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An electrical experiment for Easter

Chris Pearson

If I have missed something, please let me know.

Introduction

Electrocution has been reported due to the use of a mobile phone in a bath whilst it is plugged into a charger (https://www.rte.ie/news/2025/1001/1536213-inquest-anne-marie-ogorman/). The aim of this study was to establish whether the output of a mobile phone charger can present a risk to the user.

Method

A 5% solution of sodium chloride was placed in stainless steel bowl. The bowl was connected to the earth pin of a twin BS 1363 socket-outlet via an ammeter with a 10 mA full-scale deflection (Avometer Eight Mk 6). A USB charger (Apple Model A1696) was plugged into the adjacent socket-outlet. A USB-C to USB-C cable was plugged into the charger and the other end was immersed in the sodium chloride solution. The charger was energized.

Correct function of the charger was confirmed after the experiment. Confirmation that the earth was effective was obtained by measuring the earth fault loop impedance at the socket outlet.

Results

No current flow was detected. The EFLI was 0.80 ohms.

Discussion

These findings do not confirm the hypothesis that the use of a mobile phone which is connected to a charger whilst taking a bath gives rise to a risk of an electric shock. It may be that the risk exists only if the individual is in contact with the charger itself, or the charger becomes immersed. Further work is required to investigate this alternative hypothesis.

  • in reply to John Peckham

    If you order off the web, and it comes through your letter box having set off outside the UK (or pre-brexit,  outside Europe), you are the importer, and it is down to you, not the person abroad you paid for it, to ensure it meets all necessary legislation and to make a suitable DOC. So not only do you get electrocuted or your house burnt down you are also responsible for grey market importing by buying direct from the  overseas supplier.

    'not a lot of people know that' When perhaps they should.

    Mike.

  • in reply to Philip Oakley

    My comment (which is wrong for the USB C case by the way as noted in my last post) was more about presence of 5V on the lead rather than the safety, or otherwise of delivering that 5V.

    Unfortunately, as noted by others, it may be quite possible for the charger to be operating as designed and still pass enough current to cause harmful effects to someone in a bath.  I would call such a design defective but others might call it "value engineered"; regulations / standards will have a third view.

  • in reply to mapj1

    MapJ Assuming that my Ipad charger is plugged in to a 30mA RCD protected circuit and I am daft enough (Darwin having a day off) to get in to the bath with the Ipad connected by a long USB lead could the stored energy in the capacitors in the charger deliver enough charge (coulombs or micro coulombs)   to kill me?

  • in reply to John Peckham

    On its own, I really doubt it.
    The minimum kill energy for a healthy adult is something over ten joules (*) for a one-shot capacitor discharge electrocution, and it has to be very unlucky so the current is directed via the right, well wrong really, body part usually the heart .

    Defibs use hundred joule sort of energies applied direct to the chest. Electric fences are limited by law to less than a few joules (fences), depending where they are and if the wires are out of reach and similar limits apply to Tasers and similar weapons. (tasers )


    The problem is that the energy from the mains is replenished every half cycle, and in the case of a full mains voltage shock carries on burning through the skin, where most of the volt drop and therefore heating occurs, until a wetter lower voltage  internal contact is made.  

    disconnected mains PSUs with defective bleed resistors can be dangerous once 1/2 CV2 exceeds 10-20 joules. 

    ( * note also that if we take the mains IEC safe disconnection times and very unlikely 1k Ohm body resistance as an example and say 250V, 250mA, 0.2 seconds that is comparable,  circa 60 watts for a 5th of a second would be 12Joules.)

    There is also a risk of the RCD not actually tripping for a two wire mains lead in an unearthed plastic bath.

    Mike

  • in reply to Philip Oakley
    Philip Oakley said:

    I greatly doubt that the charger was operating 'normally' and yet creating a fatality. 

    We may be doing that 'searching for the car keys where the light is better' rather than the darker truths...

    I agree.

    If a normal USB cable and charger presents a risk, what is the circuit?

    mapj1 said:
    and in the case of a full mains voltage shock carries on burning through the skin, where most of the volt drop and therefore heating occurs, until a wetter lower voltage  internal contact is made

    How else do you explain the full thickness burns to Mrs O'Gorman's hand?

  • in reply to Chris Pearson

    I don't - personally I'd associate areal burns with far more voltage (to force the current though wet skin) than the USB 5-20V side of things on its own can provide. 

    A full thickness burn needs a decent fraction of a watt per square mm to raise the temperature faster than the blood circulating in the body can cool it. A patient in an NMR scan, or anyone sun bathing gets about 1kW per square meter and feels a bit warm - a long way from instant burning, and due to the pumped cooling,  small parts of the body can withstand a lot more - an ear near a mobile phone gets 10-100 times that for the duration of the conversation..

    Mike.

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