Electrical Load Determination System designers must ensure that each aircraft electrical bus can safely support the load based on the electrical capacity of the aircraft’s electrical generators and distribution system. All electrical devices must be safely controlled or managed by the aircraft’s electrical system, and whenever a device is added, a load analysis should be performed to ensure that the new load on the bus can be powered adequately. Wire Selection: Size, Substrate, Plating, and Insulation
Wires should be sized so that they have sufficient mechanical strength, do not exceed allowable voltage drop levels, are protected by circuit protection devices, and meet circuit current-carrying requirements. Small gauge wires should use high-strength alloy conductors and additional support at terminations (grommets, shrink sleeves, etc.) to minimize fatigue. Wires should be plated to defend against surface oxidation. Elevated temperature degradation of tin- and silver-plated copper conductors will occur if they are exposed to continuous high-temperature operation. While there is no “perfect” insulation system for aerospace wire and cable, the EWIS designer must consider the best balance of properties (electrical, mechanical, chemical, and thermal) for each application. Determining Current-Carrying Capacity EWIS designers must verify that the maximum ambient temperature wire bundles will be subjected to, plus the temperature rise due to wire current loads, does not exceed the maximum conductor temperature rating. In smaller harnesses, the allowable percentage of total current may be increased as the harness approaches the single wire configuration. Care should be taken to ensure that the continuous current value chosen for a particular system circuit does not create hot spots within any circuit element which could lead to premature failure.
Causes of EWIS Degradation
Vibration: High-vibration areas tend to accelerate degradation over time, resulting in “chattering” contacts and other intermittent problems. It can also cause tie-wraps to damage insulation, and exacerbate insulation cracking. Moisture: High-moisture zones accelerate corrosion of interconnect components. EWIS installed in clean, dry areas with moderate temperatures hold up well. Maintenance and repair: Improper maintenance techniques can contribute to EWIS degradation—for example, leaving metal shavings or debris behind after a repair. Wire bundles and connectors should be protected during modification work, and all debris must be cleaned up after work is completed. Generally, EWIS left undisturbed will have less degradation than reworked EWIS. As EWIS become more brittle with age, this effect becomes more pronounced. Repairs that conform to manufacturer’s recommended maintenance practices are generally considered permanent and should not require rework if properly maintained. Indirect damage: Events such as pneumatic duct ruptures can cause damage that, while not initially evident, can later cause EWIS problems. When such an event has occurred, surrounding EWIS should be carefully inspected to ensure no damage is evident.
Chemical contamination: Chemicals such as hydraulic fluid, fuel, waste system chemicals, cleaning agents, deicing fluids, and even soft drinks can contribute to EWIS degradation. EWIS in the vicinity of these chemicals should be inspected for damage or degradation. Hydraulic fluids, for example, are very damaging to connector grommet and wire bundle clamps, and can lead to indirect damage such as arcing and chafing. EWIS components potentially exposed to hydraulic fluid should be given special attention during inspections. Heat: High heat can accelerate degradation, insulation dryness, and cracking. Even low levels of heat can degrade EWIS over long periods of time. This type of degradation can be seen on engines, in galleys, and behind lights. Improper installation: Improper installation can accelerate degradation. Improper routing, clamping, and terminating during initial installation or during modifications can lead to EWIS damage. FAA policy states that installation and routing instructions should be completely defined in detail to allow repeatability of installation, not leaving installation to the discretion of the installer.
Qwik Connect • April 2019
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