Probably from your physics days you will remember the magnet and iron particles on a sheet of paper. There was two obvious concentration points with a lot of lines taking different paths to the other concentration point. With two conductors in water, you have the same analogy except that it is occurring in three dimensions. There is no single concentration flow path for the current to go from one point to another. With a person in the tub, some of these paths will pass through the person, and will tend to be concentrated because of the lower body impedance. If the tub drain is insulated as in new construction, then there will be a path from one side of the on-off switch to the other. With a single pole switch, and all of the resistance wire as the other side of the line, the current path is very broad, indeed. The exposure to current flow is very great, and skin resistance is lowered since the water is in =intimate= contact on a large portion of the body. It only takes micro-amps across the heart to cause the heart to react and cause death.
Now if the drain is metallic, likely it is also a possible return path for the current flow. While the distance is much further from the switch contact, whether on or off, the switch is the source contact if the new model has the fat blade that polarizes the source. The return sources are the heating elements, the other side of the switch contact, and the tub drain. The person in the tub does not have to touch any metal to have current to pass through his body. Once the current flow starts, likely the muscles will contract, and the person is powerless to get out of the tub. Current will stop the heart by turning it into quivering jelly, and the person dies from a lack of circulation. Now lets us assume there is a GFI in the circuit, then the person still gets a shock, but it is only for a part of the cycle, and likely less than the time the heart can react to become quivering jelly. In order for the GFI to work, the current difference in differential transformer must trigger the circuit breaker. If the person is in the current path, he will get a shock in order for the GFI to work. In the newer models of hair dryers, there is a sense wire which bypasses the differential transformer, and the interrupter in the dryer plug will operate, since the current is divided between the heating element and the sense wire and some of the current bypasses the differential transformer so that the breaker operates from the current in the transformer. Since the dryer can let water pass through the device, the plastic enclosure does not enclose the hazardous current path through the water. It does not matter where the person is in the tub, or whether he make physical contact with the dryer. It does not matter whether the tub has a metallic drain or a plastic drain. If the dryer is in the water, the person in the tube will have current going through the body. The only salvation in the entire scenario is the interrupter that can sense the differences in the return path and trip a breaker. In the particular case of the dryer, I would prefer the sense wire type, as this is independent of a ground return through a plastic pipe or a metal pipe. The moral of this story is that you should become a dirty old man and never bathe again! Its safer. Manning Rose, NCR Corporation, Dayton, Ohio, [email protected] >---------- >From: [email protected][SMTP:[email protected]] >Sent: Wednesday, August 13, 1997 4:51 PM >To: IEEE Product Safety Technical Committee - >Subject: GFIs, Hairdryers, and Bathtubs ... > >Recently somewhere back in the news (couple of weeks ago), >two children died when one of them used a hairdryer in >the tub. A discussion this accident with some lead me >to a counter-intuitive result from my experience in >product safety. > >************************************************** > >Given Situation #1: > >1. Person in a tub of water sitting at the opposite > end of the tub than the drain. > >2. The tub is ungrounded with water in it. Ungrounded > meaning that the drain is plastic piping. > >3. A 2-wire hair dryer (either On or OFF) is dropped > into the water at the drain end of the tub. The person > in the tub is neither in contact with the hairdryer, > not anything that would be grounded. > > The hairdryer has a plastic case. > >Result: > >1. Since there is no path to ground from the hairdryer > through the person in the tub to ground, no current > should flow *thru* the person in the tub. The person > is in no harm. > >2. Since no current is flowing through ground, > the GFI won't trip. If the water allows sufficient > current to flow by shorting between the HOT and > NEUTRAL in the hairdryer, the breaker will trip. > >************************************************** > >Given Situation #2: > >Same as situation #1 except that the drain is now >a grounded metal pipe. > >Is there a sufficient parallel path to be lethal >to the person in the tub? Seems as though the >parallel path for the hairdryer is straight to >the drain. > >Result: > >1. Person is still unharmed. > >2. GFI trips. > >************************************************** > >Are my conclusions correct? > >Is the only time one can be electrocuted in a tub >when they are in contact with a ground and holding >the hairdryer? > >Is a person really killed in a tub with hairdryer >by drowning rather than electrocution? > >I'm beginning to doubt that I know exactly how >a person is killed in a tub with a hairdryer. > >Or, am I making this way more complicated >than it is? > >Comments? > > > > > >
