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 or cord 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
(illustration above) will illuminate when the connection turned on.
If the “Reverse Polarity” light should illuminate 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!
A momentary switch “Test Bulb” allows the user to verify bulb operation.
4F6: AC Shore Power, Disconnecting & Connecting
The AC panel shore power selector switch on the AC circuit breaker panel is used to determine
the source of AC power for the boat.
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 circuits.
Important Note: If the 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 refrigeration
.
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 some estimates of AC power consumption for typical appliances:
Section 4F: Electrical Systems, AC 4.13