Aircraft Electrical Diagram
Aircraft Electrical Systems, IV
Most electrical devices, if not all, use power from the aircraft electrical system. They are connected with suitable wires. Kirchhoff states that when current goes into a device it has to come out again. Therefore we need at least two wires to connect them, i.e. power and ground/return.
Metal aircraft provide shielding, which can be thought of as a Faraday cage, but this is not suitable to use as a return for our sophisticated electronic devices we have in the cockpit today. Doing that would result in stray electric currents causing noise and problems all over the system.
Power & Ground
Using a power and return line for every device makes sure that the voltage loss is at a minimum and that no stray currents enters the device (think about audible strobe pulses in the intercom). It also is very convenient to use a color coding scheme to indicate the purpose of the wire. For example: black for return, red for power and yellow for switched power. Other colors can be used for signal wiring, trim servo's and or audio.
Common ground
Lots of problems arise when the metal airframe is used as a common ground. Paint, insulation, bad rivets, corrosion help to decrease the conductance and the result is a much higher resistance and voltage drops. And to make things worse: when using high powered strobes, bad ground connections may cause radio frequency interference or noise and you may 'hear' the strobes.
By using the two wire method, power and return, color coding and connecting all return lines to one central master ground, which is then connected with a thick wire to the battery, possible electrical repairs will be much easier to accomplish and RF noise will be brought back to a minimum.
EMC / EMI
Electromagnetic Compatibility, EMC, is the art of stopping wires becoming an antenna pickup up and radiating unwanted RF and noise which you might hear in the intercom and radio.
RF Noise
Most devices use a DC (direct current), flap motors, trim servos and power connections to radio's and such. Other wiring carry AC (alternating current) intercom audio signals, and or LF/VHF/UHF radio frequencies. Shielding is used to keep unwanted signals leaving the box and or wiring.
Transponders and EFIS systems use microprocessors and these need square wave signals to operate. These signals generate noise and harmonics (a square wave is a sine plus all of its harmonics) if they are not handled properly. Shielded cables are used to connect these devices and the shield is grounded at the driven end only, preventing ground loops. In extreme cases ferrite beads are used to block or short out unwanted RF noise on cables and wiring.
For more indepth reading follow the next link which discusses how to use Ferrites against EMI by EMACS services.
Antenna's
Some antenna's like the GPS have internal electronics to amplify the signals from the satellites. Keep these as far as possible away from other high powered cables (a current running through a wire creates a EM field) and or transmitters like the transponder. A transponder can radiate some 125 to 250 Watts of RF energy and this is difficult to keep out of wiring of they are not properly shielded.
Shielding
If you use shielded cabling for microphones, headphones, CD players, mobile phones and other forms of audio cabling and connect the shield at one end to a common ground you will avoid errors like ground loops and common return/ground paths as a result this will improve the audio quality and prevent the wiring from becoming an antenna picking up unwanted RF and other noise. As mentioned above, ferrite beads or sleeves are use to stop wiring from picking up commmon mode interference and these can be helpful too in aircraft.
Again: It is strongly advisable to create a common ground bus bar to which all shielding and return wires are connected.
Bottom line: everything with a microprocessor, digital screens, or carrying a low level ac signal should be shielded. Power wiring and return lines can use ferrite beads, sleeve or toroids to suppress radiating or picking up interference.
For those of you wanting detailed information about aircraft wiring we recommend the next link: Aircraft wiring for smart people by Greg Richter. This article is based on that document.
