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post #1 of Old 06-01-2007 Thread Starter
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Shorepower and Reverse Polarity

Perhaps someone can enlighten us about reverse polarity. Don Casey's tip (copied below) is clear enough, but still fails to explain our 'current' situation.

Earlier today we went for the first sail of the season. When we arrived at the dock all was well with the shorepower that we had been connected to over the winter. Upon our return from sailing I plugged us back in and the AC panel indicated reverse polarity so I disconnected shorepower. The reverse polarity indicator lit up once last year when one of the batteries died, but it has since been replaced and this time both batteries measured 13V. The only thing that changed was the boat that has the other outlet/meter on our electrical post had plugged in and was charging. A few questions:

1) Any ideas why after 1 year a reverse polarity problem would appear? Could it involve the old boat charging next door or are marine meters/outlets usually adequately isolated from one another?

2) What implications does reverse polarity have for charging batteries?

3) How can one adequately protect themselves from bad shorepower? Is an older Protech 4 enough or???


Don Casey Tip #40

Alternating Current/Reverse Polarity

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 stll 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.
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