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Phone charger bath accident

  1. Jul 13, 2017 #1
    http://www.telegraph.co.uk/news/2017/07/11/teenager-dies-charging-phone-bath/

    The poor girl got electrocuted, I assume, from the USB case being live at mains voltage.

    I don't understand - between you and the mains, there should be a transformer, class Y capacitor, and an optocoupler. No direct reference.

    Now, you could argue it was a clone charger, with poor isolation. However, I can measure 135Vac (our supply is 240V) between my Apple charger USB plug's metal case and earth ground. This, however, drops to 13V when I touch the case. I assume an Apple product is well isolated electrically.

    The scope shows a 50Hz (our frequency) sine wave 400V p-p. Is this just coupling through the class Y capacitor? Do you think the girl's charger had an underrated cap that broke down when given a good reference to earth ground? Is it OK for the manufacturer to supply lethal equipment with impugnity? She shouldn't have been using it in the bath, but anyone can get a good ground reference by leaning on a radiator or kitchen sink.

    There's a frustrating lack of technical detail in the article, so I'd love to hear your take on the tragedy.
     
  2. jcsd
  3. Jul 13, 2017 #2

    anorlunda

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    I've seen the articles and there is a lack of detail. Because of that, we can't rule out the possibility that she brought an extension cord into the bath to plug the charger into. Speculation is fruitless until we have the facts.
     
  4. Jul 13, 2017 #3
    Absolutely right. But from what I can gather, I'm pretty sure the phone dropped into the bath - its metal case being connected directly to the USB chassis.

    Since this happened in the US, I wonder anyone there can find out more detail. My working hypothesis is that the charger lacked a class Y capacitor. This does happen - BigClive (youtube) tore down a switching supply with a simple ceramic disc cap where the Y should have been. No prizes for guessing where the thing was manufactured, or the auction site it was bought from.
     
  5. Jul 13, 2017 #4

    anorlunda

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    You can't depend on a statement like that being reliable. The phone and extension could have fallen. I would consider popular press accounts as having zero evidentiary value.
     
  6. Jul 13, 2017 #5

    berkeman

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    Yes, I've read that she used an extension cord, but I'm not finding that news article at the moment. Looks like the parents have changed their story to claim that the phone was plugged into a bathroom outlet (which of course would have had GFCI protection).
    That seems more likely, or even just water running down the USB cord to the extension cord outlet.
     
    Last edited: Jul 13, 2017
  7. Jul 13, 2017 #6

    russ_watters

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    Thanks for posting this - I saw the same article and had the same doubts.
     
  8. Jul 13, 2017 #7

    russ_watters

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    Agreed. My problem with the scenario presented isn't the electronics, it's the wires; I don't see how such small wires could carry enough current to electrocute someone.

    This may be another misundertanding tech issue, like last week's false Alexa called 911 story.
     
  9. Jul 13, 2017 #8
    Well, a typical USB phone charger can carry 1A at 5Vdc. Just 30mA of AC can reportedly cause fibrillation of the heart, and of course therefore loss of consciousness. If you're in the bath, drowning becomes a real risk at this point. Also, in many cases, it's not the shock that directly kills you, but what the shock causes you to do - fall off a ladder etc.

    Further testing of my Apple charger reveals that the 135Vac between USB chassis and earth ground will only drive 37.5 uA of current, and drops to 10 mV p-p when doing so. Whether this is a 'ghost' voltage and reactive current/power I'm not sure, but it seems pretty safe.

    Anyway, as you say above, the point is moot until we know the exact circumstances of the accident. But we really should know, because if it turns out the charger coupled the fatal current to the phone case, a tragic accident like this could just be the first of many.
     
  10. Jul 13, 2017 #9

    anorlunda

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    Use common sense. Ordinary AAA, AA, and 9V batteries can also supply much more short circuit current than that. How many deaths have you heard of about people handling those? No doubt if you dig you can find reports of fatal falls from a height of 30 cm, but that doesn't make us worry about standing erect.
     
  11. Jul 13, 2017 #10

    berkeman

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    I think the question came up when Russ mentioned low-current-carrying capability of the USB cable. The voltage conducted along the cable would have to be high to cause a lethal shock, so somehow there had to be a conductive path back to the (non-GFCI-protected) AC Mains supply...
     
  12. Jul 13, 2017 #11
    Ouch! I was really just brainstorming about whether a USB cable is beefy enough to deliver a fatal current (30mA, for argument) at 240V or 120V, as per Russ's question in post #7. I would guess it probably is, and wouldn't trust a USB cable to fail before I did.
     
  13. Jul 13, 2017 #12

    anorlunda

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    Sorry if I was harsh. But you were trying to equate the danger of 30 ma @ 120V to 1 amp @ 5V. That's invalid. 1A @ 5V is akin to AA battery range of numbers and we know by common sense that those aren't dangerous.

    This article discusses the lethality of currents. https://en.wikipedia.org/wiki/Electric_shock
    But the currents discussed are defined as passing from hand to foot. It takes substantial voltage to drive 30 ma from your hand to your foot. That's why 5V is not dangerous. A USB can deliver 1A to a load, but it won't drive 30 ma through your whole body. If USB ports were lethal shock hazards, they would have much more safety in the way they are packaged and handled.
     
  14. Jul 13, 2017 #13
    Further to this, I rigged up a 2ft/60cm length of USB cable between a mains socket and two GU10 LED lamps in parallel. The lamps consumed 55mA of current at 240V for a good 20 seconds. I concluded the USB cable is more than capable of delivering a lethal shock. It didn't even get warm.
     
  15. Jul 13, 2017 #14
    Ah, yes, OK. I meant that if the cable can carry 1A at 5V, it can surely carry 30mA at 240V. But see post above too.
     
  16. Jul 14, 2017 #15

    CWatters

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    If it can carry 1A then it can carry 1A at voltages high enough to kill you.

    Not really any more info but..

    http://www.dailymail.co.uk/news/article-4690766/Tearful-father-stepmom-warn-electronics-bathroom.html#ixzz4mpQORVo5

    If the only thing that fell in was the phone then there must be some problem with the charger.

    Most countries have regulations that require anything electrical you can tough to be safe. Think crawling baby putting the USB cable in their mouth. I would hope all countries have regulation in place that make it unlikely such a child would come to any harm.

    Sadly this isn't the first time it's happened...
    http://www.mirror.co.uk/news/uk-news/man-killed-after-electrocuting-himself-10044838

     
  17. Jul 14, 2017 #16
    Indeed. I even started a thread a while back about the last story you mention. The consensus at the time was that the mains extension cord must have fallen in too.

    Are fatal shocks are being coupled across poorly isolated USB supplies? You may not be in the bath, but the USB chassis is just waiting for you to ground yourself with a wet hand on a radiator.

    I've taken apart a few USB supplies that I had lying around. The transformers are wound such that the primary is wound in with, or sometimes sandwiching, the secondary. You are relying on very thin insulation as you hold your metal phone chassis, or your young child sucks on the cable end.

    One of the cheaper ones had a ceramic disc, not class Y, bridging the USB chassis to the mains supply. I commoned the live to neutral, then attached the +ve of my insulation tester. The -ve went to the USB chassis. The resistance at 500V was less than 1 Mohm. If I gripped the -ve lead of the tester, and with the same hand touched the USB chassis, I got a moderate shock. (Intra-hand, not hand-hand - I'm not that stupid!). This did not happen when I lifted a leg of the cap.

    (500V is the standard for testing UK appliances - At the normal 240V rms, the peak will be around 340V.)

    All the opto-isolators were tested right up to 1000V and showed no sign of leakage.

    Was my testing reasonable?
     
  18. Jul 14, 2017 #17

    CWatters

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  19. Jul 14, 2017 #18

    jim hardy

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    What ? That's why we have "Split Bobbin" power transformers...

    splitbobbin.jpg


    Some manufacturer is stumbling up the learning curve. They need more gray hair in design department.

    old jim
     
    Last edited: Jul 14, 2017
  20. Jul 14, 2017 #19

    OmCheeto

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    Not necessarily. I just did a Zillow scan of the houses in the area of the incident (800 block of West Avenue, Lovington, NM), and they were built in 1965. The GFCI requirement for bathrooms didn't come into effect until 1975. [seemingly reliable ref]

    Also, I've been mowing my neighbors lawn for the last 2 years, and they don't have a GFCI outlet on the outside of their house. Those have been required since 1973. They just moved in a few years ago, so there seems to be some loophole about GFCI rules. Not sure if they are renting or own the house.

    When I bought my house in 1989, it still had its original 1945, two prong outlets, in every room.
     
  21. Jul 14, 2017 #20

    berkeman

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    Holy crap. That's scary, Om. Time for a PF educational campaign?
     
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