AC Reverse Polarity Indicator

An important part of a boat’s AC shore power system is the 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 is “neutral” line and 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 crossed 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 causing the indicator light to illuminate.  This light may also come on in a situation that is not reverse polarity, this is also very dangerous and must be carefully investigated.

- Lyle, PYS Electrical Technician