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[John Roberts {JR}]

The human exposure thing is tricky as the possible effects depend on far more than just current. I thought it was neat that the level needed to get the slightest flicker from a 40 watt bulb was 10 times larger than what I would get from soaking an untreated hand in distilled water. You have to consider the resistance of the bulb in series with the resistance of the water. The meter is a far better indicator of conductivity.

The resistance of the bulb is probably rather low (ohms). To pull 1A from 120V requires 120 ohms of effective resistance. To pull 5mA from 120V only requires 24,000 ohms. So you can get a shock without enough conduction to make much light.

Yes human risk profile is complex, but they generally characterize it as current (single digit mA, assumed at the heart). Importantly the human body resistance is also in series with any shock hazard. A significant fraction of human resistance is the outermost layer of relatively high skin resistance ( our core is more like low resistance salt water). Dry skin is higher resistance than wet skin, so sticking you hand in a conductive water solution is especially risky since it reduces the impedance of this higher resistance part of our total human conductivity.

While pure water may be an insulator, the standing water in my yard from a leaking water main was adequately conductive for me to get a shock from a mis-wired extension cord. The flight attendant in China who died when her smart phone battery charger fell into the bath tub with her must not have been bathing in pure water. The water we typically encounter should be considered a shock hazard unless we determine otherwise.   

It also suggests that my stills are ready for a good cleaning.

I knew an electrician who tested for hot by licking a finger and tapping the black wire. He did it for 30 years with no problem. Then one time it caught his heart at precisely the wrong spot in its rhythm and knocked him flat on his back.

Generally we don't need to lick our fingers to feel 120v unless they are very callused (I've had my share of 120V stingers with dry hands). I suspect your electrician friend inadvertently was exposed to another path to ground so the current flowed from his hand through his core instead of just flowing from finger to finger in the same hand. I hope he wasn't seriously injured, and figured out what he did wrong.   

JR

[Jay Barracato]

The resistance of the bulb is probably rather low (ohms). To pull 1A from 120V requires 120 ohms of effective resistance. To pull 5mA from 120V only requires 24,000 ohms. So you can get a shock without enough conduction to make much light.

The light bulb tester was just a visual. My point is that there is a giant gap between the 24K-ohms you mention and the 18 M-ohms of pure water. In other words, plenty of room for the video trickery originally mentioned in this thread.

(single digit mA, assumed at the heart). 

That is a big and variable assumption; in other words, another case of a complex situation reduced to a single value which may or may not be informative.

The water we typically encounter should be considered a shock hazard unless we determine otherwise.   

I never claimed otherwise. I don't know enough about the China case to know if the shock was the AC side of the charger or the DC

I suspect your electrician friend inadvertently was exposed to another path to ground so the current flowed from his hand through his core instead of just flowing from finger to finger in the same hand. I hope he wasn't seriously injured, and figured out what he did wrong.   

JR

The difference in possible paths is one of the variables I was talking about. While I would not want to trivialize any safety issue, the general public is far more likely to over guessimate the conductivity of water than to underestimate it. The other missing piece is looking at these things as a system. In your extension cord in a puddle example, there would be a great difference between simply dropping the end of a cord in a puddle, and having an exposed connection in a puddle while some piece of equipment was operating on the circuit.

[Mike Sokol]

I knew an electrician who tested for hot by licking a finger and tapping the black wire. He did it for 30 years with no problem. Then one time it caught his heart at precisely the wrong spot in its rhythm and knocked him flat on his back.

One of the electricians I worked with as a young EE would rest his thumb on the grounded box and brush a wire with his fingertip. Claimed he could tell 120 from 240 or 480 volts that way, depending on how high his hand jumped. Seemed like a suicidal trick at the time, but apparently he got away with it for decades. Of course, if his foot was wet on the concrete and he didn't have a firm connection on the grounded box with his thumb the current could go through his heart and kill him, so I told him to stop doing it or I was going to report him to my OSHA contact. 

[John Roberts {JR}]

The light bulb tester was just a visual. My point is that there is a giant gap between the 24K-ohms you mention and the 18 M-ohms of pure water. In other words, plenty of room for the video trickery originally mentioned in this thread.

I am just very uncomfortable about any inference that water and electricity can mix, if you spray some who-hah on your power tools.  I remember the high school science class demonstrations with distilled water. While they may have added acid (lemon juice) instead of salt to make it conductive.

That is a big and variable assumption; in other words, another case of a complex situation reduced to a single value which may or may not be informative.

I am too lazy to spell out the long version where there are thresholds first for feeling a shock, then a painful shock, then possible heart fibrillation, then likely heart beat disruption etc. With the additional variable that different people will have stronger or weaker heart response based on health, or genetics, or age, or whatever.

5 mA is a low number and generally ballpark for GFCI/RCD type current threshold.. Medical devices are even lower (ASSuming sick patients) more like 1mA. To stop your heart dead you probably need 50-500 mA, more or less. Since voltage and skin resistance will also make a huge different in how much current you will actually experience your mileage may vary..

Just for chuckles grab two probes of a VOM one in each hand. Then squeeze the probes harder and see how the resistance changes. For a fixed voltage this means the pressure of the contact you make, and also the area of contact will modulate the exposure current greatly.

I never claimed otherwise. I don't know enough about the China case to know if the shock was the AC side of the charger or the DC

I am pretty sure it was the AC. What is a smartphone battery a few volts?  Apple did a huge public free recall-swap on chargers over there even though it is likely that she was killed by a counterfeit knock-off charger, not an apple. I doubt we ever will get a complete investigation into this and apple is managing it more like a PR problem than an actual product safety/design issue. 

The difference in possible paths is one of the variables I was talking about. While I would not want to trivialize any safety issue, the general public is far more likely to over guessimate the conductivity of water than to underestimate it. The other missing piece is looking at these things as a system. In your extension cord in a puddle example, there would be a great difference between simply dropping the end of a cord in a puddle, and having an exposed connection in a puddle while some piece of equipment was operating on the circuit.

Well my (borrowed from a neighbor) killer extension cord was mis-wired with hot and ground swapped on one end, My 3 wire grounded sump pump that I dropped into the mud hole full of water around my leaking water main, had it's grounded chassis now energized to 120VAC. I was just standing by the muddy hole not in it, as i have respect for the combination of water and electricity.   I still got the shock through my feet as the current flowed back to the service panel ground 60' away. I was holding the cord that I had just plugged in so I quickly unplugged it. At first I thought the sump pump was seized up, but I fairly quickly determined that it didn't work because the extension cord was mis-wired. I guess i should have metered my own power  . How often do you see an extension cord with hot-ground swapped?

Respect water around electricity.. i would prefer that people be over careful, than not careful enough. I was not careful enough, I should have checked the borrowed extension cord, especially before using it in my muddy yard. 

JR

[Jeff Bankston]

my landlord works for ladwp. he told me long time ago they use distilled water to clean live power stuff like insulators on stuff at power stations. he said its the metals in water that conduct electricity and when you remove all the metals and anything else that conducts electricity it will not conduct water.

[Jay Barracato]

It is not just metals, it is any particle with a charge. Hydrochloric acid (muriatic acid) in water is a great conductor without a single metal present.

Sent from my DROID RAZR HD using Tapatalk

[Jeff Bankston]

It is not just metals, it is any particle with a charge. Hydrochloric acid (muriatic acid) in water is a great conductor without a single metal present.

Sent from my DROID RAZR HD using Tapatalk

yeah he didnt go into a lot of detail and just said anything else that conducts.

[Keith Broughton]

I know!!!
You can spray this stuff on Monster speaker cables and such to stop the electrons from escaping into the air.
Makes the stereo sound even better

[Mike Sokol]

Respect water around electricity.. i would prefer that people be over careful, than not careful enough. I was not careful enough, I should have checked the borrowed extension cord, especially before using it in my muddy yard. 

One of the most dangerous shocks can occur in freshwater, most notably near boat docks. If there's an energized conduit at the dock, or a boat in the water that's not properly grounded with a "hot skin", then there's a gradient electrical field that can reach out dozens of feet from the boat dock. Since it only takes about 20mA of current to paralyze your muscles, as you swim closer to an electrified boat dock, your arms get harder to move until they eventually lock-up and you slip beneath the surface of the water and drown. Since these victims are not actually "electrocuted" it seems that many of these deaths are reported as drownings rather than electrocutions. There's a really good report about this "shock drowning" phenomenon written by one of my engineering buddies Dave Rifkin on Mike Holt's forum: http://www.mikeholt.com/newsletters.php?action=display&letterID=1309

Interestingly, this voltage gradient really doesn't exist in salt water since the water is so conductive that it instantly "grounds" any fault currents. Also, you can change how much freshwater fault current is going though you body by putting your arms straight out like an antenna, which increases the current through your heart. 

These same voltage gradients can exist near downed power lines on the ground which is why they tell you to keep your feet close together if you have to jump from a vehicle with a downed power line draping it. Of course, you should stay in your vehicle if that occurs and wait for emergency rescue. But if your car is on fire you'll probably want to jump free of the vehicle while keeping your feet together when you land. Just spreading your feet apart on the ground near downed power lines can result in electrocution currents foot-to-foot. Yikes!!!

Bottom line - water and electricity don't mix! 

[John Sabine]

The danger of this type of demonstration is that people think, just as my friend did, that they can't portray anything in an advertisement that could put someone in danger because of liability issues.

[ProSoundWeb Community]