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Joined: Nov 2000
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Joe, The only system that I would let anyone I loved around would include a transformer at the boat slip. How would that change anything? The EGC of the primary and the secondary must both be connected to the case of the transformer. That means you are still directly tied to the service grounded conductor. Don
Don(resqcapt19)
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The last thing I would want is to be a connecting link for a ground loop over 700' apart at the ends, especially with one end being a body of water. It would really be a total waste to bypass the transformer's excellent isolating properties and the fact that it is now a separately derived source. I would want to maintain the feeder side ground clear into the transformer enclosure. You would definitely want a L-L or L-G fault to clear and need to get back to the source for that. The only thing I would tie that ground to might be some primary surge protection for the transformer. My goal for the secondary grounding would obviously be to be at a zero potential difference with the water. One would also think that this would be closer to the ideal ground than 2 rods in clay up on some cliff. Now, can you buy a grounding electrode boat anchor??? Joe
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Joe, It would really be a total waste to bypass the transformer's excellent isolating properties and the fact that it is now a separately derived source. While the code calls it an SDS, it is not electrically isolated and can't be if it is installed per code. The only thing I would tie that ground to might be some primary surge protection for the transformer. My goal for the secondary grounding would obviously be to be at a zero potential difference with the water. Again, I ask how can that be done and remain in compliance with the code? The primary EGC must be connected to the transformer. The secondary of the transformer is required to be a grounded system. The secondary EGC will also be connected to the transformer case completing the path back to the service grounded conductor. Don
Don(resqcapt19)
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I'm not saying that this is a code requirement anywhere BUT: The only system that I would let anyone I loved around would include a transformer at the boat slip.
Don, If we go back to my original statement, I wouldn't worry climbing onto that slip my way and wouldn't go near it the code compliant way you mentioned. I would like a specific code reference if you wouldn't mind because I'm more of a theory guy than a code guru. My only hope is that anyone going up or down that ladder now, as built, is in compliance with J3:16. Our poster has already measured a more than lethal current and our code experiment, future victim, has already felt the warning tingle. Alot of folks don't get that many chances or warnings. Joe
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Why would we treat this any different than a swimming pool or a agricultural building. If you create an equipotential ground reference at the dock with the land or the water I think the problem goes away. If you just want a quick experiment. put a ground rod clamp on a 10' stick of EMT or rigid and drop it in the water, connect that to the EGC. Test it. If that makes the trouble go away you should look for a more compliant grounding electrode near the dock.
It is really not that unusual to see a couple volts ground shift between systems 100' apart. The worst I ever saw was more like 35 volts.
Greg Fretwell
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Joe, As far as code references, you start with 250.20(B) which will require the secondary of your transformer to be a grounded system, at least for any of the commonly used voltages. Then to 250.30(A)(1) that will require a system bonding conductor to bond the secondary grounded conductor to the equipment grounding conductors. 250.110 will require the exposed metal parts to be connected to the EGC. This results in the primary and secondary EGCs both connected to the transformer case. Under the NEC it is really impossible to have a electrically isolated system with some rare exceptions. In addition the voltage in this case most likely has nothing to do with the power service to the dock. It is most likely stray voltage from utility primary grounded conductor current flowing in the earth/water. Don
Don(resqcapt19)
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Almost, With the mains to the house, garage and dock off I still have the same current flow, yet neither the new copper ground rod at the meter, nor the poco ground, nor their guy wire show any current flow. Did you remove the main bonding jumper for this test? The utility primary and secondary grounded conductors are connected together. In many cases like this the source of the voltage is the voltage drop on the utility primary neutral. Don
Don(resqcapt19)
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Just found this thread... here's how others have handled the problem and their logic concerning panel grounds to docks. http://www.lakesonline.com/Forum/sh...CB1831E2F&tid=40878&SiteID=AL008 The thread is a good read. If anyone can tell me what's wrong with this method (other than it may not be described in the NEC) please let me know.
Last edited by Plumbdumb; 03/25/11 06:24 PM.
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Refer to Gfretwells post above # 167497. The link IMHO is primarily DIY and homeowners looking for 'help'.
My opinion is to seek professional help from a licensed electrical contractor. In the event you may be located in an area that does not license electricians, seek a reference for a qualified electrical contractor or electrician.
John
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I would say that the link describes the same as Gfretwells post by establishing an equipotential ground between the dock, land, and water. This appears to be different from a ground wire back to the panel. Apparently not all electrical contractors know the solution to this problem and the solution may not be in agreement with NEC
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