Aircraft service imposes severe environmental conditions on electric wire. To assure satisfactory service, inspect the wire at regular intervals for abrasions, defective insulation, condition of terminal posts, and buildup of corrosion under or around swaged terminals. When replacing copper wire with aluminium wire, increase the gauge of the wire two sizes.
Voltage Drop in Electrical Wire.
The voltage drop in the main power wires from the generation source or the battery to the bus should not exceed 2 percent of the regulated voltage, when the generator is carrying rated current or the battery is being discharged at the 5-minute rate.
Resistance.
The resistance of the current return path through the aircraft structure is always considered negligible. However, this is based on the assumption that adequate bonding of the structure or a special electric current return path has been provided which is capable of carrying the required electric current with a negligible voltage drop. A resistance measurement of.005 ohms from ground point of the generator or battery to ground terminal of any electrical device may be considered satisfactory. Another satisfactory method of determining circuit resistance is to check the voltage drop across the circuit. If the voltage drop does not exceed the limit established by the aircraft or product manufacturer, the resistance value for the circuit may be considered satisfactory. When using the voltage drop method of checking a circuit, maintain the input voltage at a constant value.
It should be noted that the No. 14 wire should not be used if any portion of its 100-foot length is to be confined in conduit, large bundles, or locations of high ambient temperature.
Aircraft electrical wire or aircraft quality wire.
Correct wire selection is dependent upon knowledge of current requirements, operating temperatures, and environmental conditions involved in the particular installation. Copper conductors are coated to prevent oxidation and to facilitate soldering. Tinned copper wire or aluminium wire is generally used in installations where operating temperatures do not exceed 221* F. (105* C.). Silvercoated copper wire is used where temperatures do not exceed 392* F. (200* Nickel-coated copper wire is used for temperatures up to 500* F. (260* C.). Nickel-coated wire is more difficult to solder than tinned or silver-coated wire, but with proper techniques, satisfactory connections can be made.
Electrical cable Insulation.
Polyvinylchloride (PVC) is a common insulation, used as PVC cable sleeving competing with Vidaflex cable sleeving and PTFE sleeving. It has good insulating properties and is self-extinguishing after the flame source is removed. Normal operating temperatures are limited to 221* F. (105* C.). Silicone rubber is rated at 392* F. (200* C.), is highly flexible, and self-extinguishing except in vertical runs. PTFE sleeving Fluorocarbon (polytetra-fluoroethylene) is widely used as high-temperature insulation. It will not burn, but will vaporize when exposed to flame. It is resistant to most fluids. FEP cable Fluorocarbon (fluorinated ethylene propolene) is rated at 392° F. (200° C.) but will melt at higher temperatures. Other properties of FEP are similar to TFE.
Thermal and Abrasion Resistant Materials.
Glass braid has good thermal and abrasion qualities but moisture absorption is high. Asbestos and other minerals provide high temperature and flame resistance, but are highly absorbent. Moisture absorption is reduced by use of silicone rubber, TFE, or other saturants. Nylon is widely used in low-temperature wires for abrasion and fluid resistance. Polyimide, a new material, has excellent thermal and abrasion resistant characteristics.
Electrical wire selection.
Select wire for structural and environmental characteristics. Wire normally used for chassis wiring, in enclosed areas, or in compact wire harnesses protected by moulded or braided coverings usually has low abrasion resistance. Wire used to interconnect units, or in long, open runs as airframe wire, is designed to withstand normal aircraft environment without sleeving, jacketing, or other protection. Care must be taken in making all installations because no wire insulation or jacketing will withstand continuous scuffing or abrasion.
In order to select the correct size of electric wire for equipment, two major requirements must be met:
(1) The size must be sufficient to prevent an excessive voltage drop while carrying the required current over the required distance.
(2) The size must be sufficient to prevent overheating of the wire while carrying the required current.
For the selection of wire we must know:
(1) the length in feet of the actual wire "run" from the bus to the equipment;
(2) the number of amperes of current it must carry;
(3) the amount of voltage drop permitted; and
(4) whether the current carried will be intermittent (maximum 2 minutes) or continuous, and if continuous, whether it is a single wire in free air, in a conduit, or in a bundle.
Assume that we wish to install a 50-foot length of wire from the bus to the equipment in a 28-volt system. We are permitted a 1-volt drop for continuous operation.
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