Working inside a boat up on blocks is essentially the same as one in the water, if it is grounded through its three-wire power cord. If the vessel is not grounded and a fault develops in the hot lead, workers outside the boat on wet ground or contacting metal ladders or stands are in danger.

Extension cords, especially the household two-prong type, increase the risk; wearing rubber boots and rubber gloves can reduce the risk somewhat. Using an AC on the exterior of a floating boat is courting disaster. If a power drill or sander gets splashed or falls overboard, seawater will conduct current from the hot wire to the case, making the tool hot. If the grounding wire is not effective, any path to the sea via wet decks or a metal conductor makes the worker part of the circuit. Divers and swimmers in the water are susceptible to electrical shock, especially if there is a direct short such as would occur if a live power cord drops into the water.

Even a relatively low-voltage fault can establish an electrical field around the boat, which could cause a current flow through a swimmer’s body causing fibrillation. Current leakage into the water can also paralyze muscles and cause drowning with no visible evidence of electrocution.

A GFCI, or ground fault circuit interrupter, is an inexpensive (~$20 each), switch-like device that continuously monitors current in the hot and neutral conductors. When the GFCI detects an imbalance between the two, as would occur if there were a short to ground, it instantly trips the circuit.

A single GFCI can protect persons throughout the boat if it is located on the main AC feed, but because boats usually have various small current leaks, the unit may trip frequently with no indication of the source of the fault. It may be more practical to install one GFCI on each of the circuits to wet locations, such as head, galley, engine room, and weather deck.

GFCIs are extremely sensitive and can be tripped by “steam” or dense moisture in the air such as from cooking or showering, so they should be mounted in dry locations. On gas-powered boats, GFCIs must be ignition protected, or located in areas where fumes cannot collect. GFCIs should be checked at least monthly.

Minimize electrocution risk from an onboard AC electrical system by ensuring that the vessel is properly wired by a professional marine electrician, and inspecting it periodically for damage or deterioration.

If your electrician isn’t familiar with ABYC (American Boat and Yacht Council) standards, find one who is. Use only copper multi-strand wire (preferably tinned “boat cable”), of correct size for the load, with marine color coding.

Ensure that all connections are inside a panel box so that it’s impossible to touch them accidentally. Better yet, make them accessible only with the use of tools. There should be no bare wires anywhere on the boat. All connectors must be properly sized “captive” (ring-type) terminals match the size of the screws, with insulated shanks, and should be made of corrosion resistant materials.

Tension relief and drip loops should be incorporated. All AC outlets on board must be three prong type. Appliances should plug directly into three-prong wall sockets, not extension cords, and multiple socket plugs shouldn’t be used on board. Maintain correct polarity by using only approved plugs and if anything in the system has been modified or repaired, check it with a polarity tester. When making up plugs, ensure that the black wire goes to the brass or black screw, the white wire to the silver screw, and the green wire to the green screw.

Service outlets on the exterior of the boat are a potential problem and to be avoided. Never interconnect the AC and DC systems. The green wire must connect to the boat’s bonding system or metal underwater hardware, but the AC white wire must not. Don’t confuse the black insulation on an AC power lead with the negative on a DC system.

When you switch between a generator, inverter or shore power, the grounding connection must switch too. (If the boat is on shore power the green wire connects to the underwater metal hardware of other boats on the same shore power system. This creates a galvanic cell that promotes stray current and galvanic corrosion. A galvanic isolator on the green wire allows passage of AC but not DC, thereby isolating the boat from the others. A more sophisticated device for the same purpose is called an isolation transformer.)

Here are a few more tips for minimizing risk when working around an AC system:

• Turn off the breaker at the shore power box before disconnecting the cord, and disconnect from the dock end first. Connect at the boat end first and switch on the dock breaker last.
• Use only tools and appliances with three-prong plugs, and if you must use extension cords temporarily with power tools, use only cords with three-prong sockets.
• Shut off generator, inverter, and main AC panel switch before working on the AC system
• If you must work on live AC, do like professional electricians and work with one hand behind your back to avoid touching hot and neutral or ground at once.
• Remove jewelry, wrist bands, or other conductive items.
• Protective clothing, including rubber boots, rubber kneepads, and rubber gloves offer some protection from shock. Rubber or plastic insulated handles on tools like pliers and screwdrivers also help.
• When working on the end of a cord with multiple wires, tape off all but the one wire you’re working on. • Unless you’re trained in marine AC systems, leave it to a professional.