Join Date: Aug 2007
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Rep Power: 10
'Normal' polarity is that condition in which AC voltage is measured between the 'hot' (black) conductor and neutral (white) conductor. You should be able to measure that same voltage between hot and ground too, if all is well...that is, if the neutral and ground are connected together AT THE DOCK, NOT THE BOAT. A little Neon indicator will glow if connected between the hot and ground.
On the other hand, it may be that the black conductor and the white are swapped somewhere down the line. You'll still be able to measure the AC between the hot and neutral as before, only this time the indicator will glow if connected across the neutral and ground...that's what the RP indicator does.
'Older' boats and marinas rarely exhibit reverse polarity...that's because if the condition ever existed at all, it most likely has been corrected.