Wiring a reverse-polarity alarm


There was no shore power reverse-polarity light on my newly acquired vessel and I would like to install one. Looking at the connection in other boats confirmed my guess that the light can be connected between ground and the neutral (white) line. Although it makes sense, I don’t like using a ground connection as a power return, even for short periods of time. Are there other ways to connect up a reverse polarity indicator?

Also, I am suspicious of connecting any circuit to ground, since this opens up another pathway for electrolysis if power usage across supply phases is not reasonably balanced. Are there ways to alleviate this potential problemperhaps, say, using a neon bulb as an indicator?

John Hodgson

Marina del Rey, Calif.

Answer 1: The proper way to wire a reverse-polarity alarm is as describedbetween the neutral and the grounding wire. The American Boat and Yacht Council (ABYC) standards specify that such a device should have a minimum resistance of 25,000 ohms. The reason for this is that if we apply Ohm’s Law, assuming a 120-volt circuit, we find that, if the polarity is reversed, activating the device, the current flow will be 120/25,000 = 4.8 milliamps, which is just below the tripping threshold of U.S. Ground Fault Circuit Interrupters (GFCIs); they trip at 5 milliamps. This way the alarm will alert the operator without tripping the circuit. With such a high resistance in the circuit provided by the 25K-ohm resistor, there is no need to worry about leaks to ground, galvanic corrosion, etc.

Contributing editor Nigel Calder is the author of several books, including Boatowner’s Mechanical and Electrical Manual, published by International Marine.

Answer 2: The avowed purpose of the alarm is to indicate an incorrect connection from the vessel’s AC power system to the shore-power supply. A "correct" connection is desired to ensure safety for both those on the boat and other nearby boats and persons. Although the typical reverse-polarity alarm, consisting of nothing more than a small neon lamp and a 25,000-ohm resistor, is apparently simple, its function is frequently misunderstood.Understanding the function of a reverse-polarity alarm begins with recognition that in an alternating current (AC) power system neither of the current-carrying conductors can truly be called a "ground" conductor. After all, when the electrical potential (voltage) of one power conductor is increasing, the other power conductor’s potential must be decreasing.Construction of an efficient electrical power distribution system requires that connections between various conductors can be made without destroying the system or the generators that supply the electrical energy. For this reason, the power companies have developed a standard for 120-volt single-phase power distribution in which one of the two conductors is arbitrarily called the "hot" wire (the "black wire" in North America) and the other, by default, the "neutral" (white wire). As noted, neither power conductor in an AC system is permanently at either a positive or a negative potential. Clearly, their respective potential must alternate between zero, the maximum positive, zero, and the maximum negative potential.

Keeping track of which wire is arbitrarily assigned as the "neutral" is necessary in order to prevent potentially dangerous voltage differences from existing between one boat and another, connected to the same or separated shore-power systems. In a properly wired AC system, there should be no effective (current producing) potential difference between the neutral wire and the safety ground wire. Therefore, an incorrect connection may be detected by sensing a current flow between the assigned "neutral" wire from the shore power system and the electrical system’s safety ground wire.

As shown in diagram #2, Section E-8.32.1 of the American Boat &Yacht Council (ABYC) standards, the indicator is to be connected between the vessel’s safety ground wire (the "green wire") and the AC shore-power neutral, the "white" wire in North American practice. (Note: if you travel outside North America, the color coding of the shore-power wiring as well as the voltage and frequency may be differenttake note and take care!) If the polarity of the wiring is correct, no current flows, and the indicator will be silent. If the polarity of the incoming wires from the shore is incorrect, a voltage difference sufficient to activate the reverse-polarity alarm will be present.

Many reverse-polarity alarms consist of a miniature neon lamp connected in series with a current-limiting resistor. The resistor is necessary for two reasons: to limit the current flow to a level safe for the lamp and to ensure that the current flow is less than the 5/1,000 of an amp (5 milliamps) needed to actuate the vessel’s Ground Fault Current Interrupter (GFCI). A reverse-polarity indicator current requirement greater than 5 ma would likely cause the GFCI to trip.

If you plug into shore power and find the reverse-polarity indicator on your boat illuminated (or sounding, if it is an aural alarm), you should immediately disconnect the shore power at once and inquire about the shore-power system wiring on the dock. It is not all that unusual to find situations in which the polarity of the shore power is reversed from the normal or where the neutral is "floating" and not properly referenced to ground. There are also instances in which the shore power safety ground wire (the "green" wire) may not actually be connected to a proper earth ground. Connecting to an improperly wired shore-power system can create potentially harmful conditions for personnel as well as damaging galvanic currents.

The typical ground fault indicator system has two possible shortcomings. The neon lamps do not have infinite life. Leave your boat connected to shore power for months or years on end, and the lights will eventually fail. Fortunately, they usually signal their failure intent by first becoming blinking indicators. The other problem is that neon lamp indicators fail silently. Obviously, a failed neon lamp cannot indicate a wiring fault. Some of the more recent indicator systems use Light Emitting Diodes (LEDs) to signal a problem. These indicators offer two advantages: they are much brighter, therefore easier to see; and, with a typical operating life of more than 100,000 hours, they are less likely to fail. Some reverse-polarity sensors use transistor or integrated circuits powered from the vessel’s 12-volt supply to sense the presence of a problem.

By Ocean Navigator