4F5: AC Reverse Polarity
Although we tend to think of AC Electricity as having only two conductors, it actually has three.
One of these is called “neutral”; one is “hot”; and one is “ground”, that is, it is supposed to be the same
as the water around the boat and the earth ashore. The vessel and many of its appliances rely upon
these connections having the correct “polarity”, or relationship to one another and the earth; this is
essential to be sure that users of AC equipment do not get a shock when touching and AC equipment.
Now in a house ashore, it’s easy: We don’t “plug in” the house, for it stays connected to the utility
company all the time! But in a boat when in the harbor, we do plug in using our Shore Power cords
(and sometimes using extension cords). If the outlet to which we plug our cord, or if the cord itself is
mis-wired, then these connections can become mixed up, and then there is a significant chance of
getting a shock or just as bad, a chance that running gear outside the boat will be subject to rapid
corrosion, because the boat is immersed in sea water, a good conductor of electricity.
To protect the vessel and its crew from such contingencies, a “Reverse Polarity Warning” light will
illuminate when the connection turned on.
If the “Reverse Polarity” light illuminates when connecting to Shore Power,
immediately disconnect the cable and contact the harbor master advising him/her
of the problem. Do not risk shock or system damage!
4F6: AC Shore Power, Disconnecting & Connecting
The large AC selector switch on the bottom of the AC panel is used to determine the source of
AC power for the boat. The switch has four positions, “Off”, “Gen”, “Fwd Shore”, and “Aft Shore”. The
“Shore” positions represent the bow and stern shore power connectors for the shore cable.
This switch should be left “OFF” whenever you are connecting or disconnecting the boat to
shore. This is true so that you do not draw an arc from the plug due to the load of the boat on the
connector’s pins: such an arc will burn the contacts and eventually cause them to overheat when in
use, creating a fire hazard.
Once connected to shore power, monitor the AC voltmeter and ammeter to be sure you have
not overloaded the circuit.
Important Note: If the house/inverter batteries are low when you first hook up to
shore power, and the inverter is turned on (as it should be), the inverter will begin
charging its batteries at a very high charging rate, drawing a lot of shore power
current. Until this demand reduces (see “The Inverter System” below), you should
turn “OFF” other high-current AC appliances such as the water heater.
You can then turn on AC appliances as needed. Watch the ammeter to be sure you don't
exceed the dock's available supply, typically 30 amps.
Here are estimates of typical AC appliance power consumption:
Water Heater
, 15 amps;
Inverter
, up to 22 amps;
Hair Dryer
, 12 amps;
TV
, 1.5 amps;
Coffeemaker
, 10 amps;
Microwave
, 12
amps;
Toaster
, 12 amps.
Section 4G: Electrical Systems, DC 4.11
