How It Happens
The specifics of having any given boat plugged into shore power are innumerable, but in any case two things must occur for AC electrical current to flow through the water and put swimmers, divers servicing hulls, and those who fall overboard at risk. The first is an electrical fault aboard the boat. This could be a short circuit caused by deteriorated wire insulation or by a failure of the wiring within a device or appliance in use aboard.
The second is a break in the green “ground” safety wire of the boat, shore cord or dockside power source. This green wire is designed to carry fault current back to its source, and in the event of a short in a wire, or a fault within an electrical device, to trip the breaker. But if the green wire is broken, missing, corroded or improperly wired, the safety feature is gone. Fault current then seeks another path back to its source ashore, often through bilge water, metal fittings or drives, and finally through the marina water.
Test Your Dock
Industry pros liken plugging a boat into shore power to tossing a hair dryer in a bathtub. This is why ABYC devotes pages of guidance on how to properly wire a boat. While there are similarities between household wiring and boat wiring, there are a number of key differences that can mean life or death. An electrician who is not aware of these differences could create an ESD hazard by wiring a boat like he would a house. When it comes to your boat, it’s best to invest in a trained marine electrical technician for wiring, repairs and modifications.
That said, you can perform a check of your boat’s electrical connection by using a clamp meter like the Blue Sea Systems mini clamp multimeter ($150, westmarine.com), which resolves AC milliamps. Clamp it around the shore cord. If all is well, you will get a “ zero” reading. If you get any reading other than zero, that is an indication that current is leaking from somewhere in the system. It could be your boat, another boat or the marina’s wiring. Unplug the boat. It’s time to call in the pros.