Reply from: Jim Shafer
Don, You have a lot of knowledge I do not have but after working on this problem for over 8 years I have a developed a list (the only such data base in existance) of over 120 accidents, including 42 fatalities, caused by faulty bonding on boats AND docks. "Stray Voltage" from wooden docks and fiberglass boats DOES kill! You are welcome to my list : kp2r@bellsouth.net - and so is everyone else. This Newsletter has published the list twice in the last few years.
For your information I'm investigating at this time an "electric shock drowning" that occured at the end of a metal frame wooden dock last fall. I've acted as expert witnes in over a dozen of theses cases.
Mike - Now 10 days since by-pass surgery, doing fine and thanks for your prayers
Jim
Reply to Jim Shafer Reply from: Don Hartley, Hartley Electric, NY
You are right Jim the source could have been the dock if it had power on it and wasn't properly wired. One would think though in this case that the lack of concentric neutrals here would point more toward the power line in the lake than the dock, as the source. Also it would be so easy to shut off the dock power( if there even is power to this dock) to determine the actual source of the stray voltage in the first place.
Reply to Don Hartley, Hartley Electric, NY Reply from: Don Johnson
Don,
We did check the dock thoroughly and there were no problems with the wiring. Also the initial stray voltage measurements were taken just a few days after the accident by the electric utility. They measured instantaneous voltage measurements of 4.2 volts from the dock ground to the water. These measurements were taken during the middle of the day when the load currents and the resulting stray voltages were at the lowest. Then 10 days after the accident the electric utility placed a recording voltmeter and recording ammeter in numerous locations including from the water to the dock grounding system. During this recording period they measured voltage surges up to 8.6 volts and current surges up to 1.4 amps. All of these measurements were taken with the dock energized. Approximately November of 2006 the dock wiring was totally removed and the dock was even moved to a slightly different spot approximately 20 feet north of its original location. During my measurements the following year I measured up to 6.2 volts on my recording voltmeter using a 500 ohm shunt resistor from a ground rod stuck off the end of the dock into the lake water and bed back to the electric utility ground. This measurement occurred during the evening hours when the electric load was the highest. There were also multiple measurements well above 5 volts. Since the dock wiring was not even installed these stray currents and resulting voltages could not have been coming from the dock. As I mentioned previously using the human body resistance of 300 ohms when immersed in water and using the starting level of current through the human body necessary to cause loss of muscle control of 6 milliamps as specified in OSHA tables, the resulting voltage necessary to cause loss of muscle control is only 1.8 volts. As you can see my measurements were well above that level and the only possible source was the stray currents in the lake. It was also very evident from numerous other measurements that I made all around the lake that the lack of concentric neutrals in two strategic locations under and near the lake was the cause. These measurements were shown to the jury and obviously they were convinced that the electric utility was at fault. The electric utility continued to blame the dock for these currents even though we showed the same high currents and voltages even without the dock even being wired. Jim Shaefer is correct in that improperly wired docks can be a cause of stray currents in the water, resulting in electric shock drownings, but stray currents as a result of poor electric utility concentric neutral return current paths can also be a cause. Such was the case in this instance.
Reply to Don Johnson Reply from: Don Hartley, Hartley Electric, NY
And you would have convinced me had I been on that jury. Once the dock power was eliminated from the equation ( proven safe and wired correctly) there can be only one other source, the line in the lake. It really boils down to basic troubleshooting and using the process of elimination. You do great work by the way.
Reply to Don Hartley, Hartley Electric, NY Reply from: Jim Shafer
Don, Very good informaton but I have a comment.
We spent a year under a USCG grant to study the phenomenon that caused the 40 fataities we had observed and listed ("electric shock drownings" in fresh water - our coined term). The study is 165 pgs., rather long but quite complete, and with full references. I beleive the USCG would make a copy available or I can mail one for an out of pocket cost of about $45.
Based on quite a bit of available literature the nominal voltage gradient (some may say step voltage) of two volts per foot will induce the "let-go" current level in a semisubmerged (swimming) child, cause muscle paralysis, and immediate sinking in fresh water. I have investigated about 6 of these.
The absolute voltage from a ground to any random point in the water is not really relevant, although those are the only type of measurements you mention.We must know the gradient.
Between two electrodes in fresh water the field looks like a foot ball with the gradient becoming greater as you approach each electrode. (note that the current between any two electrodes depends almost entirely on the water contact area - we have plenty of data on that).
Consider this - If you had a very large electrode ( the length of the damaged and shreaded cable neutral) this would represent a very large contact area and would exibit a very LOW GRADIENT anywhere along the lengh of that electrode, and likely not lethal.The same applies to virtually anywhere on the lake.
If the dock, on the other hand, had a good bonding system to the utility (as was stated) and it was connected to the dock frame or ladder this would repesent a very small electrode, by comparison, and almost surely have a close-in lethal gradient area (2 v/ft - small end of the foot ball). We actually simulated this in more than 6 different tests detailed in our report, and it is a fact.
While ALL of our listed fatalities were caused because of faulty bonding here is the exception - if the dock had NOT been bonded, as stated, the swimmers would likely have only experienceded some discomfort, as many people do in home pools and outdoor showers (cows also if you have studied that problem).
There is a lot more to say about this problem and anyone is welcome to ask me questions - but i don't type so expect short answers.
I have spent over 8 years reseaching this specific problem.
Respectfully Submitted- Jim Shafer (kp2r@bellsouth.net)
Reply to Jim Shafer Reply from: Don Johnson
Jim,
I appreciate your comments on this. And I'm sure you're correct in your analysis. In this case the dock was very well grounded and when the electric utility first measured the 4.2 volts from the water to the dock frame the distance was approximately 2 ft. away from the aluminum dock ladder which was in the water according to the persons that made the measurement. I understand that they merely stuck their meter probe directly into the water and touched the other probe to the dock frame. Based upon these statements it appears that this measurement was within this 2 volt per foot gradient. Again remember that this voltage measurement was during the low load time of the day. When the utility put their recording meters on the dock, as I mentioned previously, the voltage level reached nearly 9 volts during the evening hours, again approximately 2 feet from the dock frame with the dock frame still well grounded. In each case the highest voltage readings were during the evening hours when the utility load currents would be at the highest. The teenagers as you will recall were swimming during the evening hours approximately 7:30 p.m. One thing I might mention is that in literally all of the readings, both recorded and instantaneous readings including the readings taken by me, the voltage and current readings were in a constant state of fluctuation. Based on my experience these constant fluctuations are typical of stray voltages usually caused by the electric utility. There was also considerable evidence described by the teenagers in the lake that when they first jumped in the water they felt nothing. However after several minutes of swimming around the dock they started feeling these surges. One boy described the currents as occurring in short pulses and then they went away and then they occurred again, constantly off and on. Maybe Jim you can tell me, when the dock wiring is a problem in your experience does it occur in pulses as these kids described and as I measured from a fixed ground rod in the lake? It would seem to me that if somehow the dock was energized, and if for some reason the circuit breaker protecting the dock did not trip, the current into the water would be more of a continuous flow instead of intermittent as was described in this case. What has been your experience?
Don
Reply to Don Johnson Reply from: Jim Shafer
Don,
Seems everything about this incident fits our data, eventhough our work was all done with a controlled source, not a remote source, as in this case, but the conclusions all fit.
Several possibilities for the shock reports by the victums. We have only one case where we interviewed a survivor - good story (this victum was subjected to over 6 V/ft and how he survived is interesting), call me. Body orientation and proximity to the ladder VERY critical. The gradient axis to the source and body alignment could mean the difference between mild discomfort and paralysis. 10 feet from the ladder you might feel very little sensation.
The field intensity was entirely dependent on system load and that is the only explaination for the reported pulses - other than the possible compounding of the effects stated above.
A breaker will NEVER trip under conditions that cause these accidents - that's why we have GFCI's in bathrooms, etc.If you look at our study you will see that fault currents into the water as high as 45 A may not be lethal (in close proximity), and yet we set up conditions that demonstrate fault currents as low as 2 A into the water from the prop on a boat will be lethal several feet away. I can't give you all the info in our 165 p report here. A least 6 factors are involved in each situation so only general statements can be made about this inicident - but all of the known data does fit our accident model.
Hope this is of some help.
Jim
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