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Occasionally people get killed in their bathtubs by having an electrical device such as a hair-dryer take the bath with them - in movies.

It seems to be a common belief that this is realistic, even though it makes no sense to me.

There are two scenarios:

  1. The device short-circuits internally (that means entirely over either both poles of the energy supply having contact with water, or the pole under electric tension together with the earth conductor - that way, the current should not leave the devices chassis).
  2. The device short-circuits over the pole under electric tension and the bathtub itself (earth eventually, but a different route).

In the first case (which I thought would be common, since the earth conductor is often connected to prominent large metal parts for safety reasons) I can't see what should happen: Clearly a human being wouldn't be affected from that local a current that takes place entirely in the chassis of the device - or am I mistaken?

In the second case, the question is whether the human body is more conductive (including the skin barrier) than the bathtub water. And even if it was, which I don't know, that would only make a difference if this circumstance would actually lead to a shortcut the current could take on its way to earth.

All that sounds like a lot of "ifs" to me, so I thought I put the question out here.

  • What do people think on the physics?
  • Do people have links to statistics, that is: does this at all happen?
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  • $\begingroup$ Note that hair dryer manufacturers are mandated to have GFCI/RCD in the cords to prevent such deaths. $\endgroup$
    – Kyle Kanos
    Jan 22, 2014 at 19:21

2 Answers 2

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The device short-circuits internally (that means entirely over either both poles of the energy supply having contact with water, or the pole under electric tension together with the earth conductor - that way, the current should not leave the devices chassis).

Think what the short will do: it will melt the hair drier parts and the live side of the incoming circuit will connect with the water and the water through the taps and supports of the tub to the ground. You will be holding something sparking and in contact with the water, all wet. So unless the fuse goes off there is great danger the sparks will include you too because they will be erratic. i.e. your second scenario

The device short-circuits over the pole under electric tension and the bathtub itself (earth eventually, but a different route).

comes fast on the tracks of the first.

5 amperes is a lot of current. Not to forget the surprise element for the person being stupid enough to use a hairdrier in the bath. People are over 50% or so water after all, in good paths like blood vessels.

btw I have seen sparks from a 220 volt meter for the house (provided by the electricity company) in which rain water had run through, and the whole meter gauge was sparking and melting everything resembling in sparks those melting clocks of Salvatore Dali, flowing down the wall.

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  • $\begingroup$ Thanks for your answer. Is there any reason why the fuse shouldn't go off in this scenario? $\endgroup$
    – John
    Jan 22, 2014 at 20:53
  • $\begingroup$ It should go off. In the old systems where bits of wire in the fuse were easily replaced by frugal people somebody might have put a really thick one which would fuse much later than the allowed amperes of the fuse. I have not heard of tampering with the new "switch off " fuses. $\endgroup$
    – anna v
    Jan 23, 2014 at 5:03
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I believe that you won't feel a shock.

The circuit needs to be closed. As you suggested the resistance of the water, which is in parallel with your body (from the electrical circuit point of view), is much lower that your body's.

Therefore the current (the responsible for making you feel the shock) will be very low.

However, I suggest that you do not try it at home (not even at other place) :)

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  • $\begingroup$ I think a circuit scheme with a voltage source feeding a circuit with two resistances $R_{water} << R_{human body}$ would help. But I cannot make it right now :) $\endgroup$
    – cinico
    Jan 22, 2014 at 19:28
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    $\begingroup$ Try this experiment with a cat but enter the bathroom only after you turn the current on. Call it a new Schrodinger's experiment. $\endgroup$
    – cinico
    Jan 22, 2014 at 19:32

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