Electronics > Beginners
Why some people died in their bath after smartphone dropped into water ?
TheDood:
The water is full of salts or ions, and so is your body (pure water is a poor conductor, needs ions), so conduction should be possible. Voltage would matter if it was great enough that the parallel current flow through your body was sufficient to meet the threshold of bodily damage.
Resistivity is the amount of resistance with respect to how far you are away from a source. Its units are Ω·m, or ohm-meter, or the amount of ohms per meter of seperation. It's based on the lead/probe surface area, and the distance of the conduction path. How many amps pass for a given voltage when probes are at certain distance apart and have a certain surface area.
ρ = (ΩA)/L
ρ = resistivity
Ω = ohms of resistance
L = proximity of conductors
A = surface area of conductors
When you're in the bathtub with a sunken energized cable, you're effectively creating a cct with yourself as 1 resistor and the bathwater as another resistor, in parallel. Depending on your body EC and the bath water EC (resistivity = 1/EC), a resistance can be calculated for both "resistors," (yourself and the bathwater), and because parallel ccts have the same V across the parallel components, one could see it's not hard to electrocute.
After a quick Google search I found 150mA to be a lethal current flow for humans, so if your cord were 120V..
V = I·Ω
120V ÷ 0.15A = 800Ω
...and the conduction route through your body were 800Ω or less, you're a goner! (Not sure how long 100mA - 200mA needs to flow though)
Circlotron:
--- Quote from: Brumby on January 30, 2020, 01:13:54 am ---
--- Quote from: KL27x on January 30, 2020, 12:30:29 am ---If the problem was shoddy chargers, wouldn't they be destroying phones, regardless of the vicinity to a bathtub?
--- End quote ---
Mobile phones are completely floating, electrically. Mains voltages would cause the entire device to attain a potential with respect to the world around it, but internally, there is no such risk.
But add a path to ground and things change.
--- End quote ---
Just like a bird perched on a high voltage wire.
KL27x:
^Yeah.
The death camping light is an example where I can maybe imagine the incompetence. One guy designs a camping light with an internal charger. Cheap as possible. Then someone comes along and decides to add a 5V USB port on it, just connecting the internal/enclosed battery charging traces to the USB port. Then you get the oopsie when someone dies.
If you buy the $10 phone charger from the airport that has $2.00 BOM and manufacturing cost, I don't think that will have mains on the DC side. Improper isolation maybe, but not direct mains short of a malfunction.
And if the charger isn't charging or is blowing up your phone, I don't think you will be trying to use it in the bathtub or anywhere else.
james_s:
The issue is silent failures, where the charger continues to work but is not providing proper isolation. I don't remember now who it was but I know I saw teardowns of several different cheap USB chargers and many of them had either really sketchy isolation in the transformer itself, or traces and components so close together that something could potentially flash over and bypass the isolation. In places with 240V mains this issue will be amplified.
A GFCI *should* protect the person from death but I certainly wouldn't rely on it to do so, and there are still plenty of houses out there without GFCI protection.
tooki:
--- Quote from: Jwillis on January 30, 2020, 04:48:54 am ---I think theirs more to these stories than what is really being told. The average USB charger cable is around 3 feet long or about a metre. Electrical code states that any power outlet must be a minimum of 6 feet or 2 meters from the bathtub or shower or toilet . So as the OP stated it's quite likely that an extension cord was used at the time of insistent . GFCI receptacles are supposed to be installed in bathrooms but I have seen these plugs replaced with standard wall receptacles.
So in a scenario of no GFCI with an extension cord charging a smart phone while taking a bath add up to a very bad day for the victim.
Darwin wins again.
--- End quote ---
Remember that most developed countries now require whole-house GFCIs, and that even in ones that don’t (or didn’t at the time of construction) a GFCI outlet can protect additional outlets connected downstream from it. So just because you don’t see a GFCI unit right at the bathroom sink doesn’t mean it’s not GFCI-protected.
But moreover, a GFCI isn’t going to protect against all failure modes. One potential failure mode of horrid cheap chargers is that low-current (but enough to kill, i.e. 50mA-ish) mains AC voltage is superimposed atop the 5V DC.
As for cable length, I use 2m and 3m phone cables daily.
--- Quote from: james_s on January 31, 2020, 03:30:07 am ---The issue is silent failures, where the charger continues to work but is not providing proper isolation. I don't remember now who it was but I know I saw teardowns of several different cheap USB chargers and many of them had either really sketchy isolation in the transformer itself, or traces and components so close together that something could potentially flash over and bypass the isolation. In places with 240V mains this issue will be amplified.
A GFCI *should* protect the person from death but I certainly wouldn't rely on it to do so, and there are still plenty of houses out there without GFCI protection.
--- End quote ---
The two channels that do lots of teardowns of horrid Chinese chargers are bigclivedotcom and diodegonewild. Outside of video, Ken Shirriff has done extensive, detailed charger teardowns.
All three of them have found chargers with insanely small creepage distances, as well as transformers where primary and secondary windings touch, with nothing but the enamel separating them. Doesn’t take that much vibration to wear through that!
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