The MIL-HDBK-454, General Guidelines for Electronic Equipment, includes EWIS components and covers 78 military guidelines for the selection, handling, and application of electronic equipment on aircraft. This article covers eight of the selection guidelines from the handbook applicable to wire system component testing, selecting EWIS components, and good wiring practices.
Guideline 3: Flammability and EWIS comnponents
All selected EWIS components should be fire retardant under the most severe conditions or worst case scenario in the operational environment.
Since components should not be combustible under any circumstances, fire retardant additives can only be used if permitted and they do not alter the composition of the component itself. Flammability testing can be performed to determine whether components are fire retardant (AS4373 Method 801). Additional information on flammability testing is available.
Guideline 4: Fungus-Inert Materials for EWIS components
The handbook specifies which materials are resistant to the growth of fungi which can affect a wire or component’s integrity. According to Table 4-1 in MIL-HDBK-454, polyimide, a commonly used insulation material, is fungi resistant, though in modern constructions (e.g. AS22759/87) it is found on the inside layer of the wire insulation. Fungus testing should be performed to determine which components are not nutrients to fungi and are recommended for application use.
Listing of Fungus inert materials in all modified states (MIL-HDBK-454):
|Acrylonitrile-styrene||Polyester-glass fiber laminates|
|Acrylonitrile-vinyl-chloride copolymer||Polyethylene terephthalate|
|Asbestos||Polyethylene, high density (above 0.940)|
|Fluorinated ethylenepropylene copolymer (FEP)||Polysulfone|
Guideline 10: Electrical Connectors
Connector selection should be in accordance with MIL-STD-1353, which states that the following factors should be considered when selecting a connector:
- Application type: Is the circuit for RF, signal or power distribution?
- Number of circuits required
- Electrical capabilities required: This is application dependent and should include electrical load, signal loss, etc
- Vibration and environment: high vibration environments (e.g. engine area) require different considerations. These environmental factors should be evaluated
- Mechanical data: Frequency of mating/demating by maintenance
- Quality and Reliability: Consider the number of cycles before component wear-out
- System Standardization: Standardization improves logistic supportability, life cycle costs, obsolesces, and device reliability
- Maintainability: Frequency of mating/demating by maintenance
Connector selection is an important factor when evaluating component life cycle and replacement activities. It is necessary to assess the conditions that the connectors will be exposed to during its application using these selection factors.
Guideline 11: Electrical Insulating Materials
This focuses on the additional supporting materials for wire systems included potting materials, tubing, and other insulators. Insulating materials should be selected upon the following factors:
- Temperature rating
- Moisture endurance
- Fungus resistance
- Dielectric strength/constant
- Mechanical strength
- Dissipation factor
- Ozone resistance
Insulating materials are vital to ensuring EWIS integrity throughout its life cycle. The selection factors are guidelines to consider when selecting wire and insulation types for your application.
Guideline 19: Wire Terminations
Termination selection should be in accordance with MIL-HDBK-1277. The following are recommended practices when using terminations:
- There should not be more than three wires in any single terminal or lug. According to AS50881, multiple power distribution wires should not share a terminal
- There should be no more than two lugs connected to a single terminal on a screw-type terminal board and no more than four terminals connected to a single terminal on a stud-type terminal board
- Terminal lugs should be crimped in accordance with the resistance and tensile strength requirements from SAE AS 7928
Guideline 26: Arc-Resistance Materials
Materials selected for insulating electrical circuits should be arc resistant; therefore it must pass the requirements of the ASTM D 495 Arc Resistance Testing (Note: this does not mention the typical arc track resistance testing required of most wire constructions – e.g. of AS4373 method 508/509). According to the handbook, arc-resistance materials should also be assessed on their thermal and electrical characteristics to predict their performance in the operational environments.
Guideline 45: Corona and Electrical Breakdown Prevention
Corona, a discharge caused by ionization of the air around a conductor, is affected by a variety of factors including temperature, humidity, and pressure. Corona can dramatically reduce the remaining life of insulation and is a common cause of electromagnetic interference. The minimum partial discharge extinction value should be 150% of the peak circuit voltage.
Guideline 69: Internal Wiring Practices
When designing EWIS routing, it is suggested to keep the routing as neat and simple as possible in order to accommodate frequent maintenance activities. Wires should be routed through the use of bundles. Due to frequent bending during wire routing and performance of maintenance activities, stranded conductor is preferred. A single uninterrupted wire is preferred over the usage of splices and/or connectors. These junctions should only be used for reasons such as manufacturing, maintenance or reliability issues. Wire bundles should not be sagging, however should be given enough slack to prevent unnecessary stress or loss of components. Wire bundles should be properly secured in place once routed to prevent shifting which could lead to severe damage to wiring.