I think you may have missed something critical here guys. The boat is in brackish water! Assuming he has standard zincs, they may not actually be providing the protection he needs in the first place. This is a big problem here in New Orleans where people install zinc anodes when the recommendation for brackish water is either aluminum (truly brackish) or magnesium (more fresh than salty). It is possible to overprotect a boat with magnesium but just this simple step could solve your problems.
Of course if you do have a stray current that could be it as well, but brackish water doesn't have the same properties as salt, and with zinc leads to under protected boats. Heck after the spring flooding when they dumped the Mississippi river into our normally brackish water you would be amazed at how fast boats were corroding. Must have been all that stray current in the Mississippi.
Salesman - Allied Titanium
Would an ELCI gfci 30ma breaker have detected and tripped?
For NEC and ABYC in the USA.
If the boat does not have an isolation transformer breaking the grounding conductor, then the ELCI should trip on an imbalance (between the ungrounded and grounded conductors) in shore power AC current greater than 30mA rms.
If a Galvanic isolator is used, for grounding conductor current to flow, the AC peak voltage level must exceed the diodes forward voltage. I would also have to check on the parallel current path through the cap at 60Hz to be complete.
As I understand the problem description there was a connection to the water from the neighbor at 120VAC. The return was through the boat to the grounding conductor at levels in the water that exceeded the Galvanic Isolator forward voltage. Because the boat was not the source of the AC leakage the ELCI would not trip.
If the boat had an isolation transformer there would be no AC return through the boat to the grounding conductor.
I think many who do not do comprehensive ABYC Marine corrosion surveys are unaware of the step-by-step approach to diagnosing both Galvanic and Stray Current corrosion.
One of the first basic tests, with the boat disconnected from shore power, is to take the AC voltage at the boat dock between the water and the Dock Power AC Grounding conductor using a DMM. The problem described in this thread would have been discovered in the first 15 minutes after the Tech reached the dock.
For basics of what is involved see Professional Boat Builder #100, and for ground faults PBB #94. IMHO the minimum credentials for a comprehensive marine corrosion survey are ABYC Cert for Electrical, Corrosion, Standards and Systems. It is also helpful to have an EE background or other ship corrosion experience. There are some electricians who are also very good at corrosion surveys, but usually work at or support a vary large Marina complex.
Each experienced ABYC corrosion surveyor will have their own special way of doing things depending on the problem description to save time.
In PBB Nigel Calder had his own approach and introduced the use of different testers. My procedures are slightly different and I make a lot of my own shore power tester breakout boxes rather than buy.
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