The Official Magazine of the Bluewater Cruising Association

Reverse Polarity

Jeff Cote

Pacific Yacht Systems
November 3rd, 2017

An important part of a boat’s AC shore power system is a reverse polarity indicator light.  Typically, power to a 120VAC system is delivered on three lines.

  1. The black wire is “hot” and supplies line power.
  2. The white wire which is the “neutral” line which provides a return path under normal conditions back to the source of power.
  3. The green/yellow wire is the safety ground line which is bonded to the neutral line at the source of power. It also provides an alternative path for power to return if there is a fault in the circuit and the neutral return path is compromised.

A reverse polarity situation arises when the hot and neutral lines are inadvertently cross wired making the neutral “hot”.  The reverse polarity indicator light is connected between the neutral and the safety ground lines, which normally will have zero electrical potential because they are bonded at the power source. However, in the case of reverse polarity there will be an electrical potential difference between the two lines resulting in the indicator to illuminate.  There are unusual conditions that can also lead to the reverse polarity indicator to glow without being a reverse polarity situation, but any illumination of the reverse polarity indicator should be carefully investigated.

– Lyle, PYS Marine Electrical Technician


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