Boat Dock Tingle Issue

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  #1  
Old 08-24-11, 08:51 AM
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Boat Dock Tingle Issue

Last year, I had an aluminum boat dock installed. This dock was recently wired in the following manner. A three wire cable (12/2 with Ground, direct burial) was run from a little used circuit in my house that has a 20 Amp breaker. I installed a small, metal, weather resistent box (available at Lowes or Home Depot) that contains a 20 Amp GFCI and a 20 Amp switch. The box also came with a small grounding lug attached to the box. The GFCI and switch are both grounded to the box. This box in located on a pier that is on shore. My three wires were connected as follows. Hot and neutral to line side of GFCI with ground connected to the lug. From the load side of the GFCI, I ran a hot to the switch. To the dock, I ran another 12/2 with ground in conduit to an outlet and set of lights. The ground is connected (via the lights) to the aluminum structure of the floating dock. In the box, the hot is connected to the switch, neutral to the load side of the GFCI, and the ground to the lug. I drove a separate ground rod that connected to the lug in the box along with the ground from the house and the ground from the dock.

When connected as above noted, I get a tingle when I am swimming in the lake and touch the aluminum dock structure. When I turn off the power (at the switch and/or breaker), the tingle persists. Following an investigation and discussion with the power company, I have determined that there is a stray voltage of 2-3 volts on my service neutral that is connected to my house ground which is, in turn, connected to my dock grounding system.

I can eliminate the tingle by separating the house ground from the dock ground. In this situation, I basically disconnected the ground wire from the house from the ground lug in the box. The dock wiring continues to be grounded via the ground stake that was driven at the pier as the dock ground wire is connected to the lug in the box as is the ground stake.

The bad news is that I believe the code (NEC) requires that the ground connect back to the main service ground and neutral which is the source of this problem. I have talked to the power company and they tell me that 2-3 stray volts on the neutral is not unusual and is not a concern (or their problem).

My question is, apart from the code issue, are there any safety concerns with my current wiring setup. My thinking is that the GFCI will provide protection from any type of fault associated with the dock wiring and the hot wire will continue to be protected from an overload condition by the 20 Amp breaker in the house.

Any thoughs would be appreciated.
 
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  #2  
Old 08-24-11, 09:53 AM
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I agree with the power company's assessment that this is stray neutral current and not related to the hot part of the 20A circuit. The trouble is that this can be difficult to deal with, especially if you have an overhead electrical service with a steel neutral conductor. It is actually more dangerous than a hot circuit because there is no GFCI or breaker to limit the flow of current, and there is no disconnect switch to cut it off in the event of an emergency.

Where you need to focus first is on the earth grounding of the house electrical service to try to shunt as much as possible to the earth near the house. Right now you probably only have one ground rod or just a water pipe grounding the service (which is the code minimum). Once the dock was installed in direct contact with the water, you created a path that is lower resistance to earth via the copper wire to the aluminum dock to the water so current is flowing out through the dock instead of through the ground rod at the electrical service.

A good place to start on this is to drive at least two or three 10' copper clad ground rods spaced 6-8 feet apart up near the electrical service on house, but in a spot where the soil will be moist (not under an eve). Run a continuous #6 bare copper wire from the main panel ground bus to the first rod, loop through a brass acorn clamp and continue the next rod and so forth. Drive the rods flush or just under the grade.

Note that my recommendations are above code which are necessary in your case due to the unusual circumstance.

I am also making the assumption that while the power company was there, they checked the meter box and other connections for good contact? If not either they or an electrician should check out the main panel neutral, meter neutral and weatherhead neutral to make certain there isn't a loose connection which would make this problem worse.
 
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Old 08-24-11, 11:00 AM
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Thank you for this response. I left out some information concerning the investigation that was performed to determine the problem. Yes, the power company verified the connections at the meter box with no problems noted. I also talked with a stray voltage expert with another power company that told me that this issue is a big problem around the lake and gets worse in the Summer due to dry conditions. He indicated that additional grounding to solve this issue has not been effective and would probably not eliminate the problem. He mentioned a "Ronk Blocker" to isolate the primary from the secondary neutral at the power transformer (I have underground utilities). He also mentioned using a 3 prong plug that could be unplugged while swimming (this would meet NEC code requirements). Another interesting suggestion was to bond the aluminum structure of the dock to the water by dangling grounding rods off the structure of the dock (to equalize the potential between the dock and the water).

In any case, before I get to innovative and create a real safety problem, I am wanting to determine if my current configuration (with the GFCI and the grounds between the house and dock separated) is safe for personal and hardware protection. I know the NEC wants the grounds tied together but that is what is causing the tingle and I do not feel that it is safer than my current setup.
 
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Old 08-24-11, 11:52 AM
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Originally Posted by hikersc View Post
that this issue is a big problem around the lake and gets worse in the Summer due to dry conditions.
Believe it or not I have encountered this problem a couple times around dairy farms. Cows are very sensitive to stray current in the ground and their milk production decreases when there's electrical trouble.

There was also a thread a couple months ago on this forum by a guy who lost a couple well pumps suspected to be from stray current problems.

He indicated that additional grounding to solve this issue has not been effective and would probably not eliminate the problem.
The power company could fix this problem, but it's not cheap and they probably aren't technically required to do it.

He mentioned a "Ronk Blocker" to isolate the primary from the secondary neutral at the power transformer (I have underground utilities).
Yes that would be a reasonable option.

He also mentioned using a 3 prong plug that could be unplugged while swimming (this would meet NEC code requirements).
That may be the best thing to do given how simple and cheap it would be. You could also replace the dock lights with solar powered or battery powered ones. It might be worth thinking about instead of dealing with the root problem.

Another interesting suggestion was to bond the aluminum structure of the dock to the water by dangling grounding rods off the structure of the dock (to equalize the potential between the dock and the water).
I had a similar thought. I was going to recommend driving a ground rod or two into the lake bed and bonding the dock to that, but really that only encourages more objectionable current to flow through the dock into the lake instead of where it should be. It also would be somewhat inappropriate to put that kind of grounding on your electrical system bonded only through a #12 copper wire. A minor lightning storm could really tear it up.

I am wanting to determine if my current configuration (with the GFCI and the grounds between the house and dock separated) is safe for personal and hardware protection.
In my opinion it is not safe due to the stray current issue. It has nothing to do with the GFCI at this point, and the GFCI will not protect a swimmer from harm caused by a stray neutral current. With the ground disconnected, the breaker may not trip if a hot wire shorted and that presents a different danger to swimmers. I don't think either option is a good one.

What is the layout of the property like? Where is the house, electrical service, transformer and dock and approximate distances between each?
 
  #5  
Old 08-24-11, 01:14 PM
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Could you perhaps run a #6 wire down to your dock from the main panel and add some ground rods into the bottom of the lake in the area you'll be swimming in?


I think theres an open neutral on the primary side of the transformer.
 
  #6  
Old 08-24-11, 04:17 PM
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Why is the dock grounding through the lights?

There is another issue at play: If there is current at the dock and you have a boat, the electrical current will eat away at the drive unit, outboard, or metal parts of the boat. In as little as one season it can cause thousands of dollars in damages.
 
  #7  
Old 08-24-11, 05:26 PM
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Just don't connect the dock to power ground. Use a plastic outlet box if necessary.
 
  #8  
Old 08-24-11, 05:53 PM
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Could you isolate the dock from the ground?
 
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Old 08-25-11, 09:25 AM
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Originally Posted by drooplug View Post
Could you isolate the dock from the ground?
I think the only way to do that safely would be to remove the installed circuit entirely. All it takes is a tool with a grounded plug laying on the dock for you to be right back into the same problem. When a receptacle is only a few feet away, there's no way to make sure that an appliance is never used in a way that would create an unsafe situation.

Further by isolating the dock from ground you create an even worse situation whereby you have different potentials on adjacent conductive surfaces, which is the recipe for an accidental shock. This is the exact reason why equipotential bonding is mandatory in pools, spas, fountains, bathtubs, etc.
 
  #10  
Old 08-26-11, 01:51 PM
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Let me reiterate my current configuration because what I am looking for are specific failure scenarios that would create an unsafe condition for anyone swimming or otherwise using my dock. The configuration is a follows:

I have a 20 Amp breaker in my main panel in my house that feeds a circuit with a few outlets but essentially no load. I have connected this circuit through a 20 Amp switch on the hot lead to a 12/2 with a ground direct burial wire that runs about 120 feet to small metal, weather resistant box that contains a small grounding block, a 20 Amp GFCI, and a 20 Amp switch. This box is located on a wooden pier on the shore. The GFCI and switch are both grounded via the case to the small grounding block. The hot and neutral of the 12/2 with ground cable are connected to the line side of the GFCI. The ground is covered with heat shrink and taped to prevent contact with the box. Another 12/2 with ground is connected to the load side of the GFCI and then through weatherproof conduit to an outlet and lights (also weather proof) on my aluminum dock. The ground wire for the lights is connected to a water proof metal box that is screwed to the aluminum dock (bonding the dock structure to the ground wire). A separate 8 ground stake was driven at the pier and is connected to the ground wire of the circuit that is downstream of the GFCI. Therefore, the box, ground wire, dock GFCI and switch are all grounded to this separate ground stake.

I am aware that the code specifies that the two grounds be connected but doing so gives my dock about 3 volts AC that is coming from the power company neutral .

Here is my thinking on protection at the dock or in the water. Any wiring failure (i.e. hot to ground) or contact downstream of the GFCI should trip the GFCI and remove any hot (or neutral) potential. Since I plan to open the switch (at my house) while swimming, a shock would require the simultaneous failure of the GFCI and switch which I do not consider likely or even plausible. The second failure scenario would require the hot to contact the ground inside the metal box upstream of the GFCI. A failure of this type (although unlikely) could result in the dock being energized and would not be isolated by the breaker tripping at the house since the grounds are not connected. Again, this requires a simultaneous failure within the enclosed box in combination with a failure of the switch at the house. I also do not consider this scenario to be likely or plausible.

That said, I am looking for a plausible case scenario that would result a dangerous situation on my dock with the above noted configuration.

I do appreciate any and all comments received thus far. Lots of good thoughts and ideas. Exactly what I was looking for with this forum.
 
  #11  
Old 08-26-11, 08:01 PM
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The problem is the current is coming from the Primary side of the xfrmr. Any increase in load will incerase the voltage. It is then riding in through your neutral conductor. Disconnecting the hot to the dock will not make the voltage dissappear. Yet, disconnecting the grounding from the house and using a seperate ground rod will cause a more serious shock hazard due to lack of a low-impedance fault path.

The only thing I can forsee working is sinking some ground rods on the bottom of the lake in the area you'll be swimming in and bonding them to the main panel in the House.
 
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Old 08-27-11, 06:18 AM
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One thought: Is your dock in contact with the water?

BTW - A GFCI will trip if there is stray neutral current. Touching neutral to ground on the load side will cause it to trip due to an imbalance between the neutral and hot. In a perfect world, ground and neutral has the same potential, but this is rarely the case.
 
  #13  
Old 08-27-11, 07:21 AM
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The Ronk Blocker will solve your problem if the neutral to ground voltage is generated "off-site"; that is, on the far side of your feed transformer. Was the experiment done yet to find out where the unbalance is occurring? To do this, monitor the N to earth volts at the dock (you had reported 2 to 3), then switch off your master breaker. If the 2 to 3 volts goes away, then your loads are causing the unbalanced neutral current, and you may be able to do something about that with better phase management. If the 2 to 3 remains, the POCO is sending you the neutral voltage from their side, and the Ronk Blocker would be of value.
 
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Old 08-29-11, 06:49 AM
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As an update, the local power company came last week and confirmed the existance of a small (3 to 5 VAC) voltage on their neutral that feeds my house. Basically, they turned off the power from their transformer and I reconnected my grounds and we confirmed 3 VAC between the aluminum structure of the dock and the water (Note: This dock is floating on foam filled plastic and the aluminum structure does not contact the water except one small spot where my jet ski lift is installed.). They are currently working on a solution and plan to get back with me.

Although an interesting solution, I do not feel comfortable with equalizing the voltage by putting the 3 VAC into the water. There are other docks around my dock and I could see that this might create problems for my neighbors. I like the Ronk Blocker solution but will see what the power company comes up with.

In the mean time, I plan to leave my grounds separate pursuant to my post #10. Again, I understand the code requirement and the reason for the requirement. However, I do not see a shock potential without a simultaneous failure of the GFCI and the switch feeding the hot from my house. If someone sees a specific shock potential scenario with this configuration, please share it with me so that it can be addressed.
 
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Old 08-29-11, 11:53 AM
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Originally Posted by hikersc View Post
they turned off the power from their transformer and I reconnected my grounds and we confirmed 3 VAC between the aluminum structure of the dock and the water...They are currently working on a solution and plan to get back with me.
This was a very good test for them to do. I'm glad they confirmed the neutral current from their side of the system.

Although an interesting solution, I do not feel comfortable with equalizing the voltage by putting the 3 VAC into the water.
This is actually a good thing to do as long as it is done in the right way. Running the current through your dock is troublesome because it can severely corrode metal objects; however if it is done with the right combination of ground rods and bonding conductors it might be a good solution. Hopefully the power company will offer a better alternative.

I do not see a shock potential without a simultaneous failure of the GFCI and the switch feeding the hot from my house. If someone sees a specific shock potential scenario with this configuration, please share it with me so that it can be addressed.
The hots and the GFCI are completely unrelated to this -- in fact the power company test proved it. Even with all power to your house disconnected, there was still measurable voltage between the dock and the water meaning that a person climbing out of the water will have current flowing through their body. Any time you can feel a tingle there is current in your body and you have the possibility of a dangerous shock. It's all based on the path the current takes through your body, and if it happens to go through your heart or brain even a very small current can be deadly. Around water it is even more troublesome because shocks can lead to temporary paralysis resulting in passing out, hitting your head, etc and drowning.
 
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