Capacitive coupling?

[ggunn]

This is probably a silly question, but is there any reason why you couldn't just ground the dock solidly and be done with it?

 

[LarryFine]

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.

What that means is the total resistance of the current pathway is high enough that the additional load of the bulb displaces the voltage drop to other, more-resistive sections of the pathway.

It's exactly the same reason I prefer a solenoid-type tester over a voltmeter for most troubleshooting; it effectively ignores phantom voltage, behaving just like your bulb and voltmeter.

 

[hardworkingstiff]

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.

Thank you.

 

[hardworkingstiff]

This is probably a silly question, but is there any reason why you couldn't just ground the dock solidly and be done with it?

This has been tossed around, but knowing the specifics of how you "solidly ground" a wooden dock is in question. Any suggestions?

 

[hardworkingstiff]

Here is a pic from 2004. This is to give you an idea of how the docks have cables installed. The dock on this thread has deck boards all the way across the dock where this one has concrete tiles that cover the center section.

If you look at the left chase you can see a black material on the bottom of the joist. That material is a plastic filter fabric. The on site electrician thinks this material is contributing to his problem because he claims that when he cut a section of it out (about 3' long) it changed his readings. I just don't see how this could be contributing to the issue, but stranger things have happened.

With the information I have right now, I'm thinking the 2 of 3 phases being used to power the transformers is producing a hybrid magnetic field that is inducing a voltage into the deck framing (why the other cables that use 2 different phases don't cancel, I can't explain). The treatment of the wood (chemicals and some copper) and the moisture content of the wood is what allows it to happen. I wonder if that filter fabric under the deck joists can contribute to the induction some how?

So, if indeed my far-fetched theory may be true, grounding the dock might take care of it (as suggested earlier). I'm thinking a bare copper wire just dropped into the water and attached to the joists may take care of it, but I don't know if it would create some other problem. I guess it would make better sense to ground the joists from the system ground.

I am asking for suggestions on how far to space the ground attachments to the dock joists (if we decide to do this) and what size conductor would you use (I think a #12 would suffice, but...)?

 

[SG-1]

Something to try

Something to try

Looking back over your data it looks to me like there is a high resistance ground falt between L1 & the wood dock. When I add the L1 to N + L2 to N, I get 244 volts. When I add the to readings taken from the deck screw to each line I get 242 volts. Very close even if the square root of 3 should have been used.

I believe L1 is grounded because the voltage to the screw is less then the L1 voltage to the neutral. Where as L2 to the screw is greater than the L2 to the neutral voltage. L3 to the screw ?

I would expect the voltage between the deck screws and the water or ground will be the greatest near the fault point. You could take deck screw measurements every 10 or 20 feet then closer to find the highest reading. The on-site electrician may be willing to do this.

Then carefully check the nearest wired devices.
I suppect a very small amount of current is flowing, in the milli-ampere range.
You would have to de-energize ( un-wire ? ) just that device or un-mount it from the dock so it does not touch the wood, sit it on something non-conductive.

If you loop a wire around the dock it will not fix the root cause, it may move the voltage gradient to a different location for a while. Suppose the bow of a small boat goes under the dock at low tide and breaks the wire ? To feel good about it you would have to use a cable. It may also move the voltage gradient just far enough from the dock to to be undetectable yet disable a swimmer. I am sure it is posted no swimming, but...

 

[hardworkingstiff]

Looking back over your data it looks to me like there is a high resistance ground falt between L1 & the wood dock. When I add the L1 to N + L2 to N, I get 244 volts. When I add the to readings taken from the deck screw to each line I get 242 volts. Very close even if the square root of 3 should have been used.

I believe L1 is grounded because the voltage to the screw is less then the L1 voltage to the neutral. Where as L2 to the screw is greater than the L2 to the neutral voltage. L3 to the screw ?

I would expect the voltage between the deck screws and the water or ground will be the greatest near the fault point. You could take deck screw measurements every 10 or 20 feet then closer to find the highest reading. The on-site electrician may be willing to do this.

Then carefully check the nearest wired devices.
I suppect a very small amount of current is flowing, in the milli-ampere range.
You would have to de-energize ( un-wire ? ) just that device or un-mount it from the dock so it does not touch the wood, sit it on something non-conductive.

If you loop a wire around the dock it will not fix the root cause, it may move the voltage gradient to a different location for a while. Suppose the bow of a small boat goes under the dock at low tide and breaks the wire ? To feel good about it you would have to use a cable. It may also move the voltage gradient just far enough from the dock to to be undetectable yet disable a swimmer. I am sure it is posted no swimming, but...

Thank you for reviewing the data and commenting. I'm going to make some time towards the end of the week to check this out some more. I'm going to look on my time (unless I can find the problem/s and fix them).

I couldn't get the relationship between the data I posted and I'd like to thank you again for reviewing it again and giving your opinion.

Any ideas on why the voltage continues to increase as the feeders to the transformers are energized? I can't help but feel that it's the 2 of 3 phases in the cable feeding the single-phase transformers, but I can't explain why there isn't some cancellation as more and more transformers are energized.

This is going to take some time.

PLEASE, if you come up with any other thoughts, post them. Thanks.

Edit: I only checked at one transformer on the secondary side, it's possible I get the same readings at the other transformers (9 or 10) too. It's also possible I don't (but I believe I will).

 

[big john]

Just curious, did this ever see any sort of resolution?

-John

 

[hardworkingstiff]

Just curious, did this ever see any sort of resolution?

-John

I spoke with the engineer last week. He told me they found the 600-amp panel on the dock was single-phase. The service is 3-phase. Parallel (3 sets) W cables were pulled from the service disconnect to the MDP on the dock. The cables were 4-wire with 2-hots and ground being used leaving one unused conductor.

They grounded one end of those unused conductors and reworked the grounding on the transformers on the dock and got the stray voltage lower. They did determine it is some kind of capacitance and the engineer thought the plastic filter fabric might be acting like an insulator in a capacitor (but he thought it was a stretch).

The voltage (deck screws to water) gets lower the closer you get to the transformers. The EE thinks the transformers being bolted to the dock might be pulling the voltage down since the transformers are bonded by the EGC.

The EE said he didn't think it was a big problem and a real head scratcher.

I suggested he have the electrician lift the line conductors at the transformer and see if the voltage goes away. If the voltage is still there with the line side of the transformer disconnected then you can concentrate on the feeders ahead of the transformer. The EE seemed to like that idea and he was going to try it. I haven't spoken with him since then.

Having only 2 of 3 phases down there makes me feel even stronger about my theory of the out of balance feeders developing a small field that is being induced into wet, treated with copper wood.

 

[Electric-Light]

You could megger it on DC to really make sure that there's no insulation breakdown caused by being wet. Any current flow at DC is a leakage. The difference in AC & DC is caused by capacitance.

The input impedance of a normal DMM is 10 to 20MOhms.

Get one of this adapter, which is basically a resistor block that goes parallel to meter input.

http://www.globaltestsupply.com/c/3/Fluke_TL225_SureGrip_Stray_Voltage_Adapter_Test_Lead_Kit.html

Either that, or you can get a Fluke that has low-input impedance function like the 116.

 

[jcole]

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?

If the other side is not connected to L2 (no current flow) then you would not have an electromagnetic field. It would be an electrostatic field (capacitive).

 

[hardworkingstiff]

If the other side is not connected to L2 (no current flow) then you would not have an electromagnetic field. It would be an electrostatic field (capacitive).

Thank you.

Do you think that electrostatic field could induce a voltage into treated lumber?

 

[Smart $]

I spoke with the engineer last week. He told me they found the 600-amp panel on the dock was single-phase. The service is 3-phase. Parallel (3 sets) W cables were pulled from the service disconnect to the MDP on the dock. The cables were 4-wire with 2-hots and ground being used leaving one unused conductor.

They grounded one end of those unused conductors...

I fail to see any purpose in grounding (bonding) one end.

...and reworked the grounding on the transformers on the dock...

Without the how, that bit of info is almost useless.

...and got the stray voltage lower. ...

This is the only info that keeps the "reworked" info from being totally useless. Doubt bonding one end of the extra condutors contributed to this. In reworking the grounding, they apparently decreased the resistance from the dock to ground and water.

...They did determine it is some kind of capacitance and the engineer thought the plastic filter fabric might be acting like an insulator in a capacitor (but he thought it was a stretch).

I'd determine the level of system leakage before persuing the capacitive improbability. Even if there is a capacitve effect between the dock and water, coupled with varied resistance between dock and grounded metal parts, the dock has to be getting it energy from somewhere other than the water and grounded parts. That leaves system leakage or inductive coupling between dock and energized conduuctors, or a combination thereof.

The voltage (deck screws to water) gets lower the closer you get to the transformers. The EE thinks the transformers being bolted to the dock might be pulling the voltage down since the transformers are bonded by the EGC.

That's a reasonable deduction being the dock is not a low resistance path.

The EE said he didn't think it was a big problem and a real head scratcher.

Until the voltage source is determined it is a big problem. Until such is known and remedial action taken, the possibility of the problem getting worse rather than better also exists.

I suggested he have the electrician lift the line conductors at the transformer and see if the voltage goes away. If the voltage is still there with the line side of the transformer disconnected then you can concentrate on the feeders ahead of the transformer. The EE seemed to like that idea and he was going to try it. I haven't spoken with him since then.

Need to determine and follow a controlled shutdown with documented testing at each stage.

I'd start by opening each secondary disconnect working from the farthest back, and measuring local voltage from dock (screws) to ground and water before and after each disconnect opening.

If that don't expose the problem, do the same process while deenergizing the primary feeders one tranny at a time. If the problem didn't go away, you have stray current from another system. If the problem goes away, disconnect the feeders from each tranny and megger the trannies and feeders.

Having only 2 of 3 phases down there makes me feel even stronger about my theory of the out of balance feeders developing a small field that is being induced into wet, treated with copper wood.

With what info has been provided so far, I certainly cannot rule it out...