Basic Electricity - Page 7 - AC Circuits

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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 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.

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 on boats is NOT the same as AC on shore. There are significant differences. So make sure you hire someone who has passed the ABYC certification exam.

DC will give you a tingle but AC will fry you in an instant. Do not fool around with it. 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.
Using Ground Fault Circuit Interruption to protect people.
Isolating the system to prevent galvanic corrosion. 
Installing the system so current doesn't leak into the water or be impressed on a metal hull.
Being sure there is an effective path back to ground.

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  Marine cords are designed to fit in a specific type of socket depending on the amperage rating of the cord. Here is link to a Marinco shore power inlet .  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 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 Circular 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. Here is another article, in Professional Boat Builder Number 100 April/May 2006 about shore power connections, Ground Fault Interrupted .

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.  But in AC it becomes even more 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. 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 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 in your house, 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 electrical equipment has the connectors color coded, 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 in to 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 check by a certified marine electrician.

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