|
Index to this Page
Basic Electricity - Page 7 - AC Circuits
Basic electricity for boat builders,
boat repairers and owners. What you need to know about the electrical
systems on your boat. Intro to AC Circuits. Marine Cords, Polarity.
I suppose we all remember our mother telling us not to stick things in the wall socket, and not to put our
fingers in the light socket. Hopefully we all learned from this. Because
Alternating Current (AC) electricity is very common. Like gasoline, we use it all the time, and
although it is dangerous, we become complacent. What we have in our homes
is installed in a way to keep us safe from harm. Alternating current
systems are very dangerous. If you have the slightest doubt about your
knowledge and ability to understand them and install them correctly,
don't. Hire a professional.
In the following pages about onboard AC systems I am only
going to deal with 120V Single Phase AC. There
are other systems, up to and including 120/240 Volt Three Phase. If you
don't know what this means then see the next paragraph. 120V AC is often
reffered to as 110V or 115V. I will always use 120V.
I strongly recommend you hire a marine engineer, naval architect or
certified marine electrician to help you design your AC system for your
boat, and have a certified marine electrician install it. By
certified I mean, they have passed the ABYC certification course for
marine electrical systems. I cannot emphasize this enough. An
electrician may be very competent at his job and may have installed wiring
in thousands of homes. But AC circuits on boats are NOT the same as
AC on shore. There are significant differences. So make sure you hire
someone who has passed the ABYC certification exam.
Direct Current will give you a tingle but AC will fry you in an instant. Do not fool
around with it. It can kill people in the water near your boatand it can also burn your boat to the waterline, as can DC, so
be careful and make sure it is installed correctly, to the latest ABYC, or
ISO Standards.
Please forgive my attempts to scare you. I am a bit of a purist and
frankly, would prefer to have a strictly DC system. But most boats over
25 feet today have AC systems on board. This is mainly for convenience
because they can then use AC appliances and air conditioning. If
installed and maintained properly AC
can be safe and reliable. The key is installing it correctly and
maintaining it properly.
Things that are most important in installing an AC System:
Picking the right service level, that is, 20 amp, 30 amp or 50 amp.
Picking the right shore power connection and cord for the boat.
Keeping the Black, White and Green wires separate and of correct
polarity.
Sizing overcurrent protection. Picking the right
amperage
for Circuit Breakers.
Using Ground Fault Circuit Interruption (GFCI) to protect people.
Isolating the system to prevent galvanic corrosion.
See
Corrosion On Boats.
https://newboatbuilders.com/pages/corrosion.html
Preventing current leakage into the water or current
impressed on a metal hull. This results in Stray Current Corrosion and can
cause Electric shock deaths to anyone in the water. See
the following:
Corrosion On Boats.
https://newboatbuilders.com/pages/corrosion.html
Electric shock
Drowning
https://newboatbuilders.com/pages/electricity15.html
Creating an effective path back to ground.
Marine Cords
I covered where AC comes from in the beginning of this series. The power
lines from the substation extend out onto the dock, where there is a power outlet. A cord is
used that has plugs on both ends and plugs into both the outlet and the
boat. This is not just an extension cord. These cords are designed
specifically for marine use. They should be Marine Rated or UL -
Marine Listed. Do not use any thing other than marine cords.
Here is
a link showing a cord, shore side receptacle and boat inlet receptacle.
This is not an endorsement of their product although Hubbell is
widely used.
Hubbell Cords and Receptacles https://www.hubbell.com/wiringdevice-kellems/en/hubbell-marine
Marine cords are designed to fit in a specific
type of socket depending on the amperage rating of the cord. The blades are a
different configuration for 20 amp, 30 amp and 50 amp. This is so you
cannot plug in the wrong cord. They also have a mechanism that locks
them to the socket and prevents water from getting inside and the plug
from easily pulling out. Male ends have molded on boots at the
connector, and female ends have a locking ring. Here is typical
shore power inlet.
The socket, or shore power inlet, on your boat has to match
the plug on the cord, for the same reasons as given above. This inlet will
have a cap that covers the socket and keeps water out when it is not being
used. Here is a link to a
Coast Guard Boating Safety Circular81
https://uscgboating.org/library/boating-safety-circulars/BSC81.pdf#ac that has an informative
article on AC Electricity on boats, page 1-4. It also shows diagrams of
the various socket and plug connections. This is public information. You
need Adobe Reader to read and print it because it is a pdf file.
There is a section about AC power cords and why they must be the correct
cord.
Since I first wrote this in 2007 a new shore power plug has
come on the market. It complies with both ABYC Standards and
USCG regulations. Here is a link to the Smart Plug.
https://smartplug.com/ The inventors of this plug
claim it solves many of the problems of conventional shore
power plugs. It is being widely used on boats and on
RVs. It is up to you to decide if you wish to use this type
of plug. It does the same thing as the other plugs;
transferring power safely onto your boat. However, keep in
mind if you buy a cord with a Smart Plug on it, you must
also buy the socket it goes into. This will not work
with a conventional shore power socket.
Polarity
One of the primary reasons for the types of sockets is to
maintain polarity. In DC systems we learned that one side is positive and
the other side of the circuit is negative. This is polarity. In DC the
polarity must be maintained. Otherwise the equipment simply won't work.
AC polarity is important for two reasons, the proper operation
of the equipment, and for your safety on the boat. In AC circuits you
have three wires. The black is the hot, the white the neutral, and the
green the grounding or safety wire. On the shore side the black is never,
ever, connected to ground. The white wire is connected to ground. If at some point you
accidentally switch the wires, now the black wire is connected to ground and
the white is not. Now both the black and green are hot (meaning
current is flowing in the wire) and you
have a serious potential for shock because the metal case of
everything on the boat becomes hot, and if it is a metal hull, it is hot.
So throughout the AC System, from the power plant to onboard your boat the
polarity must be maintained.
This is why appliances either
have three prongs, or one prong is bigger than the other. This is to
maintain polarity. This is also why you should never cut the third prong
off. The third prong is the green wire. If you cut it off and a short
occurs, now the current has no way to get to ground, except through you.
All AC electrical equipment has color coded
connectors, just like the
wires. The hot side is black, the neutral side is white or silver,
and the grounding side is green. In 220 and 440 systems there is also a
red wire and red connector. Fortunately there are polarity indicators you
can use to tell you if it is right. These range from small handheld
devices you plug into a socket, to indicators that are built into the electrical
panels and shore power inlets. These will tell you if something is hooked
up wrong. Do not use the circuit until the polarity problem is corrected!
I cannot emphasize this strongly enough. Reverse polarity is dangerous to
you, and some AC equipment can be damaged. Other equipment simply won't
run. I recently spoke to a person who upon plugging into shore power
got a reverse polarity indication. He also noticed his batteries were not
charging. The charger simply would not work with reverse
polarity. The problem was on the dock. The white and black were
switched in the shore power socket.
Reverse polarity most often occurs when either the shore side is wired
wrong, or someone has added an appliance on the boat and wired it in
reverse. Have the system checked by a certified marine electrician.
Links to Offsite References:
Wiring Your Boat https://newboatbuilders.com/docs/WiringYourBoat.pdf
© newboatbuilders,com 2007 All Rights Reserved Revised 02/23/2022
|
Everything Boat Building
