This article was published in Escapees Magazine, July/August, 2010. Copyright Kirk Wood
There are two types of electrical systems in most modern RVs and it is helpful if owners understand at least a few basic things about them. This is intended for the beginner who has no real knowledge of electricity to help them to understand those basics.
The first type is the same as is found inside of every house today and is called 120V-AC power. The second system is 12V-DC and it is the same as what is found in every automobile. It is good to understand a few things about electricity in general. All electric circuits must be a complete circle that allows the power to leave the source, perform some electrical function and then return to the source that supplied it. In every case, there must be a conduction path from the supply, through the load and back to the supply. If any part of that system is broken, the electrical flow will stop and equipment fails.
Another basic is that electricity is lazy and it will always take the easiest path to return to the source that supplied it. When a shortcut develops it allows the electricity to bypass the intended load and return to the source by a shorter path and is called a “short.” If there is a break in the path at some point that the electricity can’t cross, that is what electricians call an open. Those are the two most common problems in electrical circuits.
In electrical systems one of the terms we frequently hear is voltage. All electricity has some value of voltage and to work properly it must be the correct one. Voltage is the driving force behind the electrical energy. It is the equivalent of the pressure inside a water pipe which pushes the water through that pipe. A switch in the electric circuit does exactly the same thing as does a valve in the water system, but backward. Close a valve to stop the flow and you open a switch to stop electricity. But the effect is exactly the same.
Current is another common term and it is the part that moves in electrical circuits. It is measured in amperes or amps, and it is a specific number of electrons that pass a given point in a fixed amount of time. Comparing this to water, it is exactly the same as gallons per minute would be. Both measure volume versus time.
Watts are the measure of work done, and are determined by the current multiplied by the voltage driving it. That means simply the amps of current times the voltage that is driving it. It is frequently expressed in KW which is kilowatts, or 1000 watts per KW. Kilowatt hours, which you pay for are the number of watts over time. A device that uses 1000 watts of power when used for one hour has consumed one kilowatt hour of electricity.
A fuse is a device placed into a circuit to limit the maximum amount of current which will be allowed to pass through that circuit. The size is chosen as a safety device to protect something. If in a fuse block the limit it is protecting is usually the wire size of the circuit to prevent a short circuit or similar problem from causing a fire. When the fuse is inside of a device it is often there to protect the device from more damage in the event of internal failure. Fuses are most commonly used in 12V systems in RVs. They are not resettable and must be replaced when failed. A good fuse will read 0 ohms with an ohm meter.
Circuit breakers are devices which are basically a switch that will open automatically if some predetermined level of current is exceeded. They are most common in the power distribution panels of 120V-AC circuits but can be found occasionally in 12V-DC circuits. They are usually resettable and can also be used as a switch to prevent power from entering a circuit. A typical circuit breaker if tripped due to an overload will usually have the actuator moved only slightly and can be difficult to detect. To close a tripped circuit breaker, you must move the actuator to the open position and then close it. They must be fully opened to reset and close.
The difference in the two kinds of electrical systems is important. The 12V system is direct current, meaning that the current leaves the source, moving out through the wires and to the load, then returning to the source in one continuous direction. It always travels in the same direction and is usually considered to travel from the positive side of the source and return to the negative. While there is disagreement on this direction, it is of no concern unless you plan to design electronic equipment. What does matter is that the positive side of any supply is connected to the circuits of the RV and the negative side is always connected to the ground, or chassis of the RV.
In the RV and automotive 12V circuits, the chassis is used to provide the return path to the source requiring only one wire from the battery or generator and sometimes a short one to the chassis. That also means that 12V-DC circuits are much more prone to short circuits because any path will return the power back to its source. As previously mentioned, electrons are lazy and will avoid work if at all possible!
Batteries can only store direct current and that is one reason for the two systems in our RVs. The reason for 12V supply is that it is common to the automobile and thus is readily available and can draw current from the automobile system to recharge batteries and supply power.
The 120V-AC power is called alternating current because it changes directions constantly. What we have in common use is called 60 Hertz, and that means it changes its direction of travel sixty times each second. The sign wave that is commonly seen associated with it is a graph of the voltage as it changes over time and to the RVer is of minimal concern. If that frequency should be wrong it can cause problems, but you are not likely to work with that as an amateur. Always remember that 120V is much more dangerous than is 12V electricity.
With most 120V circuits there are three wires, one that supplies the current, a neutral lead that supplies the return path and one bare wire which is a safety ground. The polarity of these wires is important because the power switch will typically only break the hot, or supply side and not the neutral, so if the switch is connected wrong it will leave power in equipment even when turned off.
A 12V shock is not likely to do serious injury to the person who feels it, but it is possible. On the other hand, 120V-AC power kills more people than all other voltages combined. That is partly because of the common use but it is also because people get careless. It is never wise to work on an energized circuit of any voltage but the risk is far greater with 120V. Wise electricians only work on energized circuits when they have no choice.
Most RVs today have a power cord that is rated to supply 120V at either 30A or at 50A. The difference between the two can be easily seen by simply looking at the power plug on the end. The typical 30A plug will have three pins, one that is U shaped and is the ground and two flat pins to supply the neutral and the hot, or power side. These are not the same as the home dryer outlet that also has three pins and it will not plug into one if properly wired. It is always wise to know how to check the voltage on any outlet and to do so before you connect your RV to it.
The 50A plug will have four pins because it has a ground, two hot leads and one neutral. The 50A plug is actually two 50A legs and in theory would supply up to 100A of current flow, but in practice it seldom will exceed 80A. Even so, it supplies more than twice as much power as does the 30A RV cord.
RVs have a device called a converter which changes the 120V power from the cord into 12V power for the appliances and to charge the batteries. Some RVs have an inverter/converter which changes electricity both to and from 120V-AC and 12V-DC.
The lessons of this article do not make anyone an electrician, but should help to understand some of the basics of the power we take for granted in our RVs today.
Last Updated on Friday, 06 August 2010 11:12