Essential aid for traveling boaters when connecting to unfamiliar dock side power. The quality of marina wiring can vary greatly from place to place and from plug to plug. That is why most boats designed to plug in to shore power have a polarity indicator. It is usually a warning light on the AC electrical panel, although it can also be an audible alarm. It is an important safety feature that few boat owners fully understand or appreciate.
With a couple of exceptions, it should be standard equipment on every boat that is capable of plugging into a common 120 volt AC shore power outlet. Its purpose is to allow the boat owner to verify that the electrical outlet on the dock is wired correctly.
When polarity is incorrect or reversed, it is because the hot and neutral wires are switched. Without an indicator light you will not know when this condition exists.