Aircraft Electrical System Grounding
Most, if not all, electrical devices on an aircraft derive their power from the aircraft electrical system. These devices are connected with suitable wires. As any current going into a device must also come out, at least two wires are needed to connect these devices. Therefore, every electrical system has a power and a ground wire. Metal aircraft use shielding to protect the wiring, but this is not a suitable option for many of the sophisticated electronic devices within modern aircraft. Shielding in a modern aircraft would result in stray electric currents causing noise and issues throughout the entire electrical system. Ground return lines, or grounding, reduce the risk of stray currents from entering the airframe where they can cause issues with the intercom system, radio communication system, or navigational system.
Using a power and return wire for each device ensures that the voltage loss is kept to a minimum, and that no stray or induced currents enter the device. Grounding is also helpful in color coding schemes to indicate the purpose of a wire. For instance, many electrical systems use a black return wire, red power wire, and a yellow switched power rire. Other colors are used for signal wiring, trim servos and or audio. The goal of grounding is to achieve electromagnetic compatibility, or EMC. EMC refers to the act of preventing wires from becoming antenna pickups and/or transmitting unwanted radio frequency and noise. Radio frequency and noise can be detrimental to the performance of the intercom and radio systems.
A wide range of problems arise when an aircraft electrical system uses the metal aircraft as a common ground. Paint, insulation, faulty rivets, and corrosion all combine to decrease the conductance, resulting in higher resistance and undesired drops in voltage. Furthermore, when using high powered strobes, bad ground connections can cause radio frequency interference or noise that can be heard over the intercom. When using two wire power and return methods, with color coding and connecting all return lines to a central master ground, electrical repairs are much easier and radio frequency noise is kept to a minimum.
The majority of devices use a direct current (DC) system for flap motors, trim servos, and power connections. Other wiring carries alternating current (AC) like intercom audio signals, wing strobe lights, or any type of radio frequency. Shielding is used to keep generated signals from leaving the box and wiring radiating into other nearby devices. Transponders and EFIS (electronic flight instrument system) utilize microprocessors which need square wave signals to operate. These signals generate noise and harmonics when handled improperly. Shielded cables are used to connect such devices with the shield grounded at only one end, thereby preventing ground loops. In extreme cases, ferrite beads are used to block or short out unwanted radio frequencies or other common mode noises.
External mounted antennas, such as those used in GPS receivers, usually feature internal electronics to amplify the relatively weak signals received from the satellites. It's important to keep these antennas as far away as possible from other high current conducting cables or transmitters such as the transponder or DME. For example, a transponder can radiate between 125 and 250 watts of radio frequency energy. If wiring is not properly shielded, this radio frequency energy can be incredibly difficult to keep out. For use in a metal aircraft, it's important that the bottom of the antenna is connected directly to the metal ‘skin’ of the aircraft. Paint, oil, or other materials can diminish the quality of the electrical connection.
Whether you are using shielded cabling for microphones, headphones, CD players, mobile phones, or other forms of audio cabiling, the shield must be connected at one end to a common ground. This way, errors such as ground loops and common return/ground paths will be avoided. As a result of this, audio quality will improve and the wiring will be prevented from becoming an antenna and picking up unwanted radio frequency interference or other noise. Ferrite beads or sleeves are used to stop wires from picking up interference like this, making them crucial in aircraft.
An important factor in grounding is also understanding the size of your wires and breakers. If your system has a load that draws more than eight amps, it should be put on a separate breaker. There are very few systems that draw more than eight amps - alternator outputs, landing lights in 12 volt systems, and some gear pumps being some of them. Breakers should be loaded to less than 80% of their capacity for optimal performance. To avoid an overload, understand that eight amps is equal to 24 volts or 192 watts. The electrical limits of a system could be listed in any of these figures.
Finally, any device with a microprocessor, digital screen, or one that carries a low level AC signal should be shielded. Power wiring and return lines can use ferrite beads, sleeves, or toriods to suppress interference, but the user must select the correct suppressor for the intended frequencies. It is also worth noting that ferrites, as they are not especially lightweight, will add weight to the aircraft that must be accounted for. Grounding is a critical part of any aircraft electrical system, so it’s imperative that you get your equipment from a trusted source.
At Fulfillment by ASAP, owned and operated by ASAP Semiconductor, we can help you find all types of aircraft electrical system & aircraft airframe parts and deliver them with short lead times and competitive prices. We’re always available and ready to help you find all the parts and equipment you need, 24/7-365. Fulfillment by ASAP is an AS9120B, ISO 9001:2015, and FAA AC 00-56B accredited business, as well as the only independent distributor with a No China Sourcing Pledge, ensuring each part you purchase from is fully traceable or comes directly from the manufacturer. For a quick and competitive quote, email us at firstname.lastname@example.org or call us at +1-920-785-6790. Our team of dedicated account managers is standing by and will reach out to you in 15 minutes or less.
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