ECN Forum Forums Code Related Discussion NEC & other Code issues Shore Line supplies to boats.

A while back I was involved in a discussion of shore line electric power that supplies wiring built in to boats. Further discussion has indicated to me that the problem that the marinas are having is corrosion that is caused by the split return path the neutral current takes when the neutral of the boats system is bonded to the engine or to the hull grounding plate. In a couple of cases I have observed in the time since the earlier discussion the zinks seemed to be boiling off the hull. The production of bubbles ceased instantly when I unplugged the shore line. The only remedy I have developed so far was to use a short pigtail with a male cord cap to connect the on board wiring to the on board power source and use that same short pigtail to connect the on board wiring to the shore line extension cord. This leaves the on board wiring's Equipment Grounding Conductor (EGC) not bonded to the boats ground when on shore power. This generates a couple of questions in my mind that I would like comments on.

Since the US NEC stops at the dock receptacle I believe that this technique cannot be an NEC violation. Would you agree?

Do you see any real safety issues with not having the EGC bonded to the metal objects, such as the engine, on the boat?

How could we guard against a fault in the cord, such as from chafing, energizing the lifelines or metal railings on the boat.

A Ground Fault Circuit Interrupter (GFCI) might prevent a tragedy but it would not prevent the problem from occurring in the first place. The universal installation of multi pole transfer switches that would switch the neutral with the ungrounded conductors across an entire marina would break the circuit without opening the EGC but every boat would have to have a switched grounded current carrying conductor for that to work.

Could we equip the boat with an isolation transformer that would break the neutral current return path that is going through the boat. The transformer would be in one of those plastic battery trays to keep it aloof from the boats grounding electrode system. The neutral of the transformer secondary windings would be bonded at the boats distribution panel rather than at the transformer.

I have never seen any complete answer to the marina stray voltage problem and I'm looking for ideas.
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Tom Horne

I can think of no reason to bond the neutral to the boat DC ground. Yiu are actually creating a hazard that doesn't need to be there when the boat is underway. I would also drive a local ground rod at the dock to bleed off any objectionable currents in the EGC.

gfretwell Wrote:
I didn't follow your response so I'd like to ask some questions. If the vessel in question has an AC electrical system that is supplied by an on board derived system it would need to have a grounding connection to the hull grounding plate. That same plate is the grounding point for the DC system, the lightning protection, and the radio / radar systems. The AC wiring of the on board system will be supplied by a shore line when the vessel is at the dock. Even if a multi pole transfer switch that switches all the current carrying conductors is installed the Equipment Grounding Conductor of the shore line will continue to carry some of the neutral return current of other vessels in the marina that have their neutral conductor bonded to the hull plate during shore line operation. If I keep the EGC of the on board AC wiring aloof from hull ground then there will be no fault clearing path if an AC conductor faults to the metal hardware of the vessel during shore line operation. Given these conditions how do I guard against a fault to the vessels metallic systems during shoreline operation?

What would having a supplementary grounding electrode bonded to the EGC at the dock do? The objectionable current is divided neutral return current that is getting into the water from neutrals that are improperly bonded to hull grounding plates during shore line operation. Even if the wiring on some boats is corrected to avoid this connection it is in the nature of the marina industry that vessels will come and go. It would seem that the marina operator does not have an effective way to prevent this stray current flow.

If the receptacle that supplies the shore line is equipped with Ground Fault Circuit Interrupter (GFCI) protection then any fault is going to trip the GFCI because the water is at least likely to carry the five milliamperes that are needed to trip out the circuit. If all the outlets to which a shore line can be connected in the marina are GFCI protected then vessels that have the neutral bonded to the hull plate would not be able to operate from those outlets because there would likely be enough stray current through the water to trip the shoreline GFCI. That would avoid having neutral current flowing through the water but it also might cut down the use of that marina by visiting boats.
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Tom Horne

OK let's back up. There is no reason, legally or logically why you would ground one side of the onboard generator.
Once you connect to the shore tie one side will get grounded, via the neutral in the cable but if that was bonded to a ground rod at the dock you would still be star wired so there is still no current on the EGC. If you do bond the generator on your boat and tie to a bonded sevice you have created a regrounded feeder ground loop and made salt water the electrolite to insure boatloads of loop current through your metal fittings to ground.