Capacitive coupling?

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hardworkingstiff

Senior Member
Location
Wilmington, NC
Responded to a request today to look at a dock. Parallel 3/0 type W cables feed from the service disconnect (480/277V) on shore down to a floating dock and then a few hundred feet to a MDP. The dock is installed parallel to the shore line and at low tide, the muddy bank is visible on the land side of the dock (I guess this info doesn't matter, but ..?). From the MDP, type W cable feeds to single-phase transformers and then to power centers to provide power to the boats. From the MDP to the farthest transformer is about 600'.

The trouble call is that the customer had someone get a shock when they reached into the water from the dock. The electrician on site (certified boat electrician working on getting his EC license) said he found 80-volts from a SS deck screw to the river water. The EC that installed the system (about 6 years ago) has been checking it out for a week and can not find a problem.

I could not get 80-volts today, but I did see about 30-volts from the deck screw to the river water using a DMM. The voltage went away when the voltage was checked in parallel with a small load (light bulb).

I also found voltage (from 17-volts to 41-volts) from the river water to the fresh water outlet on power center. That outlet is fed by plastic pipe and has brass ball valve and tee on the outside of the pedestal (connected to the plastic pipe feeding to the pedestal). There was no voltage reading from the river water to the ground on the pedestal 50-amp receptacle. The voltage went away again when we checked it in parallel with a light bulb load.

When the circuits in the MDP are opened, the voltage goes away. The voltage steps up a little at a time as you close the breakers. The (2) farthest transformer circuit put the most voltage on the location we were checking.

I told the customer I don't know how to get rid of this voltage, but I doubt it is an issue. I also told him I was concerned about the people getting shocked. They are going to try to recreate the issue.

The odd thing I noticed which I'd love to learn why is the following:

At one specific location (I just picked a spot) I measured from a disconnect mounted on the side of the transformer (connected to the load side of the transformer) to the river water and to a deck screw. Here are the measurements:

L1 to L2 = 244V
L1 to N = 122V, L1 to G = 122V, L1 to river water = 122V
L2 to N = 122V, L2 to G = 122V, L2 to river water = 122V
N to G = 0V (same to the river water, 0V)

L1 to deck screw = 113V
L2 to deck screw = 129V
N to deck screw = 32V
G to deck screw = 32V

OK, so here come the questions.

Can the W cable impress a "ghost" voltage onto a plastic water line? How can a SS deck screw (wood dock with some galv. hardware) have those "offset" readings on voltages?

I tried to keep the information succinct, but I'm sure I left something out, so ask any questions you might have.

Thanks for any input.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
It sounds like the dock is energized.

Remember, when you have a voltage difference, you still need to know which is energized relative to the world.

Ideally, I'd like to run a conductor from the shore service to measue voltages against. The dock MDP is next-best.
 

SG-1

Senior Member
Another test would be to place a clamp-on meter around all the conductors that feed the dock and make sure the reading is zero.

Nobody should go into the water until this is resolved.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
What is used for the grounding electrodes for the dock mounted transformers? How are all of the various grounding conductors interconnected?
 

xformer

Senior Member
Location
Dallas, Tx
Occupation
Master Electrician
Is it any possibility that the river might be the cause of the problem? Just

thinking that the static build up on the boat house due to river flow? Is there

such a thing???
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Is it any possibility that the river might be the cause of the problem? Just

thinking that the static build up on the boat house due to river flow? Is there

such a thing???
The water would have to be an insulator, I believe. If it were, you could have lightning underwater. :grin:
 

JWCELECTRIC

Senior Member
Location
Massachusetts
Lou,

Check out EC&M magazine Nov 2009. There was an article about a drowning at a boat dock. It had to do with an underground direct buried primary cable where the neutral had corroded. You may have another system in the area affecting the dock that you don't know about.

- JWC
 

ghostbuster

Senior Member
I would suggest this entire area be cordoned off and the power be immediately turned off before someone gets hurt or killed.Bring in the diagnostic electrical professionals including all the electrical safety groups in your area.

Here are some links.

The first link, check out page 6 and 7 (utility current in a community pool),it also has an additional link to this lake electrocution death via the utility.

Good Luck

http://www.ebmag.com/images/stories/PDFarchive/2009/12ebdecjan20092010bluebook.pdf



http://www.boatus.com:80/seaworthy/kritz.asp
 

big john

Senior Member
Location
Portland, ME
I just want to make sure I have the setup correctly in my head:
-Service disconnect feeds 3 phases & equipment ground down to MDP.
-MDP sends out 2 phases & equipment ground to each transformer.
-No other loads at MDP.
-No neutrals anywhere on the 480V system.

Is the type W in a raceway or is it free hanging? If it's free-hanging, are the conductors grouped together?

I would disconnect all the outside loads connected to this power system. That would at least eliminate the "badly wired boat" concern.

I agree with Don's comment about checking out the transformers. Are there any disconnects on the transformer secondaries you can open while checking the voltage to ground?

I would also verify the continuity of your equipment ground.

If I still had no clue, I'd start meggering feeders.

If this voltage goes away when you kill power at the MDP then this is a problem within that system. If you had stray utility voltage flowing on this system, it would keep flowing even after the MDP was opened.

-John
 

hardworkingstiff

Senior Member
Location
Wilmington, NC
I just want to make sure I have the setup correctly in my head:
-Service disconnect feeds 3 phases & equipment ground down to MDP.
-MDP sends out 2 phases & equipment ground to each transformer.
-No other loads at MDP.
-No neutrals anywhere on the 480V system.
All of the above are accurate.
Is the type W in a raceway or is it free hanging?
The W lays on top of floats or lumber (2x6 laying flat, float attaches to this sideways turned 2x6 and the joist are installed on top the sideways 2x6, and the deck boards are attached above the joist)
If it's free-hanging, are the conductors grouped together?
Yes.
I would disconnect all the outside loads connected to this power system. That would at least eliminate the "badly wired boat" concern.
I've been told (by the on site electrician) that this was done with no change.
I agree with Don's comment about checking out the transformers. Are there any disconnects on the transformer secondaries you can open while checking the voltage to ground?
I did that only once, check earlier post for the info.
I would also verify the continuity of your equipment ground.

If I still had no clue, I'd start meggering feeders.
I was told (on site electrician) the EC that installed it performed both of those during the week he was on site. (The install is actually about 5 years old.)
If this voltage goes away when you kill power at the MDP then this is a problem within that system.
Agreed, and that is what's happening.
If you had stray utility voltage flowing on this system, it would keep flowing even after the MDP was opened.
The voltage does go away when the MDP is opened.
 

big john

Senior Member
Location
Portland, ME
Are there any disconnects on the transformer secondaries you can open while checking the voltage to ground?
I did that only once, check earlier post for the info.
I'm a little unclear on this one: It sounds like you checked voltage-to-ground while you de-energized each circuit at the MDP. What I'm thinking is: Can you de-energize the circuit on the secondary 240V side of each transformer? If the problem remained then this would eliminate the 240V system as a suspect and then you could focus on the 480V side.
I was told (on site electrician) the EC that installed it performed both of those during the week he was on site.
Personally, I would want to talk to the guy who did the tests and find out what the readings were.

The next thing is to start taking current measurments to see how much is flowing and what it's flowing on.

Was it raining or foggy when the shock occured? I'm wondering if there's a bad spot on some insulation that wouldn't show up during a megger on dry cable, but when the cable jacket gets damp it might be enough to energize the dock.

-John
 

hardworkingstiff

Senior Member
Location
Wilmington, NC
I'm wondering if there's a bad spot on some insulation that wouldn't show up during a megger on dry cable, but when the cable jacket gets damp it might be enough to energize the dock.

-John

I was wondering that too. But it seems to me that the stepping down of voltage from the dock to the water as the feeders from the MPD to the transformers are opened indicates the issues applies to all of the feeders to all of the transformers.

I didn't open the transformers (was a short trip), but I'm pretty sure the equipment ground in the feeder is the only ground connection to the transformer, there is not a separate ground to a ground rod or....? I questioned one of the electricians about this during the installation years ago, and the response was "that's the way the engineer drew it".
 

SG-1

Senior Member
Has there been any repair work done on the docks in the last few months near the cables ?

As John has suggested look for current flowing.

The first rule of troubleshooting is to start from scratch. Do not trust what was supposed to have been checked before by somebody else.
 

big john

Senior Member
Location
Portland, ME
If I stretch out a conductor for 500', then connect the beginning side to L1 of an AC source, will there be a magnetic field around the conductor?
Absolutely. But in a system where all the conductors are grouped together, these fields cancel out. That's why a non-contact voltage sensor can easily detect 120V on a single conductor while still several inches away, but might not detect 120V on an extension cord even if placed right next to the cord: The electromagnetic fields on the hot and neutral in the cord cancel each other out.

That's why I asked how close the grouping was on the type-W conductors: If they were spaced far apart, you might well have some induction.

I know you said the voltage increased as each transformer was turned on, so that makes it unlikey the problem is in the secondary circuit, but if the installer wasn't sure how to wire those transformers, it's possible he made the same mistake in all of them (I've seen it before).

I second SG-1: Double-check that first guy. You might chase your tail because someone told you it was "right."

-John
 
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hardworkingstiff

Senior Member
Location
Wilmington, NC
I'd like to first say thanks to all of you that have posted replies.

I've recommended the owner call in an EE to asses this problem. I'm going to stay in touch and when I don't have other work to do, I plan on going down and continue testing/checking to see what I can determine.

Someone saying they were getting "shocked" is the thing that really bothers me. Talking to that person, I was told it felt like the tingling you would get with a pinched nerve. (They were on the dock and reaching into the water)

Everywhere I found "stray voltage", it looked like a phantom voltage because when I tested the voltage points with a light bulb in parallel with my DMM, the voltage went to zero.

If I ever find out what's going on, I'll let you know. Thanks again.
 

ELA

Senior Member
Occupation
Electrical Test Engineer
I plan on going down and continue testing/checking to see what I can determine.

Someone saying they were getting "shocked" is the thing that really bothers me. Talking to that person, I was told it felt like the tingling you would get with a pinched nerve. (They were on the dock and reaching into the water)

Everywhere I found "stray voltage", it looked like a phantom voltage because when I tested the voltage points with a light bulb in parallel with my DMM, the voltage went to zero.


Scary situation near water when someone is getting shocked.
I would suggest that when you take voltage readings from the water to the dock that you use a resistor of a value from 1000 ohms to ~20K ohms in parallel with your meter rather than a light bulb.

Phantom voltage is a relative term and you want to simulate the human body as closely as you can in this type of testing.
 
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