I'll be honest. Most of the articles I've seen on electrical wiring in boats numb me into insensibility by starting out with electron theory. My feeling is that since I can't see electrons, I can't even be sure they exist, let alone that they have the capability of doing useful work on board. I don't know how the transmission in my car works either, but I can still put it in drive and add fluid to it.
The point here is that you don't need to be a rocket scientist in order to maintain your existing electrical wiring or upgrade it to include more appliances or circuits. All you need to know is that the mysterious electrons live in the battery and that once they leave home, all they want to do is get back there again. If you'll allow me to mix my metaphors, the electrons can be thought of as runners on a baseball field—home plate is the battery, first base is the circuit breaker, second base is the appliance, third base is the ground buss, and back to home again. Since the electrons run in a circle, the electrical base paths made of wiring are logically called circuits.
Maintaining a boat's electrical system is not difficult. What is needed most is an eye for obstacles along the base paths that will prevent the electrons from arriving at the next base. First and foremost among these roadblocks in the marine environment is corrosion. Terminals on the ends of wires, splices or butts where two wires meet, or even the green crumbly copper corrosion that can penetrate deeply into an electrical cable will hamper the electron on its journey. So the wise boatkeeper spends some preventative maintenance time periodically walking along the base paths, looking for chafed wires, crippled terminals, or corroded connections. If these hurdles are removed before they can interfere with the electron's circular journey, they will be allowed to travel unimpeded.
The time will come, however, even with due diligence, when you turn on a light and, voilŕ—nothing happens. All you know at this point is that something is preventing the electrons from getting through. Before you head off on a wild goose chase looking for the break in the circuit, study the problem for a few more clues. Is it just the one light (or pump, radio, or other appliance) that isn't working, or is everything on the circuit inoperable? Look for the obvious—if nothing on the entire boat is working, maybe you forgot to turn on the battery switch. If it's just the one light, perhaps it is nothing more than a "burned out" bulb (burned out being a misnomer for a broken wire filament inside the bulb stopping the electrons dead on their path).
Once the fault has been assessed, you have a pretty good idea of which base paths to start searching. Let's take an example and say that you noticed that your stern running light is out. The first thing to check is the condition of the other lights on the circuit, namely the port and starboard running lights. It they work, the problem is limited to the stern light only, but if the side lights don't come on either, the entire running light circuit is probably broken—somewhere. Your job is to find out where.
There are two tools of the trade that every sailor ought to have on board and know how to use. The first is an electron pressure gauge (all right, it's really called a voltmeter) and the second is a base path checker (normally called an ohmmeter). Little portable multimeter models containing both these tools are inexpensive and available at every marine, department, and automotive store—spend a few extra bucks to get a good one, preferably digital. You can use either one of these tools to help find the cause of your problem. Here's how:
Voltmeters These devices measure the electrical pressure behind the electrons flowing past the gauge. If there is no pressure, no electrons are running past. Think of the electrons like water flowing through a garden hose—if the water stops coming out the end of the hose, you notice right away and go looking for a pinch or kink along the length of the hose. Since you can't see the electrons, you need a voltmeter to be able to visualize the flow pressure. Voltmeters are designed to have one probe put on the positive side of a terminal and the other on the negative—if you reverse them on most multimeters, they will simply read "-12.68," or whatever the voltage is as a negative.
Set your multimeter to read a 12-volt flow and start diagnosing your running light fault by testing across the battery terminals. This creates a standard for the system (and also tells you if the battery is dead). Write the battery voltage down or commit it to short-term memory.
Now, test the first-base path by leaving the negative voltmeter probe on the negative battery post and moving the positive probe to the input side of the running light circuit breaker. You may need a length of wire with alligator clips on the end as an extension if the distance between the two is long. If the voltage is the same, or very nearly the same, as it was at the battery, then the electrons are getting to first base just fine. If the voltage is substantially reduced or zero, then the roadblock is on the first-base path (bad wire or cable, faulty battery switch, bad connection, etc.) If the circuit is not allowing the electrons through, back up along the base path following the wires back toward the battery until you can find voltage. Between the last place you find voltage and the first place you can't find any, the circuit is broken (or "open" in the strictest electrical terminology).
If you find voltage on the input side of the circuit breaker, try the output side before moving on. Circuit breakers do fail. Don't forget to turn it on before testing though.
If the electrons are still flowing through first base at the circuit breaker, take the multimeter out to second base at the faulty stern light and test the positive terminal of the bulb socket for voltage. Same as before—if there was power at the circuit breaker, but none at the light socket, the break in the circuit lies along the second-base path.
If the socket is OK (assuming the bulb really is good), you have successfully narrowed your search down to the ground side of the system. Move your extension cable over to the positive terminal on the battery, connect it to the positive probe on your multimeter, and locate the ground buss to your system—it's generally right alongside or behind the circuit breaker panel. Find the running light circuit on the ground buss and put the negative probe on the terminal. Same story again. A voltage reading here close to the original battery standard means that the third-base path is clear, while reduced or no voltage means that the blockage is between the light socket and the buss bar (make sure that the bulb is back in the socket!).
By deduction, if there is voltage at the ground buss, and all the other circuits are working off that buss, then the fourth-base path back to home at the negative terminal of the battery must be operational. If nothing on the boat works, the ground cable from the buss to the battery is the third place I look after checking for a dead battery and a main switch turned off.
Ohmmeters These devices measure resistance. They tell you whether there are obstacles along the path between two bases that are causing the electrons to slow down or stop. They read in the obverse—a reading of 1.000 indicates there is total resistance (meaning there is literally no connection) between two points. A reading of 0.000 means there is a perfect connection with no resistance. Using an ohmmeter to test the base paths is called checking for continuity. Follow the instructions for your multimeter to adjust the ohmmeter to zero before beginning.
A perfect place to check for continuity is on that bulb for the stern light. You can hold the bulb up to the light and squint at it for hours without knowing whether the little filament wire inside is intact. But by putting one multimeter probe on the positive bulb connection and the other probe on the negative, you will know instantly if the bulb is good.
To test a circuit for resistance and continuity with an ohmmeter, start at home plate and test the first-base path to the circuit breaker for the inoperable running light circuit. Put one probe of the ohmmeter on the positive battery post and the other probe on the circuit breaker input terminal. A reading of zero, or a very small number, means there is little resistance on that base path and the electrons should have no problem running along. A high number indicates there are substantial obstacles. A reading of 1.000 indicates there is no connection at all. You can test an entire circuit, or any piece of it with an ohmmeter. I routinely use mine to test fuses, bulbs, circuit breakers, and diodes.
Note that there is a relationship between voltage and resistance. A circuit with a corroded or kinked wire, or some other obstacle to electron passage, will display this resistance as a relatively high ohmmeter reading, but also as a reduced voltage reading. Again, visualize your knotted garden hose—on the side before the knot there is full water pressure, but on the downstream side of the knot, water pressure is reduced. So when you use the voltmeter to check a circuit, you need to be sensitive to small amounts of voltage drop. Those electrons are valiant and will try gamely to slip past an electrical obstacle so they can get home, but the reduced voltage behind a high-resistance area will belie the difficulty they are having returning to the battery.