[billybek]

Found this article by searching for isolation transformer for marine applications.

SAFETY AND ELECTROCUTION PREVENTION

Contrary to popular belief, electricity does not seek ground; instead it will attempt to return to its source. If the source is the utility company (or the transformer at the head of the dock), its power is referenced to earth, which means faults will return to earth in that case, or the water in which a vessel floats. If a fault develops aboard a vessel that is not equipped with a transformer, the current could leave the vessel via a bonded/grounded through-hull fitting, shaft, or rudder, and travel through the water on its way back to its source.

If a person enters that return path, a swimmer, diver, or someone who falls overboard while dockside, they could be electrocuted—it happens every year. Last Independence Day, in three separate incidents, five people were electrocuted on lakes in the United States in this manner. Make no mistake about it, swimming around boats or docks equipped with shorepower is dangerous; it is prohibited in many marinas with good reason. My advice to readers is don’t do it.

Because both isolation and polarization transformers behave very much like a source of power, fault current seeks a path back to them rather than to the shore. This means that onboard faults will not result in current flow through the water. Current will return to the transformer, ideally tripping a circuit breaker and rendering the fault inert in the process, making transformer-equipped vessels inherently safer.

Why wire a transformer in polarization rather than isolation mode, if the former requires the use of a galvanic isolator, whose effectiveness is limited? Essentially, there is a measure of added risk—the level of which is debated by experts within the world of marine electrical systems, in wiring a transformer in the isolation mode, in that the metallic case of the transformer is not referenced to both shore and dockside ground.

Therefore, if the hot, primary, or shoreside input cable of the transformer were too chafe against the case, because the case is connected to the boatside ground, it could become energized without tripping a circuit breaker. (Remember, it can’t be connected to both boat and shore ground without creating a path for corrosion current.) If a person were to then touch the transformer case and a through-hull or rudder stock for instance, they could complete the path, and be electrocuted in the process.

While this is a real concern, if an isolation transformer is properly installed in accordance with the manufacturer’s and ABYC’s guidelines, it will remain safe. Currently, transformers may be wired as isolation or polarization, and remain fully compliant. Among the most common defects in transformer installations are those involving a failure to use manufacturer-specified strain relief connectors for cables entering the transformer’s case.

Under no circumstances should a transformer be wired without the benefit of proprietary strain relief devices, which prevent chafe as well as isolating tension or strain from electrical connections within the transformer.

Finally, shorepower transformers used in marine applications should carry a UL Marine approval decal (many transformers are UL approved, few carry a UL Marine approval) as well as complying with ABYC E-11.17, last revised in 2012. Many older transformers carry an ABYC E-8 approval, which is outdated.

If you have a shorepower transformer, or are considering having one installed, or are purchasing a vessel that is so equipped, make certain it is installed in full compliance with current ABYC guidelines.