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[kavokcinque]

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One further ABYC requirement for RPI's is that they contain a minimum 25,000 Ohm Resistor. A full treatment of the reason for this is beyond the scope of this article. It is sufficient, however, to say that because the Safety Green wire is connected to devices aboard the boat that may contain stray currents, the link created by an RPI could provide a path for stray currents via the grounded Neutral wire. (See also Note 3)

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.

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.
http://www.cruisersforum.com/forums/f14/...-1180.html