my technical school did. Not having to think about maintenance even when more than 1000 (angry/bored/agitated/otherwise afflicted) students touch these things, says to me that you have a quality product in your hands.

Cherish these, you hardly come by these type of products these days. Such a meter would last me my whole life-time if I would be the sole user.

Then again, I am a cheapskate in the view of most people. Keeping up with their maintenance I can say that my army boots still server me well even though these are already 18 years old. And yes, I still maintain them with their original maintenance set. As I only have need of these boots about 2 months a year here in Paraguay, I expect these to last my whole lifetime as well.

[Stoic Joker]

(b) Telephone lines:[/b] Yes, you need to take care with those. Playing around with modems can give you a healthy respect for the voltages/currents involved. When a ringing signal is being sent...

-IainB (January 12, 2013, 02:30 AM)

All things in IT tending to be a panic... I once found my self running out of hands while under/behind some office cabinetry while tracing out a large ball of (evil elf macramé) wiring and (not thinking) stuck one of the wires in my mouth so I could better address a rather nasty tangle. And damned if the phone didn't decide to pick just then to ring.

That's a 4 sec pause for you to try swearing in, then your eyes light up for 2 sec, repeat...

-Stoic Joker (January 12, 2013, 09:42 AM)

There was a day I was fixing some phone wiring and couldn't find my tools. So I simply bit the end of the wire to use the notch in my teeth to strip the insulation.

Go figure it would be right then that the phone rings, full blast 90V.

Couldn't talk for about an hour, my tongue was numb.

-SeraphimLabs (January 12, 2013, 03:31 PM)

  (Ouch!) Thanks for not letting me share by myself.