Boat Dock hazard.

[WCEI]

Situation: large fresh water lake, (17 miles long).
floating aluminum boat dock.

Boat dock was originally installed a couple of years ago with no power installed.

Customer had us install a few lights and 120 volt receptacles. Installation was installed to code and inspected.

Customer started reporting a tingling sensation when he was in the water and reached within a few inches of the aluminum ladder. Prior to installation of power, he had not noticed this sensation.

We entered the water and could feel the same sensation. Of course we disconnected all power to perform this test. But with the power on or off, we could feel the same sensation at the same intensity.

There are no other docks within close proximity of this customer’s dock.

However, the local power utility company has a set of high voltage transmission lines crossing this body of water approximately 400 yards away. ( I say this because a few miles away, a different power company has an even larger set of transmission lines that run adjacent to a major county sports complex. The parking lot is partially under these transmission lines and the steel parking lot light poles will give you a good jolt if you are grounded. Before anyone ask, we tested this with the power off to the poles. It’s obviously induced current from the transmission lines).

The customer assumed that the problem was with our electrical installation, since he did not have this problem prior to our work. Honestly, we kind of feared the same thing. But after quadruple checking every inch of the installation, and replacing all GFCI devices, we felt confident that the problem was not with the installation. We believe it became noticeable after our installation because prior to our work, there was no affective ground path between the floating dock and earth ground. We are thinking that the dock and water were previously at the same potential.

For clarity, there are only two separate 120 volt, 20 amp branch circuits feeding the boat dock. Both originating from a small sub panel on dry land, in a storage shed, approximately 100’ from the water’s edge.

At the customers request, (and because we were running out of ideas), we drove two ground rods on shore, and connected the ramp and all sections of the dock, including the ladder, with braided stainless steel mesh straps. None of this changed anything, nor did we expect it to.

I would appreciate comments and suggestions.

 

[hillbilly1]

Yeah, it’s going to be hard to trace. The dock wasn’t grounded previously, so it was a poor return path for the fault is the reason they didn’t have that problem before. Could be a failing poco underground feed, somebodies irrigation pump, lots of possible points of failure. If the voltage is high enough, an Amprobe current tracer will show you the source.

 

[winnie]

This sounds like a classic 'stray voltage' issue, where the connection to the utility grounded neutral, or connection to 'remote grounding electrodes' is coupling current flowing through the soil to the metal structure of the dock.

The source of this 'stray voltage' might be the utility 'multi-grounded-neutral' system, or in might be a fault on the customer's premises or at a neighbor. We recently discussed an issue where a neighbor had a fault in a well which was energizing the well casing, and the resulting soil current was showing up on issues caused by the connection of the dock to grounded metal/grounding electrodes ( https://forums.mikeholt.com/threads...s-earth-ground-potential.2574100/post-2832956 )

I would suggest using a clamp on current meter to measure current flowing on the EGC feeding the dock. If you get significant current flow then start trying to trace it back to the source.

The customer assumed that the problem was with our electrical installation, since he did not have this problem prior to our work. Honestly, we kind of feared the same thing. But after quadruple checking every inch of the installation, and replacing all GFCI devices, we felt confident that the problem was not with the installation. We believe it became noticeable after our installation because prior to our work, there was no affective ground path between the floating dock and earth ground. We are thinking that the dock and water were previously at the same potential.

In the above I think you are spot on. With the dock simply floating it was at the same potential as the water. After you added the circuits you also were required to connect the dock to the electrical system ground (via the EGC) and from their to the service grounding electrode and the utility neutral.

If you can identify the source of the problem, you might be able to force a fix; but a risk with any dock is that a problem completely external to the customer's electrical system system can energize the dock. There are alarms which can be installed to detect this condition and alert the dock users of the problem.

-Jon

 

[Krusscher]

I believe my dad found a stray voltage problem by attaching a 50 or so feet of wire to a ground rod and use a volt meter and checking at different points in an arc in the ground until he found where it was the highest which pointed in the general direction of where the the issue was which ended up being a neighbors well pump shorted like mentioned by winnie.

 

[LarryFine]

I would also make a break in the EGC path and measure for voltage across the gap.

 

[infinity]

I've never wired a dock but is there a specific requirement to connect a metal dock to an EGC? If not installing everything in PVC would isolate the EGC from the dock.

 

[WCEI]

I've never wired a dock but is there a specific requirement to connect a metal dock to an EGC? If not installing everything in PVC would isolate the EGC from the dock.

I don’t have a code book in front of me, but I’m sure it’s specifically addressed in the article that covers “metallic” and/or “floating boat docks.“ But, the clause that says, “likely to become energized” would certainly apply with respect to being grounded.

 

[WCEI]

I would also make a break in the EGC path and measure for voltage across the gap.

Larry, I guess you know I’m talking about Lake Chesdin?

 

[Buck Parrish]

I wouldn't do any thing other then exactly what the NEC says to do. As you likely know additional rods could make it worse.
I would call the power company . They have metering equipment to.

Their was a case where some teens were electrocuted while swimming near a boat dock that had power.
The trial was to determine who was at fault, the underground high voltage transmission lines, the power company or the electrician.
I'm not sure what the out come was.

If you read that same article that I read where the teens were killed from electrocution. It is scary. It might seem like an over reaction. But I would take all the wiring that you installed off of the dock, all of it. Then tell him this is not a safe area to have additional power on the dock. If necessary give him his money back.

 

[WCEI]

I wouldn't do any thing other then exactly what the NEC says to do. As you likely know additional rods could make it worse.
I would call the power company . They have metering equipment to.

Their was a case where some teens were electrocuted while swimming near a boat dock that had power.
The trial was to determine who was at fault, the underground high voltage transmission lines, the power company or the electrician.
I'm not sure what the out come was.

If you read that same article that I read where the teens were killed from electrocution. It is scary. It might seem like an over reaction. But I would take all the wiring that you installed off of the dock, all of it. Then tell him this is not a safe area to have additional power on the dock. If necessary give him his money back.

Good points, and you can bet that I am more than a little concerned. We have document with words and pictures, everything we have done.
Involving the power company is a touchy subject, too. It has to be done, but it needs to be done in a way that they provide documentation on their research and findings. God help us if the worst happens. It will be he said vs she said.

We all know that electrical drowning is, and has been one of Mike Holt’s most prominent topics for most of his career. Mike has persistently made us all away of the hazard, and consequently has saved countless lives. In fact, what I’ve learned from Mike is why we are taking this problem so seriously.

 

[LarryFine]

Larry, I guess you know I’m talking about Lake Chesdin?

No. How would I know that?

 

[Little Bill]

No. How would I know that?

He just told you!

 

[LarryFine]

He just told you!

Mayhaps because of where he lives.

 

[ramsy]

because we were running out of ideas

No mention of opening utility neutral?

If that helps Golddigger's Neutral Isolator idea may also work

 

[Fred B]

Just because the HO states they "never noticed" the "tingle" prior to your installation doesn't mean it didn't exist. I would totally isolate the dock electrically (putting it back to the electrical state before your installation) then measure via a remote to earth connection for voltage presence. This would tell if it is from another source including the POCO.

This is part of the reason for EP bonding at a pool, creating a limited potential between the points of reference that can create a "feel" or worse actual shock from the differences of potential. Not really sure how one would be able to create such in relation to a dock. But this is also the reason for signage at a dock indicating potential hazardous voltage may be present and no swimming, NEC 555.10 (2023).

Use of long leads and ground probes you can actually create a compass of sorts to give a general direction of the possible NEV. If no other homes in the direction of strongest reading and the POCO and transformer is in that direction, higher than average odds the source is the POCO. Good luck in getting any satisfaction from POCO if they decide it is "not from our system" tested or not, or even getting cooperation from neighboring home owners.
mike Holts video on NEV

 

[WCEI]

No. How would I know that?

Just figured you had a little familiarity with the area, that’s all.

 

[Electromatic]

I don't have any sage advice, but here is a link to a similar thread that discusses neutral blockers and isolation transformers:

Isolation transformer for residential dock power

For a freshwater residential dock with convenience receptacles, lights, and boat lift: Is it possible to connect an isolation transformer (such as Charles Marine ISO-G2) to conform to NEC bonding requirements AND to have a true isolated grounding conductor? Issue is stray voltage on the existing...

forums.mikeholt.com

 

[winnie]

I didn't remember that I commented in that other thread. But the point at the bottom of this post bears some further research:

Isolation transformer for residential dock power

That's how I read it also. How are the marine transformers getting by this? Is it due to the fact that they are "intended" to be installed on the vessel? I came across this attached proposal for code change from a Georgia Tech EE. This is an issue that can be solved on site and needs to be...

forums.mikeholt.com

And the apparent response:
"As previously written in CMP-19's action and statement on Proposal 19-107, the installation of a transformer intended to isolate stray voltage (current) is not presently prohibited. The NEC is not intended as a design guide.
CMP-19 is very sympathetic to the evident safety concerns surrounding electrocution hazards at marinas and boatyards."

I don't see how a transformer could be installed which would provide the desired neutral isolation to mitigate stray current, _and_ still be NEC code compliant. But CMP-19 seems to believe it is possible.

A transformer installation which breaks the electrical connection between the primary EGC and the secondary grounded conductor would isolate the dock grounded metal (and the dock electrical system) from the wider utility neutral. Does anyone see how to do this in a code compliant fashion?

-Jon

 

[Electromatic]

Could one use 250.6(B) to "Stop Objectionable Current" by removing the primary to secondary bonding?

 

[winnie]

I think there might be a fight about the exact meaning of the term "Objectionable Current", but I'd personally read the term expansively to include utility created 'stray current'. In which case you would be permitted to isolate primary and secondary grounding as long as you ensured:

[...]provided that the fault current path is permanent, electrically continuous, capable of safely carrying the maximum fault likely to be imposed on it, and it has sufficiently low impedance to facilitate the operation of overcurrent devices under fault conditions [250.2(A)(5)].
[...]
2. Change the locations of the neutral-to-ground connections.
3. Interrupt the continuity of the conductor or conductive path interconnecting the grounding connections.

You would need a transformer that somehow isolates primary and secondary terminals, and provides a separate 'ground fault current path' for each. I don't know if suitable hardware is available off the shelf, but it could certainly be built.

-Jon