Allport Marine Survey
Tony Allport, SAMSŪ SA Serving the Puget Sound Region |
Your AC Polarity Indicator, Its Purpose and Importance, by Tony Allport, SAMSŪ SA
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 because your electrical system still works normally. AC electrical lights and appliances don't care which direction the current is flowing, they work fine either way.
When polarity is reversed it means that electric current is coming in the back door (neutral wire) of the circuit instead of the front (hot wire). You may have closed the front door to keep electricity out of the "house," so to speak, but in reality, the "house" is still full of electricity coming from the opposite direction. This greatly enhances the possibility for an electric shock or fire; and makes it impossible to isolate the individual branch circuits labeled on your electrical panel.
Why don't we worry about the polarity of AC power coming into our homes? The reason is that houses are typically only connected to the grid once, by an experienced professional. If you spend much time afloat you might plug your boat into a dozen or more marinas in the course of a season.
There are two situations that eliminate the need for a polarity indicator on a 120 volt AC electrical system. One is if the boat is equipped with an isolation or polarization transformer. These are large and expensive pieces of equipment that are generally only found on larger yachts. The other exception is a boat that is equipped with double pole circuit breakers on every branch circuit. This type of breaker is designed to connect or disconnect both the hot and neutral wires for a given circuit simultaneously. This again represents a significant upgrade over what a production boat builder is likely to install and very few boats are so equipped. Some larger boats are equipped with a 240 volt AC shore power interface. Polarity indicators are not required on these systems because if the problem exists it will be abundantly obvious as things begin to blow when the connection is made.
The main breaker on your AC panel is required to be the double pole type regardless of how the rest of the system is configured. The polarity indicator should be installed upstream of this, or between the main breaker and the boat's shore power cord inlet. The proper way to connect your boat to shore power is to turn off the main breaker on the boat and the breaker at the shore power pedestal. Then make the connections with the shore power cord. Once the connections have been made turn on the power at the pedestal/dock and then check the polarity indicator. If it indicates that everything is OK then, and only then, turn on the main breaker on the boat.
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The problem is that safeguards built into your boat's AC system will not function properly if the current is not flowing in the right direction. Reversed polarity can also be deceptive. Branch electrical circuits are typically designed with a circuit breaker on the hot wire at the beginning of the circuit. Imagine turning a circuit breaker off in preparation for doing some work on the wiring. Your expectation is that the circuit is now dead and safe to touch. If the shore power polarity is reversed you will likely be shocked and rudely surprised to find that the wiring is still hot. This can result in serious injury or death. Needless to say, maintenance on your electrical system should only be done by a qualified person. 
Some of them might be rustic affairs in remote locations where a well-intentioned handy person added an outlet at the end of the dock in an effort to accommodate a few more boats. To test it they plugged in their electric drill and it worked. Everything is OK, right? With electricity, the fact that it works doesn't make it right. The fact that it works when it is wrong increases the likelihood that you will someday plug into a pedestal where the polarity is reversed.
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Installed August 15, 2006 - Last updated March 20, 2008 - Pages Hosted & Maintained by Don Robertson