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ELECTRICAL SYSTEMS SIMPLIFIED PART FOUR BY RON ALEXANDER from Aircraft Spruce
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ELECTRICAL SYSTEMS SIMPLIFIED PART FOUR BY RON ALEXANDER

The next step in our electrical process is to connect our equipment to a power source.

Connect Equipment to Power Source

Care should be taken to properly connect all equipment to the power source. Most electrical problems encountered during the initial installation phase can be traced to a poor connection. Crimp on connectors must be carefully installed on a wire that has been properly cut. Cut about 3/16 inch of insulation away from the wire. Use a good quality crimping tool. The correct size connector should be used. The connectors are color-coded—red for 18-22 gauge wire, blue for 14-16 gauge, and yellow for 10-12 gauge wire. Taking your time as you crimp each connector will save you hours of troubleshooting when you power up your system for the first time. Check the connection physically by pulling on the wire and connector. Protect the connection with the use of heat shrinkable tubing. This tubing will insulate the connection protecting it from electrical faults. Check each connection electrically using an ohmmeter. Equally as important are solder connections. Proper soldering techniques must be used. Practice your soldering before you attempt it on your aircraft.

Routing of wires is also very important. After marking the wires you can group them into bundles. This is often very convenient when running several wires from one area to the bus bars. Several acceptable practices for routing of wire follows:

  • Support wires and bundles of wires using MS21929 cushioned clamps.
  • Do not allow bundles to have too much slack. Using normal hand pressure you should not be able to deflect the bundle more than one-half inch.
  • Separate wires and bundles from flammable fluid lines. If this is not practical, always run the fluid lines below the wire bundle and separate the two by 6 inches or more.
  • Protect wires in high temperature areas by using sleeves.
  • When running bundles or wires through cutout areas such as bulkheads, protect from chafing by using a rubber grommet.
  • Splicing of wire is permitted but should only be used if necessary.
  • No more than one splice per connection is recommended.
  • The splice should not be within 12 inches of a terminal end.
  • Be sure wires are at least 3 inches from control cables. If not possible, use a guard to prevent contact.
Proper routing of cables is necessary to prevent the wires from coming in contact with other parts of the airplane. This can easily occur as a result of movement or vibration. A wire can obstruct movement of a control or it can be damaged causing a short. A shorted wire can be very difficult to locate. This is one other reason to be very careful during the entire installation process.

Grounding and Bonding Grounding is defined as the electrical connection of a conducting object to the primary structure to provide a return path for the electrical current. Bonding is the electrical connecting of two or more conducting objects not otherwise connected. The difference between the two can be confusing. A simple explanation is that bonding may not always result in a ground. It may only connect objects. Metal aircraft have a built in ground—the airframe. The main frame of a metal aircraft can be used as a ground. Many builders will use a grounding strap and connect the piece of equipment to be grounded to the frame itself. Another method of grounding is to use a common grounding bus bar. This bar can be located under the instrument panel and used to connect all ground wires from equipment throughout the aircraft. This is referred to as a central grounding point. The grounding bus bar is grounded to the engine of the aircraft. In this case two wires must be run to each piece of equipment—the power wire and the ground wire. This type of grounding is more common in composite and wood aircraft. Both of these aircraft require some form of ground other than their airframe. Wood and composite material do not conduct electricity.

Many wood and composite aircraft builders will use a bonding network that interconnects all grounds and then terminates at the engine providing a ground. It is a metallic network installed specifically to provide a ground. This helps explain the difference between bonding and grounding. The bonding network must be grounded.

The wire used for grounding must be adequate to carry the electrical load. A braided grounding strap is often used. You should also check each grounding connection with an ohmmeter. To be sure you have a good connection between two metal parts the surfaces must be completely clean. Any protective coating should be removed. This can be accomplished by sanding the surface. After the bonding connection is completed, the area can be protected using a varnish.

Install Instruments to Monitor the System

You want to know the status of your electrical system as you operate your aircraft. Proper instrumentation can alert you of actual or potential problems. The battery ammeter is a necessary instrument. You want to know the state of your battery at all times. The ammeter indicates the amount of current flowing into or out of your battery. The instrument you install should show both charging and discharging current. You can then interpret the condition of your battery and its charging system with this instrument. After 1 hour of flying a typical battery should reach a fully charged state. The indication of this would be a slight indication on the positive side of the ammeter. The ammeter may be used to detect a number of problems. Be sure to install this instrument.

Another instrument you may want is a voltmeter. This instrument will monitor the electrical system voltage. In a 14-volt electrical system, a reading of 13-15 volts would be normal. Readings less than 12 volts are usually indicative of a problem. For instance, a very low reading could mean that the battery is failing.

A warning light may also be installed as an added method of monitoring the electrical system. This can be in the form of a light used to monitor an overvoltage or undervoltage condition. This could be very helpful in detecting a high voltage condition that might require shutdown of the electrical system or warn of alternator failure. An overvoltage light or a higher than normal load could also be the first warning of an impending electrical fire. The light may allow you to detect the problem before it becomes too serious.

The importance of monitoring the electrical system becomes even more critical if you are going to be flying at night or IFR. You will want to know the status of your battery, in particular. If you lose you primary source of electrical power, the alternator, you want to know that you can depend upon your battery to supply power to the essential items necessary for an immediate landing.

Complete a Schematic Diagram

Your schematic diagram is nothing more than a road map of the electrical system. The symbols used in electrical diagrams vary from one manufacturer to another. Some of the symbols are fairly universal but you still may find differences. These symbols are actually a language of electrical components and are sometimes difficult to understand. I would not be too concerned about the symbols. More importantly, you want to map out your system very simply so you can understand it. Mapping it out allows you to trouble shoot electrical problems during installation and at a later time when you may encounter a problem. Use as many common symbols as possible so that when another person tries to find a problem at a later date they will be successful. You can indicate the length of wires along with the size on the schematic. Define each piece of electrical equipment along with part numbers if you desire. Show where bus bars and circuit breakers are in relation to the system and physically in the airplane. Several common symbols are found in Figure 2. Anything you can do to map out your system will be most helpful at a later date. Build your schematic diagram as you build your electrical system. This will be much more effective than trying to remember everything after it is installed. Build it to suit your needs but build it so that it can be understood by someone else who may be helping you with a problem or a new owner.

Hopefully, the articles presented will take some of the mystery out of electrical systems as they apply to amateur-built aircraft builders. You can certainly dig in much deeper than I have in these articles. There are several books available that will explain in depth electricity and the installation of an electrical system. I have chosen purposely to keep this as simple as possible. I do not think you have to understand all of the intricacies of electricity to properly design and install your electrical system. Only a very basic understanding is necessary along with proper planning of the system. Planning, as always, is key during this phase of construction. Plan your electrical system early in the building process. Talk to other builders of similar type aircraft to see how they have overcome electrical problems.