Since alternating current, by definition, flows in one direction then the other, what is meant by polarity when applied to an AC shorepower connection and why is polarity so important on a boat? Even though the current flow reverses, the "hot" wire is connected to the generator at the power plant and the "neutral" wire is connected to ground there. That means the electricity flows to us through the hot wire. All switches and circuit breakers must be in this side of the circuit to disconnect the load from the power.

Now suppose connections to the dockside receptacle are reversed. That puts all the AC breakers on the boat in the neutral side of the circuit. An overload might still trip the breaker, but since the breaker is in the neutral side, the circuit is unprotected from a short. Current will continue to flow until the circuit burns open. A fire aboard is the likely consequence.

Reversed polarity also presents a serious shock risk. Turning off a breaker appears to remove power from the circuit because it turns off all appliances connected to that circuit. But with reversed polarity you have disconnected the appliance from ground, not from power. The circuit is still live!

If your AC switch panel does not have a polarity tester, buy a plug-in tester and use it. Most also detect an open grounding wire and other dangerous conditions.

Note: This is Don Casey, Tip #40. Don Casey is the best-selling author of "Do-it-Yourself" books, including "This Old Boat". His books provide valuable and money-saving solutions to hull and deck repair, boat electronics, sail repair, and much more. This article is a reprint from the April 3, 2007 edition of the Boat U.S. Weekly Web Specials. This, and more articles by Mr. Casey can be found on the Boat US web site at: http://www.boatus.com/howto/howto_lib.htm