Recent tragedy

[dreamsville]

Unfortunate event. It will be interesting to find out the facts of this incident after an investigation.

http://www.kansascity.com/2012/07/04/3690674/siblings-electrocuted-while-swimming.html

 

[kwired]

Unfortunate event. It will be interesting to find out the facts of this incident after an investigation.

http://www.kansascity.com/2012/07/04/3690674/siblings-electrocuted-while-swimming.html

Police SGT needs to let someone who knows electricity better make the comment of whether or not this is something unusual.

Sure it may not happen often but if you bring electricity near the water like is done around docks the risk of something like that happening increases quite a bit, and even more so if codes and standards are not followed.


It is unfortunate, and the chance of unqualified persons being involved in the electrical work is probably high.

I have been around a few lakeside areas and am willing to bet - especially away from public areas that there is code violations at more than 60-70% of the installations.

 

[K8MHZ]

This type of tragedy is not as rare as people think.

http://www.huffingtonpost.com/terry-gardner/esd-hidden-danger-in-fres_b_693454.html

 

[kwired]

From near the end of the article:

If you do nothing else, please write on the NFPA's Facebook Wall requesting they take action to make boats, boat marinas and river areas with electrical equipment safer

NFPA already has articles 553 and 555 in the NEC. Like any other article there is likely room for improvements, but this area has not been totally overlooked by NFPA.

If one does an investigation of nearly every incident by an investigator that is competent in the electrical hazards area, there is a good chance they will find something not in compliance with NEC.

The other situation that is outside of following NEC that happens sometimes is a voltage drop problem on POCO's MGN neutral conductor. The way we distribute power in North America makes this one harder to deal with, but it is a problem. You can have all power turned off to the dock and the voltage on the MGN is still there via the equipment grounding conductors of the installed feeders and branch circuits.

I don't buy the fact that this doesn't happen in salt water, I will buy the fact that more current is shunted around potential victims in salt water.

 

[charlie b]

This situation is not as easy as it might seem. Let?s start with the basics.

The person in the water is essentially a resistor that is in parallel with the resistance of the water. The voltage source is driving current through the one resistance (i.e., the water), before the person starts swimming. Once in the water, the person feels a shock because the resistance of the body has formed a current divider with the resistance of the water. Whichever of those has the lower resistance will get the greater share of the current. Since salt water has a lower resistance than fresh water, one would think that more current will flow around the person?s body in salt water than would flow around the person?s body in fresh water. So, at least, goes the theory.

The problem I have with this theory is that it does not tell the whole story. For example, since the resistance of salt water is lower, the total amount of current flowing through the water will be higher than you would get in fresh water. So even if the percentage of the available current that would go through the person?s body is smaller, it is a smaller percentage of a larger number. I just did a quick and simple circuit model, and the numbers told me that swimming in salt water was a higher danger. But that was just one set of numbers, and it might not represent a real-world situation.

If anyone wants to play with the numbers, the circuit model consists of a voltage source that is connected to, in order, Resistor 1, then the parallel combination of Resistor 2 and Resistor 3, and then this combination is in series with Resistor 4, which in turn connects back to the voltage source.

• Resistor 1 represents the resistance of the water that is between the victim and the voltage source.
• Resistor 2 represents the resistance of the victim?s body.
• Resistor 3 represents the resistance of the water that is in contact with the victim?s body in the direction of the current flow.
• Resistor 4 represents the resistance of the water that is beyond the victim?s location.
• The values of Resistors 1, 3, and 4 would be lower in salt water than in fresh water, but the value of Resistor 2 would remain the same.

 

[kwired]

This situation is not as easy as it might seem. Let?s start with the basics.

The person in the water is essentially a resistor that is in parallel with the resistance of the water. The voltage source is driving current through the one resistance (i.e., the water), before the person starts swimming. Once in the water, the person feels a shock because the resistance of the body has formed a current divider with the resistance of the water. Whichever of those has the lower resistance will get the greater share of the current. Since salt water has a lower resistance than fresh water, one would think that more current will flow around the person?s body in salt water than would flow around the person?s body in fresh water. So, at least, goes the theory.

The problem I have with this theory is that it does not tell the whole story. For example, since the resistance of salt water is lower, the total amount of current flowing through the water will be higher than you would get in fresh water. So even if the percentage of the available current that would go through the person?s body is smaller, it is a smaller percentage of a larger number. I just did a quick and simple circuit model, and the numbers told me that swimming in salt water was a higher danger. But that was just one set of numbers, and it might not represent a real-world situation.

If anyone wants to play with the numbers, the circuit model consists of a voltage source that is connected to, in order, Resistor 1, then the parallel combination of Resistor 2 and Resistor 3, and then this combination is in series with Resistor 4, which in turn connects back to the voltage source.

• Resistor 1 represents the resistance of the water that is between the victim and the voltage source.
• Resistor 2 represents the resistance of the victim?s body.
• Resistor 3 represents the resistance of the water that is in contact with the victim?s body in the direction of the current flow.
• Resistor 4 represents the resistance of the water that is beyond the victim?s location.
• The values of Resistors 1, 3, and 4 would be lower in salt water than in fresh water, but the value of Resistor 2 would remain the same.

Grabbing a metal ladder to climb out of the water could possibly put resistor 1 in parallel with resistor 2 depending on where/what the voltage source actually is.

 

[augie47]

http://abcnews.go.com/US/missouri-t...-kill-children/story?id=16714407#.T_YNG_XHn2k

Additional coverage also shows a similar tragedy in TN during the same holiday.

 

[charlie b]

Grabbing a metal ladder to climb out of the water could possibly put resistor 1 in parallel with resistor 2 depending on where/what the voltage source actually is.

True, given that the electrical problem might be in your own boat, or in someone else boat, or on some land-based equipment. In fact, one of the characteristics of these situations is that the owner of the boat causing the problem might never become aware that their boat was the culprit.

 

[jmsokol]

True, given that the electrical problem might be in your own boat, or in someone else boat, or on some land-based equipment. In fact, one of the characteristics of these situations is that the owner of the boat causing the problem might never become aware that their boat was the culprit.

Guys, I actually know a lot about how this happens having worked closely with David Rifkin, who provided the technical information for this MSNBC article just published today: http://usnews.msnbc.msn.com/_news/2...ible-killer-electrical-shock-to-swimmers?lite

Basically, it only takes about 2 volts per foot gradient in water to paralyze a swimmer, and that quickly leads to a drowning death. If you extend your hands out 5 feet, then the 10 volts from finger tip to finger tip can exceed 10 mA current through your body, which is enough to cause paralysis. This is why you can be shocked unconscious in the water even if you're not directly touching a conduit or electrical box with a hot-skin condition on a boat dock. This gradient field produced shock potential is mostly a freshwater problem and can extend out dozens feet from a dock or boat hooked to shore power. Under proper freshwater conductivity conditions a 120 volt potential hot-skin on a dock or boat could reach out 60 feet and still meet the 2 volt per foot gradient where a swimmer can be paralyzed and drown.

Since salt water is highly conductive, any hot-skin voltages will quickly be "grounded" via the low resistance of the water, thus the volts per foot gradient is too low to paralyze you. However, you can still receive a deadly shock while reaching up from the saltwater and touching an outlet box or boat that's hot-skin due to a wiring problem, but it's unlikely you'll be shocked out in the salt water itself.

I'm now conducting experiments with David Rifkin to determine if a NCVT like a Extech DV50 will indicate a gradient voltage in the water beneath you while standing on a wood dock or in a boat hooked to shore power. Stay tuned for more info in a few weeks.

In the meantime, if any of you would like to confer directly with Dave Rifkin about how this all works or specific water shock situations you've witnessed, (he's the expert on it) PM me and I'll hook you up with him.