AC current must alternate between two points making a circuit.

Coming from the power source is the “hot” wire, which normally has black insulation, and returning is the neutral or “grounded” conductor, which is white. “Neutral” carries the same current as the hot wire. As long as the current remains in this closed circuit there is no danger, but if it should escape (a “fault” or “short circuit”), it will attempt to go directly to ground.

Most modern AC circuits have a green third wire, which is a “grounding” wire. It is connected to the third prong of the common three-prong plugs; it parallels the white wire and it connects to neutral at the power source. It’s supposed to ground the circuit when a short occurs.

In a household system the third wire works well as long as three-prong plugs are used and the grounding wire is intact. (Note: it won’t protect you if you touch the hot and neutral wires at the same time.) Household electrical systems are grounded through a metal rod driven into the earth under or next to the building. Between that rod and the people in the house are many layers of wood, concrete and other electrically insulating materials.

In a boatyard, where workers may be standing or crawling on wet ground, there is a potential for electrocution. When a boat is floating, the water is the ground and any metal that has an electrical path to it, including the hull of a metal boat or the engine of a glass or wood boat via the shaft, becomes a path to the ground. Touching any of these items and a hot wire at the same time can send current through the body.