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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.

Parents

  • I had a go at finding an actual report form the Coroner's Court but couldn't find it. Various media articles refer to the Consultant forensic engineer stating that the charger gave 2 amps, which seems an odd thing for them to say (since presumably that's 2 amps at 5V)? I'd have thought what would be more relevant would be the leakage capacitance across the convertor which may cause the DC output to be something like 115V ac to ground (if it's symmetrical from both L and N to the output) with a source impedance that will allow a few milliamps to flow - I've seen that happen quite commonly.

    The trouble is there's a number of stages of interpretation going on here - the husband (who of course I do feel sympathy for) interpreting the consulting engineer's report, and the journalists then interpreting that. It would be interesting to find out what the Consultant forensic engineer actually said.

  • in reply to Andy Millar
    Andy Millar said:
    I had a go at finding an actual report form the Coroner's Court but couldn't find it.

    I believe it can be requested if one had a good reason for doing so.

  • in reply to gkenyon

    Yes, I'm sure it can be, unfortunately the only reason I could give would be "morbid engineering curiosity"!

  • in reply to Andy Millar
    Andy Millar said:
    I had a go at finding an actual report form the Coroner's Court but couldn't find it.

    That was one of the first things that I did too.

    As a general rule, Court and Tribunal hearings in England and Wales are open to the public: the law is not only done, but seen to be done. I think that a report is issued only in a minority of cases. Otherwise, there is not much more than a death certificate stating who the deceased was and how, when and where they died. I suspect that similar rules apply in Eire.

    Andy Millar said:
    presumably that's 2 amps at 5V

    My A1696 charger specifies 5V [symbol for smooth or pulsed DC] 3A or 9V/2A. Model A1299 specifies 5V/1A and A1357 is 5.1V/2.1A. The first two are designed for BS 1363 socket outlets, so they have an earth pin, but I suspect it is only there to open the shutter. The third one is a world travel adapter kit. There is a convertor and six different plugs. It is clear that the earth pin on the BS 1363 part is connected to nothing.

  • in reply to Chris Pearson
    Chris Pearson said:
    so they have an earth pin, but I suspect it is only there to open the shutter.

    I'd expect, given some reports of an expected tingle from the phone case while being charged, is that the Apple design does include an RF filter, reference to the expected Earth connections (as is normal for RF / EMI filters).

    These RF filters deliver half supply voltage, via the Y-capacitor divider, to the earth connection. It's that half supply voltage that delivers the tingle when you are the earth. If the Y-suppression capacitor has a fault, then it can deliver even more...

    --

    I also suspect that the 2A in the newspaper report is a red-herring that is actually a reference to the output current capacity of the USB C standard, albeit at a low voltage. 

  • in reply to Philip Oakley
    Philip Oakley said:
    the Apple design does include an RF filter

    When it comes to electronics, my eyes glaze over, but the charger says (in such teeny tiny writing that I had to use a pair of loupes) "EMC 3258".

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