More on shore power
To the editor: All your articles are interesting and I read almost every one completely. In a recent article on shore power ("Hot marinas, corrosion and safety," Issue 120 March/April 2002), I have a question regarding the schematic diagram on page 72 showing the ABYC endorsement of a 230 VAC shore power hookup. I failed to see the "added wrinkle" in the diagram of needing to disconnect the neutral and grounding conductors when switching from the genset back to shore power. Shouldn't there be a fourth terminal on the transfer switch that does this?
And the diagram also showed a conductor going to the engine negative terminal. Please correct me if I'm wrong, but wouldn't that connect your AC and DC systems together? The text, and common sense, says don't do that.
Brian Ford writes from Chicago where he is refitting his Kalik 44 for living aboard and voyaging.
Steve C. D'Antonio responds:
Your questions are excellent, as well as being intertwined, and I will attempt to answer them as simply as possible.
All grounds, whether AC safety, DC negative, bonding, lightning, etc., should be common or at the same potential. In that respect, you are correct, this does connect the AC and DC systems together, in a manner of speaking. There are several reasons for this but the primary is to ensure protection against electrocution, and then for corrosion protection and lightning prevention/dissipation. This is why the schematic shows a connection between the AC safety ground and the engine block. If the engine block were to become energized with AC hot current, and it were not so grounded, the breaker would not trip. It would remain energized, and a person could then complete the path to ground.
There is no need, nor is it desirable, to separate the connection between the genset's ground and the rest of the boat's AC safety ground system. The primary concern is to prevent a connection between neutral and ground in all locations other than the source of power, i.e., the generator, utility supply on the dock, inverter, etc. when any of those items are acting as the source of power. This prevents an alternate return path for AC current through the water surrounding the boat. When the transfer switch is opened, disconnecting the genset from the boat's AC grid, this is achieved because the neutral connection is then opened, thus separating the AC safety ground and neutral systems. Furthermore, it is mandatory that the genset chassis remain grounded at all times to prevent the above mentioned scenario of AC hot energizing the block. It doesn't matter what the object is — engine block, generator, fuel tank, etc. — once energized, the potential for electrocution is the same.
Contributing editor Steve C. D'Antonio is a freelance writer and the boatyard manager of Zimmerman Marine in Mathews, Va.