Most electrical devices, if not all, use power from the aircraft electrical system. They are connected with suitable wires. Kirchhoff's current law 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 current 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.
Installing ground return lines in our electrical system makes sure that we reduce any possible stray currents in the airframe where they do not belong and could cause interference in the intercom system and or radio communications and navigation.
And: clear communications contribute to safety, so read on!
Using a power and return wire for every device makes sure that the voltage loss is as close to zero as possible and that no stray or induced 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.
What we are trying to accomplish here is Electromagnetic Compatibility or EMC, and this is the art of stopping the wires becoming an antenna pickup up and/or radiating unwanted RF and noise. Which you might hear in the intercom and radio and there are a number of ways to get good results and keep the electrical system clean of any interference.
Lots of problems arise when the metal airframe is used as a common ground. Paint, insulation, bad rivets, and corrosion help to decrease the conductance and the result is a much higher resistance and unwanted 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 flashing over the intercom.
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 or non-existent at all.
Most devices use a DC (direct current) system for flap motors, trim servos and power connections to radio's and such. Other wiring carry AC (alternating current) like intercom audio signals, wing strobes and or LF/VHF/UHF radio frequencies. Shielding is used to keep generated signals from leaving the box and wiring radiating into other nearby devices.
Transponders and EFIS systems use microprocessors and these need square wave signals to operate (basically 1's and 0's). These signals generate noise and harmonics (a square wave is a sine wave plus all of its odd 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 or common mode noise on cables and wiring.
For more in-depth reading follow the next link which discusses how to use Ferrites against EMI by EMACS services. EMI stands for Electro Magnetic Interference.
External mounted antennas such as used for GPS receivers usually have internal electronics to amplify the very weak signals received from the satellites. Keep these as far as possible away from other high current conducting cables (a current running through a wire creates an electro-magnetic field) and or transmitters like the transponder and DME. For example: a transponder can radiate anywhere between 125 to 250 Watts of RF energy and this is difficult to keep out of wiring if they are not properly shielded.
If you use shielded cabling for microphones, headphones, CD players, mobile phones and other forms of audio cabling you must connect the shield at one end to a common ground. This way you will avoid errors like ground loops and common return/ground paths and as a result it will improve the audio quality and prevent the wiring from becoming an antenna picking up unwanted RF and other noise. As mentioned earlier, ferrite beads or sleeves are use to stop wiring from picking up common 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 in a star configuration to one central point.
Everything with a microprocessor, digital LCD screens, USB chargers 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 but you need to select the correct one for the intended frequencies you wish to filter.
These ferrites will add to the aircraft empty weight, as they are not very light weight.
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.