Water as a conductor.

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haskindm

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Maryland
I think we can all agree that pure water is a very poor conductor. I saw Mike Holt dip his "plug strip" into a pitcher of water to prove that nothing would happen. I also saw a case once where the electrical panel was submerged and all the electrical systems in the house were working properly. I was explaining this to a class and was challenged by a question.
If water is such a poor conductor, then why is it so much easier to be shocked or electrocuted if the person is wet or standing in water? I wasn't sure how to answer. I came up with something about the water lowering the resistance of the skin, but I would like to give them a better answer. Can anybody help?
 

raider1

Senior Member
Staff member
Location
Logan, Utah
I could be way of base (probably am), but I was taught that it was the electrolytes in the water that make it a good conductor. The minerals, salts and such are what conduct electricity in water.

I believe that this is why you are only supposed to use distilled water in a battery, so you don't change the content of electrolytes in the battery.

If I am wrong please feel free to correct me. :D

Chris
 

charlie b

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Location
Lockport, IL
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Retired Electrical Engineer
I see two answers to this (very good, IMO) question. Roland gave one, and Chris touched on the other.

First of all, I should mention that you seldom encounter "pure" water, meaning H2O with so few dissolved impurities that it is a nearly perfect insulator. But the key to understanding the phenomena described in the question is to remember that water can conduct enough current to kill a human, and not (by a long shot) carry enough current to trip a breaker. A current of 0.1 amps can kill. But it could take 25 or more amps to trip a 20 amp breaker.

It should not be too much of a surprise that a panel submerged in water could continue to function in an (apparently) normal manner. What are the paths by which current leaving the source can return to the source? Well, all the loads that are turned on will have their usual current paths. In addition, within the submerged panel, there are paths from the "hot" bus bars (and from each circuit breaker) to each other and to the neutral bar. These paths include water as the conductor. But the water is not a very good conductor, so the currents will not be high enough to trip any breaker.

However, there will also be paths for current to flow from the energized parts within the submerged panel, through the water, to water pipes and other metal objects in the room. If a person tries to walk or swim to the panel, in an effort to turn off the main breaker, then that person will become a current-carrying path. Here again, there can be enough current to kill, but not enough to trip a breaker.
 
I recently talked with an old electrician who load tested generators with water. He would fill a horse trough with water, put the conductors from the generator in the water and fire up the generator. He would add salt to the water and monitor the current in the conductors. When full load was reached he was happy the generator was good. One note though--he had to add a lot of water as the load went up.

And several years ago, a university professor was using three electrodes down a well to generate steam. He would control it by metering a saline solution injected into the well.
 

lile001

Senior Member
Location
Midwest
Conductive Water

Conductive Water

stormywyo said:
I recently talked with an old electrician who load tested generators with water. He would fill a horse trough with water, put the conductors from the generator in the water and fire up the generator. He would add salt to the water and monitor the current in the conductors. When full load was reached he was happy the generator was good. One note though--he had to add a lot of water as the load went up.

And several years ago, a university professor was using three electrodes down a well to generate steam. He would control it by metering a saline solution injected into the well.

We used to have a machine that generated pure water by reverse osmosis. This is essentially the same as distilled water for practical purposes. The filter change was measured by a neon light in series with a container of the water. If the ions began to go up, because the RO filter was going bad, the light would go on with just a tiny amoutn of current.

You rarely encounter pure water outside of a laboratory. Even rainwater has some ions in it due to dust in the air. Pure water will eat up copper pipe, because it scavenges copper ions. Regular tap water does no such thing! Just add a little dirt, and you've got plenty of conductive ions. Any water you encounter in the real world enhances conductivity, whether it be skin, switchgear, or substation.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
Pure water is pretty uncommon. Not many people take baths, or showers, or swim in pure water. Even if you take pure water, and pour it on your skin or on a concrete pad, the water will absorb salts and no longer be pure water.

Pure water is made of H2O molecules. These are electrically neutral - neither the H's or the O's give up any electrons. Instead, they share a few of their electrons. Therefore, when a voltage is applied, the molecules don't go anywhere since they are neutral. And the sharing of electrons holds the electrons in place very tight. (Although a very high voltage might be able to pull the electrons off. Then the pure water would conduct.)

Now lets add some salt or chlorine to the water. These both produce ions - they loose electrons and become positively charged. Apply a voltage and you have an "ion current". The entire molecules move and create electric current.
 
Pure water (H2O) is an insulator.
Example ...the water that is surrounding the reactor core in a Nuclear Plant is purified H2O.
Theres several steps they take to add & remove chemicals and treatments before flooding the reactor.

little tid bits of useless information.............
 

ramdiesel3500

Senior Member
Location
Bloomington IN
Had an older electrician tell me this many years ago. He was running a large job where a very VERY large 3-phase 480V motor was being installed. Just before landing the conductors, someone inquired about rotation. As it turned out, major damage to the equipment would result if the rotation was wrong. They had no meters to determine rotation direction back then and they were short on time. So he took three glass cake pans and filled them with distilled water. Then they laid out temporary conductors from each of the three phases supplying the motors and from the motor leads, and stripped them so as to route the power through the water in each of the three pans (one phase per pan). He, then, closed the motor contacts and began to sprinkle salt in the pans. The motor began to slowly turn allowing him to determine rotation.
 

eric stromberg

Senior Member
Location
Texas
As far as the original post is concerned, Roland answered it.

Water lowers the resistance of human skin. So, according to the IEEE, humans represent a resistance of about 1000 Ohms (with dry skin). I don't know what the resistance is with wet skin, but it is much lower.

Eric Stromberg, P.E.
 

hardworkingstiff

Senior Member
Location
Wilmington, NC
I wonder if anyone ever did a test using tab water (with contaminants) verses pure H2O (with no contaminants) to see what resistance changes there might be in the skin.

Just doesn't make natural sense, water (in a pure state not being a conductor) makes the skin conduct electricity better.
 

bikeindy

Senior Member
Location
Indianapolis IN
Reading this makes me laugh. Of course water in a pure form is not a conductor. Once it is on a person it alows an easy path through the skin to the internal organs of the body because it absorbs the salts of the body. To electrical current flow you may as well be a bowl of salt water as your body has the same content of salt as the ocean. As stated by others it is not the water that is the conductor it is the salts and other impurities which the water helps bring closer and increase the conductivity of those elements.

By the way there are stupid questions. Once had a guy ask of our instructor in an electronic theory class, "How do the electrons 'know' which is the path of least resistance?" classic stupidity.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
Interesting article on a guy who killed himself with a 9 Volt battery

I am very skeptical to say the least. I don't think a 9 volt battery would kill someone even if the probes got into the mussle in someone's thumbs. Directly across the heart could definately kill someone, but that's quite a bit different.

Although the IEEE may say the resistance of dry skin is 1000 ohms, I think that's probably at line voltages or higher. If you hold multimeter probes, you will probably measure 100's of Kohms, or a few megaohms. The resistance of things like skin are non-linear - higher voltage means lower resistance. So the 1000 ohms at 120-480 volts may actually be 1 megaohm at 9v.

Steve
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Skin resistance is the only thing that prevents death from lower voltages. It is indeed quite possible to kill ones self by application of only a few milliamps. A nine volt battery can easily produce that kind of current if the skin is broken.

What makes you think resistance is nonlinear with voltage?

steve66 said:
Interesting article on a guy who killed himself with a 9 Volt battery

I am very skeptical to say the least. I don't think a 9 volt battery would kill someone even if the probes got into the mussle in someone's thumbs. Directly across the heart could definately kill someone, but that's quite a bit different.

Although the IEEE may say the resistance of dry skin is 1000 ohms, I think that's probably at line voltages or higher. If you hold multimeter probes, you will probably measure 100's of Kohms, or a few megaohms. The resistance of things like skin are non-linear - higher voltage means lower resistance. So the 1000 ohms at 120-480 volts may actually be 1 megaohm at 9v.

Steve
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
This is from:

http://www.cdc.gov/niosh/docs/2002-123/2002-123b.html

Currents greater than 75 mA may cause ventricular fibrillation (very rapid, ineffective heartbeat). This condition will cause death within a few minutes unless a special device called a defibrillator is used to save the victim. Heart paralysis occurs at 4 amps, which means the heart does not pump at all. Tissue is burned with currents greater than 5 amps. 2

You aren't going to get 75ma from a 9 volt battery connected to opposite hands. Maybe if it were connected directly across the heart, but not through both arms. I think a 9 volt battery might even have a hard time pushing 75ma through a dead short for more than a couple of seconds.



What makes you think resistance is nonlinear with voltage?

Higher voltages break some of the chemical bonds, and strip electrons off some atoms, creating ions and electrons that can move. I can't find a reference offhand, & I've forgotten most of the stuff I learned from chemistry, but I think that's the general idea.

Once again, if you reach the breakdown voltage, even the pure water would conduct.
Steve
 

DGrant041

Senior Member
Location
Peoria, Illinois
steve66 said:
I am very skeptical to say the least. I don't think a 9 volt battery would kill someone even if the probes got into the mussle in someone's thumbs. Directly across the heart could definately kill someone, but that's quite a bit different.

Steve

STEVE! STEP AWAY FROM THE SIMPSON! :lol:
 
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