Technical article from Blue Sea Systems
In the Technical brief on Reverse Polarity the flowing diagram of a Reverse Polarity sensing circuit is shown.
It is possible for such a circuit to faintly illuminate the Reverse Polarity light even though the circuit is properly wired. It is useful for boaters to understand how this can occur.
Imagine that one small change is made to Diagram 1 that results in the circuit shown in Diagram 2.
These two circuits are, absent a ground fault, electrically equivalent. All that has been done in Diagram 2 is to represent the green safety ground wire as an extension of the one leg of the Reverse Polarity sensing LED. We now have an LED with one leg on each end of the neutral wire. Because voltage is always consumed pushing amperage through a resistance (wire), the voltage is different at points A and B on the Neutral wire when there is current flowing through it. This is called "voltage drop".
When high amperage loads are operated in the circuit, enough voltage drop in the length of the Neutral wire can be created to overcome the resistance in the 25K resistor required by ABYC that sufficient current is driven through the LED to cause faint illumination. This situation is not inherently dangerous, however, it can indicate undersized wiring in the dock, shorepower cord or ship's wiring portion of the AC circuit.
Original article from Blue Sea Systems