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What actually happens when you drop a hairdryer in bath

by Passiday
Tags: bath, hairdryer
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Passiday
#1
Nov7-11, 01:43 PM
P: 12
Hello

Ok, there's been another news in my area that a young woman was drying her hair, while she was in bath. Smart, huh. And the dryer fell out of her hands, and now she's dead.

My kids hear those news and they're shocked and all, but I'd like to explain my kids what actually happened there and why, so that they can apply this knowledge to protect themselves not only when in bath an drying hair, but in any dealing with electricity.

So, I understand that a persons health is severed when stream of electrons goes through her tissue. This, in turn, happens when the tissue is connected to electrodes with high enough potential difference to cause the electrons naturally floating there to run towards the high potential end, while there's fresh and plenty supply of new electrons from the low potential end (it's awkward that it's the "low potential" end where the electrons come from, but that's due to historical reasons, I guess).

I have problem, though, understanding where exactly a person in bath is exposed to the electrodes. If the dryer falls in, perhaps the water gets in between the + and - somewhere inside the dryer. Why doesn't it just cause short circuit and that's the end of the story? Or maybe the news are wrong and in fact the person got killed when she tried to fish the dryer out, thus putting herself between shifting + and - potential (by holding wet dryer in her hand) and the ground (non-isolated bath)?
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berkeman
#2
Nov7-11, 04:46 PM
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Quote Quote by Passiday View Post
Hello

Ok, there's been another news in my area that a young woman was drying her hair, while she was in bath. Smart, huh. And the dryer fell out of her hands, and now she's dead.

My kids hear those news and they're shocked and all, but I'd like to explain my kids what actually happened there and why, so that they can apply this knowledge to protect themselves not only when in bath an drying hair, but in any dealing with electricity.

So, I understand that a persons health is severed when stream of electrons goes through her tissue. This, in turn, happens when the tissue is connected to electrodes with high enough potential difference to cause the electrons naturally floating there to run towards the high potential end, while there's fresh and plenty supply of new electrons from the low potential end (it's awkward that it's the "low potential" end where the electrons come from, but that's due to historical reasons, I guess).

I have problem, though, understanding where exactly a person in bath is exposed to the electrodes. If the dryer falls in, perhaps the water gets in between the + and - somewhere inside the dryer. Why doesn't it just cause short circuit and that's the end of the story? Or maybe the news are wrong and in fact the person got killed when she tried to fish the dryer out, thus putting herself between shifting + and - potential (by holding wet dryer in her hand) and the ground (non-isolated bath)?
The metal drain pipe for the bathtub acts like a ground path, so there is a "ground fault" created when the dryer falls into the slightly conductive bathwater. If your body is in the water between the dryer and the drain, you may have enough current pass through your body to stop your heart.

That is why most bathroom outlets (and certainly ones near sinks and showers/bathtubs) are required to be fitted with Ground Fault Circuit Interrupter (GFCI) circuits. THe GFCI outlet senses the ground fault, and opens the circuit quickly enough to prevent injury.
Passiday
#3
Nov7-11, 05:11 PM
P: 12
GFCIs are smart indeed. I don't know how they detect the imbalance tho (one sensor on the neutral socket and other on the hot socket? what if the device radiates electrons thus causing imbalance?).
So, the well-conducting bath is to blame. Plastic baths with plastic plumbing should be safer then, although at the end of the line there will be some metal anyway. But the distance (=resistance) could be too long for the circuit to form.
But, do I understand correctly, if the body is not between the source of the current and the ground path, the dangers are not that great? Perhaps the water channel affected by current is pretty compact, following the rule of the shortest path with least resistance.

berkeman
#4
Nov7-11, 06:41 PM
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What actually happens when you drop a hairdryer in bath

Quote Quote by Passiday View Post
GFCIs are smart indeed. I don't know how they detect the imbalance tho (one sensor on the neutral socket and other on the hot socket? what if the device radiates electrons thus causing imbalance?).
There are not many devices that would "radiate" electrons...

So, the well-conducting bath is to blame. Plastic baths with plastic plumbing should be safer then, although at the end of the line there will be some metal anyway. But the distance (=resistance) could be too long for the circuit to form.
The water going down the platic drain meets up with grounded metal piping at some point. The water will be more or less conductive, depending on what-all is mixed in it.

But, do I understand correctly, if the body is not between the source of the current and the ground path, the dangers are not that great? Perhaps the water channel affected by current is pretty compact, following the rule of the shortest path with least resistance.
The "path of least resistance" is a big part of it. Still, I don't want any part of my body near any part of the path, thank you very much!
berkeman
#5
Nov7-11, 06:43 PM
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Quote Quote by Passiday View Post
GFCIs are smart indeed. I don't know how they detect the imbalance tho (one sensor on the neutral socket and other on the hot socket?
BTW, here is an article about how GFCIs work:

http://en.wikipedia.org/wiki/GFCI

.
bahamagreen
#6
Nov7-11, 07:27 PM
P: 536
The first thing to understand is that an AC outlet is not "plus and minus" like a battery. People that know just a little about these things may be familiar with simple direct current circuits where the battery has two wires from it: "plus and minus".

AC is different. The old plugs and wires have two conductors and many mistakenly assume these two wires are "plus and minus". The new ones have a third conductor that is called "ground", which does little to correct the mistaken idea of "plus and minus".

The reality is that one of the conductors called the "hot" is alternating positive and negative. The one called "neutral" is simply providing a path connected to ground back at the breaker box. The one called "ground" is a redundant connection to ground to serve as a safety in the event that the "neutral" connection fails.

Basically, the power company is sending a single line of AC to your home as the "hot". To use it, you place your appliance between that "hot" and ground, that ground connection provided by the "neutral".

So in effect, the power company is pumping AC into the ground when you intercept it with an appliance at the outlet.

The extra safety ground is designed to save people from shocks when a fault occurs and the chassis of an appliance is allowed to contact the "hot". Normally the "neutral" is connected to the chassis to serve this purpose, but if the "neutral" fails, the redundant safety ground diverts the AC to ground instead of through the person. This works assuming the person is not barefoot, standing in water, gripping a ground rod, or anything else that would make his path more attractive.

Most small appliances with double insulated chasses like hairdyers don't have the redundant safety ground, and the outer chassis is not connected to the "neutral", the protection from shock is based on the double insulation.

When the hairdyrer is dropped into the bath, the AC finds two pathes to ground - through both the internal chassis neutral and through the water, drain, and piping (and through anyone sitting in the water). Not much needs to go through the water path, it talks a miniscule amount of current to stop the heart.
gsal
#7
Nov7-11, 11:09 PM
P: 873
Let me add one more thing that may not sound quite intuitive, but you know is true...

...even if the bath tub does not have a metal pipe, the large cross sectional area available allows for conduction of electricity to ground...think about it...concrete is not known for being a conductor, neither is ceramic; in fact, ceramic is sometimes used as an insulator...yet, everybody knows that if you are barefooted on tile and touch an outlet you get a pretty good electric shock, if not electrocution...
Passiday
#8
Nov8-11, 01:06 AM
P: 12
There are not many devices that would "radiate" electrons...
Particle accelerators come to my mind, and perhaps CRT monitors (but I'm not sure).

Thanks, berkeman and bahamagreen for the clarification.
berkeman
#9
Nov8-11, 11:08 AM
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Quote Quote by Passiday View Post
Particle accelerators come to my mind, and perhaps CRT monitors (but I'm not sure).
Even in those devices, the electrons stay contained in the device.


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