Key Takeaways
- Progress has been made on defining wire/cable voltage ratings for airborne systems.
- The next revision of AS50881 is near release.
- There is new guidance on wire harneess derating.
Wire and cable standards are never proactive, but reactive to industry needs, and as the aviation world moves to high power systems, wire and cable must catch up to ensure the products are available for long reliable service life. Twice a year the SAE wire/cable community comes together to slowly push the standards and industry guidance forward; this year, New Orleans was the setting.
During this three-day meeting, several topics were discussed impacting the next generation of aircraft designs. Here, we cover a couple of these developments.
Crank Up the Voltage
As with past meetings, a fair bit of time and discussions were devoted to voltage ratings of wire. For those that have not been following the saga of voltage ratings, Lectromec has covered this in several previous articles. In short, the voltage power systems being employed and/or developed for aircraft applications are greater than previously deployed. The concern among the power distribution and Electrical Wiring Interconnection System (EWIS) component communities is the high voltages may stress the wire insulation and result in more rapid ageing/deterioration.
To address these issues, the SAE AE-8D/A and AE-7 committees held a joint session. One presentation from the AE-7 community proposed several voltage levels for aircraft power applications. These voltage levels specifically identify the maximum allowable voltage for a particular configuration (e.g. Voltage Category 4: AC 300 – 450VAC, DC 210 – 320). With this as a basis, a strong backing exists for the wire/cable committee to develop standards that fit the voltage category needs.
It is uncertain how the application of wire/cable voltage categories will evolve wire/cable standards, but it does help to set a greater level of certainty in the voltage rating arena. The AE-8D community has performed a significant amount of research investigating methods and techniques for evaluating long-term high-voltage endurance; prior to this meeting, the application was at a bit of an impasse. One hopes that the community will use this as an opportunity.
To do this, the community will need to:
- Define voltage ratings for wires,
- Define the expected performance criteria on the power system to ensure safe continued operation at those voltage levels (i.e. a Voltage Category 4 wire can last 100,000 hours at voltage assuming use below maximum rated temperature and a power system frequency at or below 400Hz), and
- Publish an Aerospace Information Report (AIR) cataloging the work already done and guidance on potential wire/cable derating when used outside set performance levels (e.g. what can be expected when a 400Hz rated wire is used at 1200Hz).
Lectromec is looking forward to assisting in the development of these standards.
Coming Soon – AS50881 Rev G
The latest round of balloting on the AS50881 standard was completed just a couple weeks ago. At the AE-8A committee, the final changes to the standard were agreed-upon and Revision G may soon be out as a final edition (assuming no hiccups occur between now and final publication).
Among the changes to the standard include:
- A prohibition on wire harness termination with the D-sub connectors. These connectors that have been popular for decades, are not suited for airborne applications. Frequently, these connectors do not have any EMI protection, environmental protection, or any of the other typical performance requirements of aircraft EWIS components.
- To be more in line with separation requirements of redundant systems in FAA EWIS regulations (25.1709), the AS50881 specifically now calls out for redundant systems (identified as equipment with the duplicate functions) shall be run in separate bundles. In summary, this requirement goes hand-in-hand with the best practices for achieving maximum reliability for flight critical systems (i.e. those systems that require redundancy).
Determining what a wire/cable’s temperature will be in-service is not straight forward. Even the best thermal modeling still runs into issues with temperature estimation. Thankfully, the generalized principles for wire harness derating are pretty good.
Tools for Harness Derating
A presentation during the AE7 meeting was on the work performed on AIR6540. This AIR on “Fundamentals in Wire Selection and Sizing for Aerospace Applications”, runs through many of the fundamental principles for the selection of wire sizes for aircraft applications. Thankfully, much of the process outlined in this document follows the methods prescribed in AS50881.
The document also goes beyond what is described in the AS50881 to ensure that the wire selection is performed in such a way that the physical and environmental conditions within the operational environment are considered. The AIR6540 goes through a thorough step-by-step process and examples to show how to properly derate a wire/wire harness.
Those that have been reading Lectromec articles for a while have probably seen several articles discussing wire derating both for voltage and thermal reasons. The AIR6540 is just one more piece of information engineers can rely upon to guide their engineering process.
Next Meeting
The next SAE AE-8A and 8D meetings will be in San Diego and will coincide with the Aerospace Electrical Interconnect Systems Symposium (AEISS). The AEISS occurs once every two to three years and brings together several groups for a day of presentations. Those interested in submitting an abstract should review the information here.
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Michael Traskos
President, Lectromec
michael.traskos@lectromec.comMichael has been involved in wire degradation and failure assessments for more than a decade. He has worked on dozens of projects assessing the reliability and qualification of EWIS components. Michael is an FAA DER with a delegated authority covering EWIS certification and the chairman of the SAE AE-8A EWIS installation committee.