Main Area and Open Discussion > Living Room

Electric shock from USB cable

<< < (4/8) > >>

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)
--- End quote ---

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)
--- End quote ---

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 :) ).

Navigation

[0] Message Index

[#] Next page

[*] Previous page