boat lift shocking

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wesley1

Member
Location
Port St. Joe, Florida
Occupation
Electrical contractor owner
read previous post but wanted opinions on this situation. Boat lift in salt water had reports of shocking people in water that touched it. Aluminum eye beams with stainless cables is make up of lift that is attached to wood pilings. From salt water to i beam digital voltmeter reads 1.3 volts. Also checked contuniuty from outlet ground that lift motors are plugged into to the i beams and checked good. However this is the only ground. I am planning on installing #8 from panel at the end of dock ground to bond I beams better. None the less meter shows they are grounded to electrical system now. Checked grounded conductor in house power panel for loose or corrossion found in very good shape. Called PoCo to open meter and check ground connections for loose or corrosion- checked good. Panel at end of dock fed with 2 pole 50 amp gfi. When disconnect the neutral and EGC that feeds that panel in house pwer panel lose the 1.3 volts at boat lift to salt water. Turned all power off at residence and 1.3 volts remain. With the EGC and neutral that feeds the dock disconected at house pwr pnl can read 1.3 volts from the neutral/ ground buss in panel to EGC and neutral wire that feeds dock. Disconnected GEC in house pwr pnl and can read 1.3 volts between neutral/ ground buss and GEC, with or without meter in. PoCo pulled jacks that fed sevaral surrounding residences and the 1.3 volts stayed. When connect everything at house pwr panel 1.3 volts shows up at lift. Thougt 1 time felt slight tingle at lift but not sure. Any advise on what to do will be greatly appreciated. PoCo says 1.3 volt normal diferential. I am not sure but wont rest till i am. Thanks WES
 

SG-1

Senior Member
What you are describing is called NEV - Neutral to Earth Voltage also called Stray Voltage. The POCO (power company) is the likely source. To prove it is, the power company neutral must be lifted when the ungrounded conductors are opened. Keep in mind the neutral is actually energized with the voltage you measured.

After the neutral is lifted the 1.3 volts should go away, except... It may still be coming in on the telephone system or the cable TV. If lifting the POCO neutral does not eleminate it, unhook the other two systems - checking for voltage each time. The cable & telephone are connected to the same POCO as your customer. (One other source could be a metal water or gas pipe that is shared by the community, because everybody is bonded to it, then to the POCO.)

Hopefully by this time the voltage is gone and you have now proved it is the POCO. They already proved that it is not the neighbors when they pulled their meter bases.

Ask them to install a neutral isolator.

Isolators are made for the cable & telephone as well.

The problem stems from the fact that the POCO primary neutral is also the customers secondary neutral. The NEV will fluctuate during the day as the POCO power system sees more or less load. There is a ton of information on this site about this subject. Go to the Home Page then to Free Stuff and look for the Stray Voltage tab.

Hopes this helps to get you started.
 

don_resqcapt19

Moderator
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Location
Illinois
Occupation
retired electrician
If 'SG-1 " is right, you will need to get an isolation transformer. Make this a priority #1 job.
Frank,
How does an isolation transformer solve the problem? If it is installed per the NEC it will have a primary EGC that will be connected to the utility neutral and to the secondary EGC and the problem will still be there.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
091015-0716 EST

wesley1:

Try this experiment.

Get a piece of metal about 1' x 1'. Connect a #16 wire from the EGC or metal frame of the lift to the metal plate. Put the plate in the water. Measure the current to the plate.

You should be able to move the plate to different locations in the water with little change in the current.

Now at a position where a person might be in the water put your voltmeter from the water to the boat lift frame. Record this voltage as you move the EGCed plate to different positions in the water from many feet away from the voltage probe to near the voltage probe.

I do not know what to expect. Salt water is highly conductive. The relative resistances between earth and the water and the water resistance will determine what kind of measurements will result.

Your metal plate could be a large stainless frying pan. There is nothing critical about 1' x 1' I just want something of moderate area.

Note: when a human body is immersed in salt water the resistance from the water to the interior of the body is quite low. This is why a 1.5 V level will cause a tingle. Be very careful in your experiments. I would suggest that you be insulated.

If you see amperes of current from the power system EGC to the water there is a very major low resistance connection from the electrical source to earth and to water.

Indirectly from one reference it appears that sea water has a resistivity of about 0.2 ohm-meter. This means that if you measure the resistance from one face of a 1 meter cube to an opposite face the resistance is 0.2 ohms. For the same resistivity in ohm-cm the result is 20 ohms from one face to the opposite face of a 1 cm cube.

For comparison silver has a resistivity of about 1.59 * 10^-8 ohm-meters.
And distilled water exposed to the CO2 of air is about 13000 ohm-meters.

References:
http://en.wikipedia.org/wiki/Electrical_conductivity
http://hypertextbook.com/facts/2004/GeorgeBauer.shtml


.
 
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benaround

Senior Member
Location
Arizona
Frank,
How does an isolation transformer solve the problem? If it is installed per the NEC it will have a primary EGC that will be connected to the utility neutral and to the secondary EGC and the problem will still be there.

Don,

You got that right !! Sorry Wes, I didn't think that one through.
 

hardworkingstiff

Senior Member
Location
Wilmington, NC
I believe SG-1 has nailed it. If you lift the PoCo neutral at the meterbase the voltage will probably go away. I've seen this before and thought the salt water was a good enough path to pickup that 1.5-v as a parallel path back to the substation.

I wonder if PoCo would feed their transformer primary with two ungrounded conductors (instead of an ungrounded and grounded) if the problem would disappear?
 

wesley1

Member
Location
Port St. Joe, Florida
Occupation
Electrical contractor owner
Thanks for all the input sorry I dissappeared after asking question but were kinda behind and i'm sure ya'll know it is. We did lift the neutral at the meter and voltage went away. Having hard time convincing POCO there is a problem and kinda at loss with corrective action. Owners are out of town and we have disconnected EGC and Neutral feeding dock panel. Thus the 1.5 volts has dissapeared. I will be studying this as hard as possible. Do you think that maybe driving some 30' ground rods at meter would help any. Unfortunately I do not have ground tester, but do have access so I dont know if i'm less than 25 ohms or not. The house is on wood pilings with no concrete encased electrode so my gut tells me its probably higher than 25 ohms. The guy with PoCo seems to think at end of dock i should drive ground rods and tie to EGC, but I dont agree due to not seperatley derived system. This is why i love electricity you learn new stuff daily. Thanks again. WES
 

benaround

Senior Member
Location
Arizona
Wes,

A ground rod will never make voltage disappear, it will just add the voltage to the area of

the ground rod.

The problem you have is not going to go away. The voltage is coming in on the POCO

neutral and unless " they " do something, it is going to remain. I've heard the POCO will

not admit freely to it.
 

wesley1

Member
Location
Port St. Joe, Florida
Occupation
Electrical contractor owner
Benaround, thanks i agree with you. I am learning more about this, but still caint understand if evrything is at the same potential why does the voltage show up. I understand that a water heater element or many items could be leaking a current to ground at any residence on the same PoCO ground, but with all transformers grounded and GEC's at every transformer and meter base it seems to me that would be the path. Not down a #4 THHN EGC that travels 300' down a dock. I am reading the info that SG-1 referred me to on NEV. I will make sure that i can explain this to my customer, instead of just telling them its the POCO problem. Just when you think that your getting good you realize just how much you dont know. What is scarry is we live in a area surounded by salt water and our POCO knows less about this than i do. I am hoping that they see the danger of this situation. After reading the post that had a link to all the recorded mishaps with boats and boat docks i sure get the message. I just dont know what course to take other than to ask POCO for neutral isolater which i will do tomorrow. This should be good!
 

dbuckley

Senior Member
Do you think that maybe driving some 30' ground rods at meter would help any. ..... The guy with PoCo seems to think at end of dock i should drive ground rods and tie to EGC, but I dont agree due to not seperatley derived system.

Your PoCo engineer is right on the money.

Ground rods at the meter will at best make no difference, and at worst, increase the problem.

Ground rods by the water (or in the water) will reduce the problem by reducing the potential difference between the water and the EGC (which is what the boat lift metalwork is connected to) but because (somewhat annoyingly) electricity will travel by all possible paths you are most unliklely to completely eliminate the voltage.

The safest approach is non-NEC compliant - an isolating transformer without a bond between primary and secondry grounds. Secondary has a GFCI on it, and a local ground rod. Rock solid safe but non-compliant.

Under these (and certain other circumstances) the NEC compliant soloution is just plain unsafe. Sure, the folks are just getting a volt or two tingling them now, but imagine if there were a decent transient fault, causing the stray voltage to rise significantly...
 

wesley1

Member
Location
Port St. Joe, Florida
Occupation
Electrical contractor owner
dbuckley, that does make sense when you drive ground rods at end of dock it should reduce potential difference between EGC and salt water which is what were after. But is that illegal by NEC or was you referring to iso xformer without bond. I think that driving rods at end of dock is code violation but not sure. I am positive that they wouldnt last a year. Thanks
 

SG-1

Senior Member
Press the POCO for the neutral isolator if lifting their neutral makes the voltage disappear. No one will be electrocuted by swimming near by, but some one may panic and drown.

Measure the voltage between the service steel and the GEC coming down the POCO transformer. You should see that everything is not at the same potential. Actually you are seeing it now, but still can't believe you eyes. Or measure between the service steel and the earth several feet away. This is called a voltage gradient.

You know if you measure the voltage at a receptiacle between the neutral and the ground it will read almost zero if the circuit is unloaded. Now plug in a hair dryer and turn it on wide open. Voltage magically appears. This is simular to what is happening at the service. The POCO neutral is carrying load current. This is a I2 R loss & the farther from the substation you are the higher the NEV will be with all other factors being the same.

Gar or someone will bust me if I have strayed from the mark with my explanation.
 
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don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Your PoCo engineer is right on the money.

Ground rods at the meter will at best make no difference, and at worst, increase the problem.

Ground rods by the water (or in the water) will reduce the problem by reducing the potential difference between the water and the EGC (which is what the boat lift metalwork is connected to) but because (somewhat annoyingly) electricity will travel by all possible paths you are most unliklely to completely eliminate the voltage. ...
The addition of grounding electrodes in the water will not make the voltage go away. It will raise the voltage of the water in the area close to the grounding electrodes, but all that does is move the area of the voltage gradient in the water. The best solution is the neutral isolator installed by the utility.
 

ghostbuster

Senior Member
Here is an article on stray voltage that just appeared in the article section of M.H. forum.The stray current is coming up from the lake to the utility primary neutral.This will give you a full understanding of all these issues.:)



The Case of Stray Voltage in a Lake


October 1, 2009 By Donald R. Johnson, P.E., Johnson Engineering (donjohn@silverstar.com)
Faulty concentric neutrals on high voltage underground cables was the cause of one drowning death and brain injury to two others while swimming in a lake.


Stray voltage is a popular term resulting from electrical currents flowing through the earth, or other conductive surfaces not normally expected to carry electric currents. Small amounts of electric currents travelling through the earth are prevalent throughout the nation primarily due to electric utilities using the earth as a grounding medium for grounded wye distribution systems. Even though these grounded wye systems do carry a neutral conductor return current path since the neutral conductor is grounded to the earth at multiple locations, (as required by the NESC), the result is the earth is a parallel path for these currents. Typically, depending upon the conductivity of the earth and the amount of return neutral current on the electric distribution system, the amount of current flowing through the earth is small. As electric loads across the nation continue to increase, these earth currents are also increasing. These earth currents became noticeable many years ago on dairy farms. The dairy farmers noticed a significantly higher mortality rate among the dairy cows along with a major loss of milk production. The culprit was directly tied to the amount of earth currents flowing through the dairy facilities. Electric utilities across the nation have found out the hard way (through multi-million dollar lawsuits) that they must reduce these earth currents to non-damaging levels.


The Scene:
Six teenagers were enjoying a cool evening in an outdoor hot tub in a home near a small lake. Several of the teenagers decided to exit the hot tub and run out onto a small dock (owned by the homeowner) and jump into the lake.



Photo of the dock where the accident occurred.


The Accident:
The teenagers were swimming near the dock for several minutes when they began to notice electric currents flowing through their bodies. One boy who experienced the phenomena at the site describes the currents as causing him to lose muscle control such that he could no longer swim and he began sinking in the water. Several of the teenagers that were on the dock saw the swimming teenagers? plight and called to the adults in the house for help. Several of the adults came running out, called 911, and began diving into the water trying to retrieve the teenagers that were under the water (Note: It was determined during later testing that the currents in the water were intermittent thus not causing damage to the adults.). After multiple dives the adults were able to retrieve the two boys and one girl that were under the water. CPR was immediately administered until the paramedics arrived. After the three were taken to the hospital the outcome was that one boy drowned and the other boy and girl received brain damage due to lack of oxygen caused by near-drowning.

The Investigation:
The first thoughts by those at the scene after the accident was that the dock wiring was somehow faulty which caused the electric current to be in the water. My services as a professional electrical engineer were then engaged by attorneys representing the plaintiffs (who filed suit against the electric utility and others) to try to determine the cause of the electric currents in the lake water. After numerous investigations by me and other experts, it was determined that the dock wiring was not the cause of the electric currents in the lake water. I later determined through discovery from the electric utility that they had major corrosion problems with the bare concentric neutrals of their high voltage underground cables that were buried under and around the lake where the accident occurred.


The electric utility realized up to nine years prior to the accident, through concentric neutral testing procedures, that the bare concentric neutrals were in a serious state of deterioration throughout the lake area and the housing subdivision around the lake. Even though the electric utility realized the problems they were having with the deteriorated concentric neutrals in the area they didn?t seem to realize the importance of immediately fixing the problem. They began a multi-year program of replacing the bare concentric neutral cables with new jacketed concentric neutral cables in the area. However due to difficulty in installing new cable in two particular sections of underground cable under and around the lake, these sections were not replaced. It turned out that these two particular sections of underground cable that were not replaced were the primary paths for the neutral current return for the entire lake area subdivision back to the electric utility substation source. The result was that the electrical load within the subdivision was essentially on an ?island? in terms of an adequate return current path. Because of this ?island? effect the currents took the least resistant path into the lake water heading to the dock which was more than adequately grounded to the electric utility system overhead neutral which was then tied directly to the utility substation source.


To verify the above scenario numerous tests were completed by me during several site visits. During one visit a Fluke Model 289 recording multimeter was installed at the dock where the accident occurred and left to record for eight to ten consecutive days (The wiring to the dock was totally disconnected and removed at this time.). The voltages were recorded by placing a ground rod off the end of the dock into the lake water where one meter probe was attached. The other meter probe was attached to the house ground which was in turn attached to the electric utility ground. A 500-ohm shunt resistor was included in the voltage measuring circuit. The currents were measured using the same technique however the 500-ohm resistor was placed in series with the circuit. The results of the testing indicated that both the voltage and current levels followed the electric loads of the utility in that the load peaks were at the highest levels during the morning and evening hours (Note that the accident occurred during the evening hours when electric loads were at the highest.). In addition the frequency of all voltage and current measurements were measured at 60 Hz. This was a direct indication that the electric utility was the source of these measured voltages and currents. The highest currents measured during this test period reached .5 amperes and the highest voltage reached 6.2 volts.


During another visit to the site, with the electric utility present to allow access to their underground cable junction enclosures, numerous readings were conducted to measure the current on both the underground electric utility energized phase conductors and the concentric neutrals. Voltage measurements were also taken from the ground system in the enclosures to a remote ground. The results indicated that indeed the concentric neutrals in the two particular critical sections were likely non-existent. In one particular section right near the lake the neutral current on the concentric neutral was less than one-tenth of an ampere whereas the energized phase current was in excess of 6 amperes. Obviously the remainder of the return current was flowing through the earth and in this case the lake water. In addition the voltage measured from these same junction enclosures was in excess of 7 volts to a remote ground rod. These measurements were a clear indication that the concentric neutrals on these underground sections were likely absent due to corrosion.


The Conclusion:
Using an assumed human body resistance of 300ohms1 when immersed in fresh water and assuming a current range through the human body where muscle control is lost in the range of 6 to 30 milliamperes2 using Ohm?s law the voltage necessary to cause a drowning in fresh water is in the range of 1.8 to 9 volts, 60 Hz AC. The above testing results show that the necessary voltage and current levels were at a level well within the range to cause the drowning and near-drowning of the victims of this case.


This case went to a trial by jury and the jury awarded the plaintiffs a total judgment of $2,325,000. No appeal was filed by the electric utility defendant.


This case shows the dramatic effects of stray voltage especially in an area where people are exposed to stray voltages when in a wet environment. Electric utilities must be vigilant in maintaining their distribution systems such that stray voltages are kept at extremely low levels such that humans and animals alike are not exposed to these dangers. As electric loads continue to increase across the nation many experts are even encouraging electric utilities to modify their distribution systems so that the earth is not used as a current carrying medium.




1. ?Stray Voltages ? Concerns, Analysis and Mitigation? National Electric Energy Testing Research & Applications Center (NEETRAC) Manual Project Number 00-092, August 2002, Page 5-24.
2.http://www.osha.gov/SLTC/etools/construction/electrical_incidents/eleccurrent.html
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
091016-1329 EST

wesley1:

From your posts I believe I can conclude the following:

At the house there is a ground electrode of some sort. 25 ohms to ground is unimportant.

If your remove the power company neutral from the main panel at the house, then measure the voltage between the EGC at the dock and the water the voltage is near zero. This means there is no stray current in the earth from the house to salt water at the dock, or the resistance in the earth from the house to the water is very low. In any event there is no significant voltage.

However, you measure about 1.3 V from the power company neutral to the house neutral, EGC, and ground electrode.

If you reconnect the power company neutral to the house neutral, then all the reconnected conductors are at about 1.3 V above the salt water.

Under these conditions and with no loads connected in the house or dock how much current is flowing on the neutral from the power company?

I think in your local earth area there is relatively little stray current until the power company neutral is connected to the house neutral and ground electrode.

If you use an electrode, a few square inches of metal, in the sea water, and relative to this electrode measure the voltage of the earth at various points from the dock to the house ground electrode I suspect there will be little voltage difference until you get close to the house ground electrode. Most of the power company stray neutral current is producing a voltage drop across the resistance of the house ground electrode to the earth. There is not significant stray current from somewhere else flowing thru this yard.

If I am correct why is there power company stray current on their neutral to the house ground electrode?

This may mean they have a somewhat floating neutral to earth before getting to the house location. The house location seems to be somewhat of an end of line location and without the neutral connection there is no voltage drop from the house to the water. The power company may have a bad connection in their wire neutral path prior to this house, or bad ground connections, or other houses in the area may have bad neutral connections.

I think it is clear if you can get the power company to provide an isolation transformer to this house with no connection of the house neutral to their primary side neutral that the problem will be solved. If they don't provide isolation and they have a major fault, then they have a major liability.

I am lucky that I have a delta primary and my secondary center tap only connects to a ground rod at the pole and then to my water line and my single neighbor's water line. My maximum ground voltages are in the 100 MV range and partly are from other currents than mine.

.
 

dbuckley

Senior Member
The addition of grounding electrodes in the water will not make the voltage go away. It will raise the voltage of the water in the area close to the grounding electrodes, but all that does is move the area of the voltage gradient in the water. The best solution is the neutral isolator installed by the utility.
Absolutely true, but this is not necessarily bad - there will be a voltage gradient across the water, but that is different to the step potential difference of water versus metalwork. Lesser of two evils.

The neutral isolator is a better solution. The NEC being ammended to sort out this danger point would be the best solution.
 

hardworkingstiff

Senior Member
Location
Wilmington, NC
Absolutely true, but this is not necessarily bad - there will be a voltage gradient across the water, but that is different to the step potential difference of water versus metalwork. Lesser of two evils.

The neutral isolator is a better solution. The NEC being ammended to sort out this danger point would be the best solution.

What ammendment/s would sort this out?
 
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