Dock and Amps

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sb1280

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While inspecting a dock using a clamp method as shown by James Shafer in
http://www.icomia.com/library/ICOMIA_File_Download.asp?BK_ID=251
or
http://www.halifaxharbor.net/Marina Grounding 6 04.pdf
I get a reading of .501 amps clamping the feed to the dock, which includes 2 undgrounded conductors, one grounded (neutral) and a grounding conductor while operating a 1500 watt hairdryer in one outlet.
This is a new commerical 14 slip dock with no shore powers, just outlets and lights, and no load at the time of the test. It has one lighting circuit and two separate outlet circuitis.
This dock is bonded to ground.
see link for detail wiring information: http://www.ameren.com/LakeOzarks/ADC_DockElectrInstal.asp
NEC 2005 with AHJ changes as listed online.I shut off the load and reading remains, fluctuating between .359 and .501.
I throw the mains feeding the dock and the current readings remain.
I had a similiar dock recently showed .10 amps but when the power was turned off, the reading left.
All outlets on both docks checked correctly for polarity and grounding.
My guess and that is what it is a guess, is that I am seeing a reading because by bonding the dock to the service and requiring a grounding electrode at the seawall the amp reading is coming from the service wiring. Whether it is stray voltage or just a better grounding system I don't know.
My question: Does this present a danger?
In the case of the commerical dock it is a 4 wire subpanel.
In the .10 amp case it is a 3 wire circuit with GFCI protection from an older house that is probably has the neutrals and grounds on a common bus.
Any insight with answers or additional test I can perform to ensure the safety of the dock owners and their guests would be appreciated.
 
sb1280 said:
I throw the mains feeding the dock and the current readings remain.
Click to expand...
This would lead me to believe it's coming from the PoCo. Try lifting the PoCo neutral (with mains off) and see if the current drops off, if it does you have your answer).
I had a similiar dock recently showed .10 amps but when the power was turned off, the reading left.
Click to expand...
This would indicate the problem is with the premise wiring, but .10-amps? You sure it's just not your meter, that's reaaalllllyyy low.
 
ceb58 said:
Where are you getting the readings from? The ungrounded legs, grounded or EGC. I think all you are seeing is a ghost.
Click to expand...

Mr Shafer's Marina Inspection procedure calls for using a clamp meter on the ungrounded legs and grounded and EGC at the same time. Looking for 0.00 under a load like the 1500 watt. If the reading any other than 0 the power is finding another route back, ie. water, dock, cables, etc.
But on the commerical dock the reading is there load or no load, power or no power.
 
Yes, stray voltage or more correctly called NEV ( Neutral to Earth Voltage ) can present a hazard. Besides a shock hazard this could present a drowning hazard. The cause of the drowning may never be linked to NEV. Until a second victim turns up.

I think you will find the current is flowing on the EGC. It is grounded at the service and again at the dock. As stated the dock is grounded. There is a difference of a few volts between the dock and the service. The service is energizing the dock. I would think it would be less than 5 volts, but it could be more. You could probably measure voltage between the dock and the earth or the dock and the water. Measuring will require a low impedance voltmeter.

The POCO should be involved because either their primary neutral or a service close by is the source. Only they can pull meter bases on other close by services to eliminate them as the possible cause.

If it is determined the POCO primary neutral is the cause a neutral isolator could be installed to correct the issue. The POCO may have a high resistance connection that is causing the voltage which they can correct. Driving more ground rods at the service or Dock is not the answer. This at best will just move the potential gradient to another area.

It is very fortunate you discovered this. Peoples lives have been lost because of situations like this. On Mikes Home Page down the left side near the bottom is a link to Technical then look for Stray Voltage if you want to read more. There is a lot of material to read.
 
sb1280 said:
Mr Shafer's Marina Inspection procedure calls for using a clamp meter on the ungrounded legs and grounded and EGC at the same time. Looking for 0.00 under a load like the 1500 watt. If the reading any other than 0 the power is finding another route back, ie. water, dock, cables, etc.
But on the commerical dock the reading is there load or no load, power or no power.
Click to expand...
With the use of a clamp on meter and the errors that are introduced by the position of the conductors within the amp clamp, it would not be unusual to see a reading of 0.1 when there is no actual current flow outside of the circuit conductors. I am not trying to say there is not a problem, as there very well may be, I am just saying that this method will not give accurate results.
 
sb1280 said:
Mr Shafer's Marina Inspection procedure calls for using a clamp meter on the ungrounded legs and grounded and EGC at the same time. Looking for 0.00 under a load like the 1500 watt. If the reading any other than 0 the power is finding another route back, ie. water, dock, cables, etc.
But on the commerical dock the reading is there load or no load, power or no power.
Click to expand...

Let me throw this out there using the SWAG method. Could you be seeing this due to galvanic action between the water and the dock. Since the dock is bonded and there is an GEC at the seawall the movement of the water in contact with the dock may produce its own current.
 
ceb58 said:
Let me throw this out there using the SWAG method. Could you be seeing this due to galvanic action between the water and the dock. Since the dock is bonded and there is an GEC at the seawall the movement of the water in contact with the dock may produce its own current.
Click to expand...

Isn't voltage involving galvanic action a DC voltage and would it show up on the OP's meter? I don't think it will, I think. :)
 
hardworkingstiff said:
Isn't voltage involving galvanic action a DC voltage and would it show up on the OP's meter? I don't think it will, I think. :)
Click to expand...

I guess it would be. That's why I'm using the SWAG method:D. However, the OP is stating he is getting AMP readings not VOLTS. If the galvanic action did produce DC voltage you should still read some amp reading. I think:confused:
 
ceb58 said:
I guess it would be. That's why I'm using the SWAG method:D. However, the OP is stating he is getting AMP readings not VOLTS. If the galvanic action did produce DC voltage you should still read some amp reading. I think:confused:
Click to expand...

I didn't explain my thinking very well. I'm of the understanding that the meters most of us use to detect amps do not detect DC amps and since DC volts puts out DC amps, well, I don't know. <shrug>
 
The approach suggested in 'Mr Shafer's Marina Inspection procedure is using a clamp on amp meter to detect any 'net' current flowing on the feeder to the marina. This approach is quite similar to the way a GFCI detects fault current, and will act to detect any current flowing on a path 'outside' of the feeder.

A significant difference, however, is that the described approach clamps everything, including the EGC in the feeder. This is useful but not a sufficient measurement. 1) A ground fault that is completely carried by the EGC won't be detected and 2) any current flow on the EGC caused by outside sources will be detected even if there is absolutely no problem with the electrical system being measured.

Given the net current flowing on the feeder, the next step is to measure both the net current flow on the feeder circuit conductors excluding the EGC, to determine if there is a fault on this particular circuit. Then measure the current flow on the EGC alone.

If the EGC is connected to earth (water) electrodes in multiple locations, it is entirely possible to see current flow on the EGC, caused by currents flowing through the earth from one electrode to another. This can happen with absolutely no flaw in the 'local' electrical hardware.

-Jon
 
SG-1 said:
Here is a related thread.

I think the real matter here is if there is any difference in voltage between the Dock & the earth, & especially between the Dock & the Water.

http://forums.mikeholt.com/search.php?searchid=2767437
Click to expand...


I can't get this link to show anything.

Also, at both docks we also checked voltage gradient using leads that held 1' apart and connected to ac volt meter touched to the water at various points and positions around the dock and inside the wells. There were no reportable readings. (.0010 to .0030).
 
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