Isolated ground from switchboard to subpanels

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leblar1

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Here is the problem. My company is building a towboat for the Corp of Engineers. The prints call for an isolated ground connecting the panels. A seperate single conductor is run between the panels. The question is where the single conductor ties in to the panel. In the vessels we build we usually don't actually run a grounded conductor on power feeds, so I instructed the foreman to tie the single conductor to the bonded ground bus.

Another manager came behind me and instructed the foreman to tie the single conductor in at the neutral bus. Now I know we NEVER tie a ground to the neutral bus. He said it was because this was an isolated ground system: which started me thinking.

What I suggested finally was to isolate the ground terminal and tie the grounds and single ground there in each panel. This would keep it seperate from the neutral and seperate from the hull which is the true ground. Thereby creating an isolated ground system.

Any thought or suggestions.

Best Regards

Andre
 
Any isolated grounds I have ever installed have landed on their own "floated" ground bar so they ARE isolated. I am not sure how wiring on a boat would be different, I assume it would be for some reason.
 
Read 250.146(D) and 408.20 exception. I don't know what you mean by "an isolated ground connecting the panels". Any sub-panel requires that an "isolated ground" be pulled to the sub-panel and that the grounding and grounded conductors be separated in the panel. You may or may not have a grounded (neutral) conductor in the sub-panel, but you MUST have a grounding conductor. An isolated ground receptacle allows you to run the isolated ground through the sub panel(s) back to the main service where it would be installed on the Ground/Neutral termination bar. You cannot connect a grounding conductor to the grounded (neutral) terminal bar in a sub-panel. See 250.142. There must be only ONE Grounding system. If the hull is used as "ground" on your vessel then the isolated ground must eventually be connected to the hull in some manner, just as on land where the isolated grounds are connected to the ground/neutral bar at the main service where the grounding electrode conductor (to ground rods, etc.) is also connected. An isolated ground does NOT mean a separate grounding system. I am not aware of any section in the NEC that will allow you to have a separate grounding system. Even a lightning arrestor system must eventually be bonded to the electrical grounding system.
 
I agree with all above- my question would be can the hull grounding attachment be made anywhere since it is complately welded(bonded) together or should wiring be attached at the same location as in building grounding?
 
The grounding point for the vessel is at the switchboard.

For the power feeds we will generally run a 4 conductor for a 3ph 208V panel w/no ground. So the panel in question would not have a grounding conductor since the panel is welded to the frame of the vessel which is ground.

So if I understand correctly, I would isolate the grounding bus in the sub-panel from the panel which would give an isolated grounding conductor back to the original grounding point at the switchboard.


andre
 
I believe the hull attchement would be treated as a grounding electrode attachement in a land based installation. 250.24(A)(1) says that this should be done at the service, where it is also attached to the grounded service conductor. This is the only spot where grounding and grounded conductors should be brought together. See the Article 100 definition of Grounded. I believe that the hull of the ship "serves in place of the earth" in this definition.
 
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