Finding a bad ground over Water

[ktrou]

Guys,

This is probably completely impossible without stepping on someone else's dock.

I live on a Lake and have a Aluminum Dock with an Aluminum ladder that is lowered into the water only when needed.

I am 99.9% sure that the issues is NOT my dock since during the summer months is the only time this occurs, after the summer residents depart the issue goes away.

When exiting the water and stepping on the ladder you can feel a slight tingle when part of your body is in the water and a body part is on the ladder.
There are a number of docks (10 or so) that are within a stones throw distance from our dock.
Some of these docks have up to a 50A service to them since they have 60' House Boats connected to shore power.

Like I said I'm pretty sure it's someones dock but it would be nice to see if there is an easy way to see how much power is leaking from their dock?

Idea's???

 

[George Stolz]

Your dock isn't connected to an EGC of a circuit?

 

[hillbilly1]

There is quite a few lakes that are "electrified", having the dock properly bonded just makes it a better return path hopefully tripping the gfi protection, the problem is there is a lot of bad installations that are not gfi protected, causing electrocution. Sometimes if the voltage is high enough, I can track it back to the source with my Amprobe circuit tracer.

 

[hillbilly1]

Guys,

This is probably completely impossible without stepping on someone else's dock.

I live on a Lake and have a Aluminum Dock with an Aluminum ladder that is lowered into the water only when needed.

I am 99.9% sure that the issues is NOT my dock since during the summer months is the only time this occurs, after the summer residents depart the issue goes away.

When exiting the water and stepping on the ladder you can feel a slight tingle when part of your body is in the water and a body part is on the ladder.
There are a number of docks (10 or so) that are within a stones throw distance from our dock.
Some of these docks have up to a 50A service to them since they have 60' House Boats connected to shore power.

Like I said I'm pretty sure it's someones dock but it would be nice to see if there is an easy way to see how much power is leaking from their dock?

Idea's???

A quick and easy way to find out if it is a neighboring dock, is to turn off each dock, checking to see if the voltage disappears.

 

[Cow]

A quick and easy way to find out if it is a neighboring dock, is to turn off each dock, checking to see if the voltage disappears.

I agree.

Time to visit the neighbors....

 

[kwired]

It does not have to be coming from a neighbors dock. It could be voltage drop on POCO MGN, which is also bonded to all secondary grounds on the supplied services, which effectively extends that MGN to everything connected to your premesis EGC's.

After summer months when many are gone the MGN probably has less load on it and therefore less voltage drop.

Not saying this is your problem, but if it is and you ignore it because you are certain it is a neighbors problem you will be looking for a long time and never find the problem.

You could also have a neighbor that has some resistance in their service neutral or a neutral - ground bond someplace it doesn't belong.

A simple non invasive start would be to test from each dock to the water for voltage. If one is exceptionally higher than the others start there, but that is not too likely without the users of that dock experiencing problems - and probably more severe than the problem at your dock.

I would think if you presented the other owners with the possibility that there is something dangerous going on and that someone may be killed if it is not resolved they would be willing to allow some investigation into the problem to take place on their property. If not, legal threats if the problem should be found on their property will probably change their minds.

 

[buzzbar]

I would be interested in hearing advice on this as well. I work on a lot of marinas, and I would like to be able to properly test for leaking voltage.

I understand that problems such as this are usually caused by faulty wiring within a boat, or the shore power cord. It's much more common in fresh water since fresh water is a much better conductor than salt water. I have attempted to look for these problems using an in-line GFI cord. I plug the GFI cord into the shore power receptacle, and plug the boat cord into the GFI cord. Theoretically, if there is a voltage leak, the GFI will trip. Would anyone disagree with this?

Somebody mentioned that you should test the voltage at the water, and unplug nearby boats until the voltage goes away. How would you test the voltage "at the water"? Would you submerge one lead from a digital meter into the water, and put the other lead on the hot conductor? I haven't tried this yet, but I'm skeptical about whether this would work or not.

Thanks.

Andrew (buzzbar)

 

[don_resqcapt19]

I would be interested in hearing advice on this as well. I work on a lot of marinas, and I would like to be able to properly test for leaking voltage.

I understand that problems such as this are usually caused by faulty wiring within a boat, or the shore power cord. It's much more common in fresh water since fresh water is a much better conductor than salt water. ...
Andrew (buzzbar)

You have it backwards on the conductivity of the water. Salt water is the better conductor. The fact that it is a better conductor makes it less of a shock hazard when you are in the energized water. When you are in energized water you are a parallel path for the current and the most current flows on the path of least resistance. There will be less current flowing across your body in salt water than there would be in fresh water (assuming the other conditions are the same).

 

[bob]

Kwired nailed it right on the head. Don also answered the question with more detail. It is very likely that it is the utility
causing the problem. They have long single phase primary conductors and the current flowing thru the neutral can set up a difference in potential that you are feeling when you are in the water. Bussbar, you can unplug everything and still have this problem. If you want to understand the problem Mike Holt answers the question in this video. It is long but worth the time.

http://www.mikeholt.com/strayVoltageVideo.php

Bob

 

[buzzbar]

You have it backwards on the conductivity of the water. Salt water is the better conductor. The fact that it is a better conductor makes it less of a shock hazard when you are in the energized water. When you are in energized water you are a parallel path for the current and the most current flows on the path of least resistance. There will be less current flowing across your body in salt water than there would be in fresh water (assuming the other conditions are the same).

You're correct. Thanks for pointing that out.

 

[hillbilly1]

Kwired nailed it right on the head. Don also answered the question with more detail. It is very likely that it is the utility
causing the problem. They have long single phase primary conductors and the current flowing thru the neutral can set up a difference in potential that you are feeling when you are in the water. Bussbar, you can unplug everything and still have this problem. If you want to understand the problem Mike Holt answers the question in this video. It is long but worth the time.

http://www.mikeholt.com/strayVoltageVideo.php

Bob

Yes, utilities do have issues with their lines that also can cause this problem, but so far I have found it to be on the customer side most of the time, so don't automatically rule out that it can be faulty dock wiring or equipment. Gfi's go bad all of the time, and will fail to trip.

 

[hurk27]

Current does not always come from other sources unless there is a common metallic path between these sources, there would have to be a metallic connection between these docks or they would have to be very close together for a voltage on one to cause a difference of potential at another dock.

This is based upon the understanding of how Earth shells work and how current takes all paths not just a straight line between two points, the voltage gradient between two points will follow the same bases of Earth shells in water as it does on land, the only thing that affects this is the conductivity of the water as it does on land, for fresh water the shells around a metal dock will be larger then if this dock was in salt water which would be much smaller thus a much higher of voltage drop at the same distance from the dock.

For a better understanding of Earth shells check this thread out: Earth Shells Experiment

pay attention to the fact that the more conductivity of the Earth or water as in this case, the smaller the size of the shell and the higher voltage drop across the shell.

Now there are many reasons for the voltage, a long service run from the utility will allow a voltage drop on this neutral and any grounding that is bonded to this neutral.

But as you said after most other people leave the area the voltage is much lower, this leads me to believe that the source of the voltage is in fact a voltage drop on the MGN or if these services are fed via a common transformer and SEC's to a point to which they split up to go to each house, it can be this service neutral that could also be causing it.

The only way to test that it is coming from your service is to isolate the grounding feeding the dock, just turning off the main will only show that it isn't a voltage drop on your service drop that is causing it, if it is still there after turning off your main, then it is ahead of the point where your service drop neutral connects to a common point of the other services. Isolating the grounding of the feed to this dock will further confirm this.

I little about a GFCI:
A GFCI protecting the circuit feeding the dock will not provide protection from an elevated voltage on the grounding, a GFCI does not monitor the current in the grounding as it does not need to to function.

I only takes a few volts to keep a person from being able to get themselves out of the water, and not much more to prevent them from being able to maintain keeping their head above water then they can drown.

The only true fix is to find the cause and find a solution, this can be very hard if you have a utility who doesn't offer much help (provided it is on their end)

Taking voltage checks at various points in the system to Earth starting at the dock working your way back to the ground running down the pole of the utility can pin point as to where the problem is, if you get to your meter cabinet and still find the same voltage then you know it is ahead of your service, if you get to the pole ground, and it is still there it is ahead of this point, just using a DVM and a screw driver stuck into Earth will show this voltage potential and is a very good way to follow it back to the source.

At anytime you find that you no longer have the voltage to Earth (and other loads have not changed) then it is between this point and the last point you had the voltage, some times when a sub panel is involved you might find a bonded neutral, or you might find a grounded neutral in the branch circuit feeding the dock, but only testing will find it.

Ok book writing is over:happyyes:

 

[gar]

120304-1721 EST

I would try an experiment with two probes connected to a high impedance voltmeter like a Fluke 27. I have not run this test in water and therefore I do not know what magnitudes to expect.

One can detect the direction of current flow in the earth or water by moving the probes to obtain the maximum voltage, and that then is the maximum voltage gradient and points in the direction of current flow. The two probes need to be supported on an insulated member and i would suggest about 12 ft apart. Each rod could be 6" long and must have an insulated wire from it to the meter. The boat should be a fiberglass canoe or something small and the probes probably should be some distance in front of the boat to minimize the disruption of the electric field in the water.

By searching for the direction of current flow you may be able to point to a source. I am not in a position to run this experiment, but I would find it interesting.

I have done the experiment in the earth. And I can identify current that flows from the earth toward my copper water supply line. I can also see in an area further away from my water line the direction and magnitude of current flow change with time in my yard or out in the country.

I have no idea what to expect to see in a lake.

You can run an experiment in a glass tray with tap water and a small electrode at each end of the tray for injecting current thru the water to learn the fundamentals.

Use an AC source like 12 V from a small transformer. In this experiment you can determine equipotential lines. Mid way between the electrodes should be about 1/2 the applied voltage by measuring from one injection electrode to a single test probe moved around in the water. There should be an equipotential line about straight across the tray perpendicular to the direction of current flow in the middle of the tray. Near the ends of the tray the equipotential lines will be curved.

Suppose this tray is about 10 by 14 inches.

If you have two probes on an insulator maybe 2 inches apart and measure the voltage in the direction of current flow the voltage might be in the range of 1/7 the applied voltage. As you rotate the probe pair to perpendicular to the direction of current flow the voltage will drop to a low null value.

I will run the tray experiment a little later an report the results.

.

 

[gar]

120304-21043 EST

Details on my tray experiment.

Glass pan about 7" x 12" with about 3/4" of tap water. 28 V AC nominal from a Stancor 28 V 2A transformer, about 32 V actual.

A 2" wide copper plate at each end of the pan. Current about 20 MA. About 1.5 V drop from copper plate to quite close to the copper plate. Means about 29 V across the water.

From one copper plate to a Fluke test probe in the middle of the pan about 16 V. To a corner of the tray about 6 V. You could try to hand sketch a field map using curvilinear squares as we were taught by A. D. Moore. Spent half of a semester doing such plots.

Two test probes near middle of pan and hand held about 2" to 3" apart, and I could null to about 20 millivolts. Probably could do better with a good mechanical fixture.

Off near a corner you can see the current flowing at about a 45 deg angle from the length of the tray. Held one probe fixed and moved the other to determine angle of maximum voltage.

.

 

[gar]

120304-2342 EST

If you do run tests from a boat, then there may be multiple currents from different directions. If these are all from the same power source, then at any point they sum to a new sine wave with a composite angle based on their points of origin.

However, if you get close to a specific source the gradient should be primarily from that source. So pointing might be fairly good.

Since you have a seasonal variation that is substantial it is not likely the problem is from a long distance source.

.

 

[Speedskater]

Side notes:

Docks can be made of any combination of conductive and not so conductive elements. The conductivity of wood elements can very with the weather or human activities.

Most sailboats have a well grounded lead keel and some motor boats are also grounded.

Never assume that a boat's ground (or internal wiring) is properly connected.