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Basic electricity for boat builders, boat repairers and owners. What you need to know about the electrical systems on your boat. Generation of electricity, grounding electrical circuits.

So where does electricity come from, or rather, how do we make electricity? What are the sources?

The first and most obvious is the electrochemical process I described as a battery. Dissimilar metals in an electrolyte form a cell from which electrons will flow between the metal plates.  But what about when the battery gets too low (discharged) and we need to recharge it?  Where does that electricity come from?  And where do we get this Alternating Current?

That comes from a different process call electromagnetic induction. Many hundreds of years ago people discovered magnets.  They also discovered that if you float a magnet in a liquid it points to magnetic north, the north pole. This led to the magnetic compass. But it wasn't until the last few hundred years that scientists determined why.  Our earth has a magnetic field. This field radiates along lines of magnetic flux that flow north and south.  Every magnet also has a magnetic field around it with these same lines of magnetic flux.
See Wikipedia on Magnetic Flux.

Then someone discovered that if you move a wire (a complete circuit, not just a short piece of wire) through a magnetic field it causes electrons to flow in the wire. You are creating electricity.  As the wire moves through the lines of magnetic flux around the magnet, the electrons in the wire get all excited and start dashing about.


Basic Generator

So after much experimentation it was discovered that if you rotate a coil of wire on a shaft (a rotor or armature), inside a magnetic field, electricity is generated in the coil. This is a DC generator

 If you are much over 40 years old you may remember that cars had generators, not alternators, up until the 1970's.  For the most part, the devices that make electricity are still called generators, even though they are now mostly alternators. So what's the difference?  A generator puts out DC, and an alternator puts out AC.
See Wikipedia on Electrical Generators

As I said on the previous page, DC loses a lot of voltage when transmitted over long distances due to resistance in the wires, and a thing called voltage drop. Someone discovered that AC did not lose a lot of voltage this way. So until recently power plants have all put out AC from very large Alternators. In recent years methods have been discovered to transmit DC over long distances as well, but power plants are still mostly AC. In generators, if you wind half of the coil, (the armature) one direction, and the other half the opposite direction, as the rotor turns the electrons go one way in one half and the opposite way in the other half. That gives you alternating current. If you spin this rotor at 60 revolutions per second you get 60 cycle AC.
See the War of Currents on Wikipedia

So, power plants put out AC. Car generators put out AC.  Generators on your boat, whether on the engine, or a separate generator, all put out AC.  This is then "rectified", that is, converted to DC and used to charge your batteries and run equipment.
See Rectifier

In the USA and around the world, there are thousands of plants generating electricity by using steam to turn the generators, water to turn the generators, wind turbines, tides, solar cell arrays, geothermal powered generators, and even nuclear power.  The plants are all connected to a power grid. In North America, the USA and Canada are both hooked up to the same power grid.  This grid is divided up into many little grids and separate power sources, but by throwing switches the power you get in your home can come from almost anywhere in North America. This power can then be used by you to not only power all of your electric appliances, but also to charge your batteries.
See Wikipedia on Electric Power Transmission.

Marine Alternator

Usually, the batteries in your boat are recharged by the generator on your engine, but if hooked up to shore power, you can use a charger to do this.  You can also put what is called a converter on your boat, convert the AC to 12 V DC and use it directly to run all of your DC equipment, as well as charge the batteries. A typical marine alternator.

Also, there are alternative methods to generate electricity. The best known is solar power. Certain types of silicon, when exposed to sunlight, put out a small amount of electric current. By connecting hundreds of these solar cells together, large amounts of current can be generated.  Many boats used for long distance cruising use solar cells to recharge their batteries.   Some cruisers use wind powered generators to charge their batteries.
See Wikipedia on Solar Cells

Also see on solar power and wind turbines. 


One of the most important concepts when dealing with electricity is the

Grounding rod 

concept of ground. It literally means the earth. In order to measure the amount of voltage in a circuit, you have to have a reference point that is considered zero volts. In most cases this is ground, or earth.  In the AC power grid the ground point is literally the earth. A large rod is driven into the earth and connected by cable to one side of the AC system. See photo for a typical ground rod. 

AC uses three wires. A black wire which is considered the hot side, a white wire which is considered the neutral, or grounded side, and a third green wire which is a safety wire, or grounding wire. If you go to your local substation you will see such a ground.  Even your house has a ground. If you look at the wiring in your house you will see that it is made up of three wires bundled in a cable. They are black, white, and green. If you have 220 or 440 there will be a fourth wire which is red. Both the white and the green are connected to ground. This is an important point, because on shore side systems the white and green are connected. But on a boat they are not. That is why the distinction between the grounded wire and grounding wire is so important. 
See Grounding.
See Video Grounding and Bonding

In DC systems, ground is usually established by using the negative side as the ground. However, some DC systems do use a third wire. The positive wire is red, the negative wire is black or yellow, and the third wire is green.

All wiring on boats is color coded. That is, for each type of equipment and for main power circuits there is a specific color used for the positive wire. The negative is always black or yellow. 
Go to Color Codes table to see the colors used. The battery is connected to the engine block and so all wires connected to the negative side of the battery are grounded to the engine block. ISO Color Codes are different than those used in the United States.
 See Wikipedia on Wiring Color Codes

Another important point is that there should be only one ground point in any system. This goes for both DC and AC. Both are grounded at the engine block. This is the only point on the boat where the two systems are connected. 2007 All rights reserved. revised 03/28/2021