boat lift shocking

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gar

Senior Member
Location
Ann Arbor, Michigan
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EE
091018-2139 EST

The information from wesley1's posts does not indicate that power company current is flowing into the salt water. This is indicated by disconnecting the power company neutral from the main panel. The EGC is still connected to the ground electrode at the main panel. Under these conditions there is reported to be no difference in voltage between the water and the EGC. Thus, no significant stray current flowing thru the ground from the area of the ground electrode at the main panel to the boat dock area.

If the power company installs an isolation transformer, two primary posts and no connection from primary to secondary, at the pole instead of the current single post transformer, eliminates the primary neutral to ground electrode at this pole, moves it away about one pole, and connects the new transformer neutral to the existing pole ground electrode, then there should be little primary neutral current from the new, at least one pole away, remote grounding point to the ground electrode at the customers transformer ground electrode. Thus, negligible voltage from the main panel ground electrode to the EGC at the dock and to the water from the power company. There does not appear to be stray power company ground currents going to the water presently.

You can approximate the direction and magnitude of ground currents with two probes slightly into the ground 12 ft apart by measuring the voltage difference. Two screwdrivers will work as probes.

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hurk27

Senior Member
I see two things pointing to a voltage drop on the utility MGN (utility neutral) if he had turned off the main breaker for this service, and the voltage was still there, and the fact he disconnected the service neutral and the voltage went away, this would only lead to a MGN that has a voltage rise above the Earth potential, this is a common problem with the cross bonding of the MGN to the secondary neutral supplying a service, the only two fixes available is the utility supplying a neutral isolator, or they find the reason if the voltage drop on their MGN, this could be a bad connection at a transformer or a lost MGN up the road in the direction of the sub-station feeding it.

Ground rod's will only produce another step potential whether in the water or in Earth, a rod will not fix this, I would believe that this boat lift has a substantial amount of metal in the water, but the voltage is still there right? so I don't think a little 10' rod will do anything different, and it can't physics won't allow it.
Try telling the utility to put an amp probe on the MGN starting with your service and go towards the sub-station, bet they find one that has very little to no current on it. trust me they should know what this is about.

Dealt with this on pools and dairy farms.:mad:
 
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mivey

Senior Member
Here is a Mike Holt Produced video, that should give you some insite as to what is happening:

http://www.mikeholt.com/strayVoltageVideo.php
OK, just finished watched it. A couple of things come to mind:

Not a lot of discussion of ground currents as it seemed to focus on neutral current. The MGN really minimizes the effect of weaker pole grounds as you have so many of them in parallel. The 3 volt example might be a fair example of a sorta-balanced 3-phase line.

But what about a single-phase line? I wonder why we don't have a ton of trouble with these? Just thinking of a small 7.2 kV, 1-phase line primary line running about a 1/2 mile. Even considering that the earth will carry 50-75% of the return current, it would seem to me that the neutral might experience a rise of quite a bit more than 3 volts or even 30 volts.

Maybe I haven't thought it through enough. I wonder what the parameters were for the computer model shown?
 

mivey

Senior Member
the only two fixes available is the utility supplying a neutral isolator, or they find the reason if the voltage drop on their MGN
It does not seem to me like the neutral isolator will always solve the problem, at least for people in the direct path between a line and a sub.

Picture a distribution line that leaves the sub, goes out a ways and makes a 90 degree turn and then runs a ways. Wouldn't the people between the sub and the line after the turn be subjected to the ground currents that make a bee-line back to the sub through the earth?

add: I'm talking about a problem other than the secondary neutral being energized by a direct connection to the primary neutral.
 

hardworkingstiff

Senior Member
Location
Wilmington, NC
I'm not sure how to ask this question, but here goes.

The video seems to indicate that earth will carry current back to the substation with little to no voltage drop (or am I misunderstanding it, I admit I did not watch the whole thing).

Is this accurate? Does the earth have basically near zero resistance back to the substation?

The problem with the salt water seems that the salt water is so much of a better conductor than earth that the voltage drop on the distribution neutral finds a better path to the substation in the salt water than it does through earth.

???? Do I understand it correctly or am I just out there?
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
091022-1150 EST

hardworkingstiff:

Sea water is going to have much better conductivity than earth. Probably true for almost all surface earth conditions. But why do you conclude this means the utility neutral return path is thru the sea water? Under some conditions maybe, but in this case there is no evidence of this path.

Remember that when the power company neutral was removed from the main panel that the voltage drop from the sea water to the EGC at the dock dropped to zero.

I have more general comments in a post to follow.

.
 

mivey

Senior Member
091022-1150 EST

hardworkingstiff:

Sea water is going to have much better conductivity than earth. Probably true for almost all surface earth conditions. But why do you conclude this means the utility neutral return path is thru the sea water? Under some conditions maybe, but in this case there is no evidence of this path.

Remember that when the power company neutral was removed from the main panel that the voltage drop from the sea water to the EGC at the dock dropped to zero.

I have more general comments in a post to follow.

.
The problem is the volt drop on the system has raised the neutral potential relative to the earth (and the salt water). When you complete the circuit from the elevated neutral to the salt water, the salt water does become part of the return path (and so do you).
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
091022-1424 EST

mivey:

Most of the following is not for you but others reading this.

That is true. But if the power system neutral is not connected, then the measurements made indicate that there is no stray power company current flowing out and thru the sea water.

In the case of the fresh water problem mentioned in an earlier post where electrocution occurred there was stray current flowing thru the lake.

The problem described in this post is that the salt water and the earth are about the same potential and the power company neutral is connected to the earth thru some ground rods. The voltage drop from a small amount of neutral current flowing into the ground rod(s) produces about 1.3 V drop from the rod to a few feet away from the rod. Effectively a voltage source has been placed between "most earth" (sea water in this case can be considered the same as "most earth") and neutral. Since the EGC is connected to neutral it is 1.3 V above sea water. Break the power company primary neutral from the EGC with an isolation transformer and now there is no voltage source from ECG to "most earth".

My term "most earth" means the earth some moderate distance from the source of current injection to the earth.

Now change the circuit so the substation is out in the middle of the salt water. Now an isolation transformer that breaks the primary to secondary neutral connection does not prevent a voltage drop from the main panel EGC and connected grounding electrode at the house to the sea water because there is a current thru the earth to the sea water independent of whether a person is in the water and touching the dock or not..

.
 

hardworkingstiff

Senior Member
Location
Wilmington, NC
The problem described in this post is that the salt water and the earth are about the same potential and the power company neutral is connected to the earth thru some ground rods. The voltage drop from a small amount of neutral current flowing into the ground rod(s) produces about 1.3 V drop from the rod to a few feet away from the rod.

Instictively, this seems ... not quite what's going on.

I think if you were to pull the GEC conductors and tested the PoCo neutral to the salt water with the earth not involved you would still have the voltage. Actually, the voltage may go up.

It seems to me that the resistance from earth to the substation is more than it is from the salt water to the substation causing the voltage difference between the GEC and the salt water.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
091022-1254 EST

I took a field trip this morning to get some quantitative data.

Measurement tools: Fluke 27, 6' Fluke meter leads, Fluke alligator clips, and 2 Craftsman 12" screwdrivers. Ground potential difference is between the two probes spaced about 12' apart.

I found a point out in the country that appears to be a tie between Detroit Edison and Consumers Power. There are three groups of high voltage lines entering from the east and one from the West. Each phase has two parallel wires and there are six phases in each group. So all total there are 36 conductors from the east. Those in the know can indicate what power level this interconnect might represent. One group is spaced about 50 ft from the center group, and the other on the opposite side of center is about 75 ft.

Just below the middle group my east-west probe pair voltage was 102 MV, and north-south was 36.5 MV. Next I went south about 2000 ft. Virtually no power lines around. EW 2.9 MV, NS 2.8 MV. Note: MV indicates millivolts.

There is no substantial ground current.

This means that ITC, that now owns these lines, has good neutral conductor connections on the towers.


Different location, my daughter's home. This is single phase supplied from a grounded primary. Her transformer is at the corner of her backyard and is a 50 KVA unit with a ground rod at the pole.

Middle of backyard north of meter --- NS 7 MV, EW 26 MV.

Her primary service runs along the back lot line, EW, and is fed from a perpendicular line, NS, on the east side of her property. Below the NS line at the side of her property the NS reading was 10-16 MV. EW was 103-185 MV.

About 150 ft south and still below the NS line the voltages were:
NS 30-40 MV, and EW 84 MV. This GPS location is 42.241998,-83.719833.


At my son's shop location the transformer is located on a pole at GPS 42.253404,-83.734585. The substation is at 42.250249,-83.738325. The substation is about 1500 SW of our transformer. Going in close with Google satellite you can see two white dots for the transformers. With one probe close to or in contact with the pole ground rod the following readings resulted:
N 21 MV
NE 44 MV
E 132 MV
SE 109 MV
S 167 MV
SW 109 MV the direction of the substation
W 80 MV
NW 48 MV

At GPS 42.253398,-83.735229
EW 9 MV
NS 12 MV
NE-SW 16 MV

In any locations I have checked I have not found substantial voltage gradients in the earth.

.





.....
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
091022-1533 EST

hardworkingstiff:

True.

There is about the same potential throughout the sea water and most of the earth and only a large difference when you are close to any of the power company ground rods connected to their neutral.

.
 

glene77is

Senior Member
Location
Memphis, TN
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,

The primary and secondary are isolated
by a capacitance shield which is grounded.
No stray voltage leaking across the Prim/Seco core.

On the Secondary load circuit, the only reference is the other leg from the transformer secondary.

Thus, the POCO reference is out of the picture.

This was the typical approach used in hospital ICU applications, where patients are connected intra-venously and are 10 times more sensitive to leakage currents than the standards used for GFCI applications. We tested at 10 microamp increments on our scale.

Comments are welcome.
 

hurk27

Senior Member
OK, just finished watched it. A couple of things come to mind:

Not a lot of discussion of ground currents as it seemed to focus on neutral current. The MGN really minimizes the effect of weaker pole grounds as you have so many of them in parallel. The 3 volt example might be a fair example of a sorta-balanced 3-phase line.

But what about a single-phase line? I wonder why we don't have a ton of trouble with these? Just thinking of a small 7.2 kV, 1-phase line primary line running about a 1/2 mile. Even considering that the earth will carry 50-75% of the return current, it would seem to me that the neutral might experience a rise of quite a bit more than 3 volts or even 30 volts.

Maybe I haven't thought it through enough. I wonder what the parameters were for the computer model shown?

The problem here is we are dealing with water, it doesn't require very much voltage to cause a problem where the human body is in immersed in water, touching something that is just 1 or 2 volts above Earth. we don't hear of all the other times because it doesn't involve the above. so it goes unnoticed.
now as far as the voltage drop problem on power lines using only one of the three phases, remember there is very little current on most power lines, 6 or 7 amps at 7200 volts will give you 180 to 210 amps at 240 volts, so if a line only has one or two houses on it, the current on the MGN will be very little, so in most cases the 4 rods per mile do there job and keep the voltage drop on the MGN down to practically nothing. so this is why stray voltage is kind of rare to be a problem, but it does happen.

I'm not sure how to ask this question, but here goes.

The video seems to indicate that earth will carry current back to the substation with little to no voltage drop (or am I misunderstanding it, I admit I did not watch the whole thing).

Is this accurate? Does the earth have basically near zero resistance back to the substation?

The problem with the salt water seems that the salt water is so much of a better conductor than earth that the voltage drop on the distribution neutral finds a better path to the substation in the salt water than it does through earth.

???? Do I understand it correctly or am I just out there?

Actually the highest resistance is found the closest to the connection point of earth, this is because of the small amount of earth around this connection point, as you get further out from this connection point the resistance of earth gets lower because of the greater amount of earth or parallel paths.
after a given distance from this connection point the sum of all the parallel path will be so close to 0 ohms the voltage drop will go away. current will still be flowing but in all directions away from the connection point. this same principle applies to water, salt water, and the earth, it only varies because of the difference in the conductivity of the medium to which the current is flowing through, but the results are the same, at some point in a distance from the connection point the medium will reach a 0 ohm equilibrium. this was proved by the test Gary did in this POST.



Instictively, this seems ... not quite what's going on.

I think if you were to pull the GEC conductors and tested the PoCo neutral to the salt water with the earth not involved you would still have the voltage. Actually, the voltage may go up.

It seems to me that the resistance from earth to the substation is more than it is from the salt water to the substation causing the voltage difference between the GEC and the salt water.

see the above explanation

The problem is the volt drop on the system has raised the neutral potential relative to the earth (and the salt water). When you complete the circuit from the elevated neutral to the salt water, the salt water does become part of the return path (and so do you).

This is what is called "touch potential" and is the same on Earth, but with water your immersed in it, and our sense of the voltage is much greater.

It does not seem to me like the neutral isolator will always solve the problem, at least for people in the direct path between a line and a sub.

Picture a distribution line that leaves the sub, goes out a ways and makes a 90 degree turn and then runs a ways. Wouldn't the people between the sub and the line after the turn be subjected to the ground currents that make a bee-line back to the sub through the earth?

add: I'm talking about a problem other than the secondary neutral being energized by a direct connection to the primary neutral.

It doesn't work this way, the reason is because current takes all paths back to the source, when we start thinking about the paths in earth or a large body of water we must start thinking in three dimensions, not two, and because almost all the voltage will be dropped in the first 26' from the contact point to earth or about 19' in salt water, and as high as 40' in fresh water, after these distances from the connection point to earth/water it can be said the resistance of the rest of the pathway is 0 ohms. now if it is at 0 ohms then the current will take all pathways equal in all directions back to the source, even around the whole globe and even through the earth.
again see the link I posted to hardworkingstiff. about Gary's experiments.
 

bob

Senior Member
Location
Alabama
now as far as the voltage drop problem on power lines using only one of the three phases, remember there is very little current on most power lines, 6 or 7 amps at 7200 volts will give you 180 to 210 amps at 240 volts, so if a line only has one or two houses on it, the current on the MGN will be very little, so in most cases the 4 rods per mile do there job and keep the voltage drop on the MGN down to practically nothing. so this is why stray voltage is kind of rare to be a problem, but it does happen.
Hurk
I suggest you watch the Mike Holt film posted earlier.
The reason that this problem exists is due to long single phase lines installed by utilities. The returning current thru the neutral raises potential of the neutral above ground. By ground I mean the single point reference point at the substation. The service neutral at this home is connected to the primary neutral. When the OP lifted the neutral he interrupted the return path and the voltage dropped to zero. I suspect that there is another long single phase line serving the other side of the lake. Since the boat lift is grounded, it connects the two neutrals together thru the water. There are two solutions to the problem. First pray the the utility is not city owned and have more than 2 linemen in a derrick truck. If this is true you are out of luck because they will never understand the problem. If the utility is well staffed, then you may be able to convince them to add 2 additional phase conductors and balance the circuit. This will reduce the returning current and lower the potential of the neutral. These additional phases may mean miles of construction and they are not going to want to do it. Another means is to install an insulator in the cables that hold the boat. This will electrically isolate the frame the the boat sits on. I don't think this is an NEC issue.
 

hurk27

Senior Member
Hurk
I suggest you watch the Mike Holt film posted earlier.
The reason that this problem exists is due to long single phase lines installed by utilities. The returning current thru the neutral raises potential of the neutral above ground. By ground I mean the single point reference point at the substation. The service neutral at this home is connected to the primary neutral. When the OP lifted the neutral he interrupted the return path and the voltage dropped to zero.
I have not disputed this, and in fact, I posted the very same thing in post #22. all I was saying is most powers company's limit their loads on single lines like this if possible, but there are times they cant. and we see the results, it's just not very often we hear about stray voltages unless it involve water, or farm animals. and the reason these are the only times we hear about it is because of what I posted before, 1 to 2 volts would go unnoticed in any other situation, but add water and it gets noticed.

I suspect that there is another long single phase line serving the other side of the lake. Since the boat lift is grounded, it connects the two neutrals together thru the water.

This can not be true, it would have no bearing on what is on the other side of the lake. as I said current takes all paths back to source, not a straight line. after your out of the sphere of influence of the contact point, the earth, water or what ever conductive medium is, the resistance is at 0, thus, current will take all paths back to source equally in all directions.

this would be like drawing a circle around a ground rod at 26', outside this circle is 0 ohms, since there is no voltage drop at 0 ohms, the current will fan out equally in 360? of direction from the ground rod to reach the 0 ohm outside of the circle, the 0 ohms outside of the circle is equal to the X0 of the source of the current, now this was an image in just 2 dimensions, now visualize this in 3 dimensions, all the way around and under this ground rod.
To understand this, read this thread on experiments that Gary did, HERE

There are two solutions to the problem. First pray the the utility is not city owned and have more than 2 linemen in a derrick truck. If this is true you are out of luck because they will never understand the problem. If the utility is well staffed, then you may be able to convince them to add 2 additional phase conductors and balance the circuit. This will reduce the returning current and lower the potential of the neutral. These additional phases may mean miles of construction and they are not going to want to do it.

Thats about the truth of it in most cases, unless the issue of liability gets into the picture.

Another means is to install an insulator in the cables that hold the boat. This will electrically isolate the frame the the boat sits on. I don't think this is an NEC issue.


You might be able to isolate the boat, but the lift frame which goes all the way into the water will still have voltage on it, and will still pose a safety risk.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Don,

The primary and secondary are isolated
by a capacitance shield which is grounded.
No stray voltage leaking across the Prim/Seco core.

On the Secondary load circuit, the only reference is the other leg from the transformer secondary.

Thus, the POCO reference is out of the picture.

This was the typical approach used in hospital ICU applications, where patients are connected intra-venously and are 10 times more sensitive to leakage currents than the standards used for GFCI applications. We tested at 10 microamp increments on our scale.

Comments are welcome.
While there are some rare installations where the code does permit the installation of a truly isolated device, this is not one of them. In most cases, the code rules prohibit true isolation as both the primary and secondary EGCs must be bonded to the metal parts of the transformer and these EGCs will be bonded to their respective grounded conductors so there is no real isolation between the primary and secondary grounded conductors.
 

SG-1

Senior Member
There are other forces that can cause current to flow through the earth. Here is one that can last for hours or days:

In the 1970s scientists at the University of Alaska Fairbanks Geophysical Institute during aurora activity measured ten amperes of direct electrical current generated by the aurora in local power lines. Almost all electrical power lines are built to carry only alternating current. Direct current generated in these lines by the aurora causes heating and other problems in the equipment that is used to distribute alternating electrical power. As aurora occurred Geophysical Institute scientists also measured over 2000 amperes of direct current flowing in the Trans-Alaska Pipeline. This electrical current flows in and out of the pipeline where the pipeline touches the ground which causes increased corrosion or loss of metal from the pipeline at that point. Although the electrical current is large the voltage is only a few thousandths of a volt per kilometer, which is not enough to feel if you touch the pipe.

If I am ever in Alaska I am so not going swimming around that pipeline !! :D

I too have been checking for NEV around the farm. Using a Fluke 287, Fluke leads, Fluke alligator clips, stanley screw driver, & leather gloves. I have suceeded in finding more DC than AC using the LowZ setting; zero on the AC & 300 mVDC at the service. No DC found inside, yet. My readings were 20-30 % higher using the high impedance setting. I live in the middle of nowhere, but all lines are above ground and most ( 3 phase lines ) have recently been replaced along with the poles. The ground is not as dry as I would like & rain is in the forecast. Why did I check for DC ? Superior intelligance, NOT. The meter displays both AC & DC in this mode, I just had to read the numbers, sorry to say. In other words I can see better than I can think !!! :D

It might not be a bad idea to check for DC around the boat lift.
Or any other place where someone says they were shocked or felt a tingle.
 
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gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
091022-2308 EST

don:

My home pole transformer is an isolation transformer. Primary side has no neutral, only two posts fed from an essentially delta system. This is not covered by NEC. Lots of these delta systems exist in our area. It may be a Y source at the substation, but from my view point it is three wires only and thus to me looks like a delta.

.
 
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