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AS4372 – Building a Wire/Cable Specification

Certification Standard & Regulation

Key Takeaways
  • While AS4373 defines how tests are performed, AS4372 identifies what test should be performed.
  • The AS4372 provides performance requirements.
  • While the push will always be for better performance, system design can compensate for component performance reduction.

Construction of a new wire specification is a non-trivial task. First, it requires being able to build a product that is novel and has a place in the market. Then comes the process of defining the product assessments (i.e. what are the means of determining the product performance and what are the performance levels). While it is possible to start from scratch and develop an independent set of tests and performance requirements, there are existing standards that can be used to expedite the process.

One such document is the SAE standard AS4372.

What is the difference between AS4373 and AS4372?

Those that have been reading Lectromec articles, are familiar with our frequent reference to the SAE standard AS4373. This standard provides the test procedures for many of the common wires and cables that are used in aerospace. These details included in the AS4373 help to reduce variability between labs by addressing many of the areas where interpretation can cause a deviation of results. Those that have ever tried to compare test results from multiple labs know the painstaking process of reviewing each step of the test procedure and connection/placement of the measurement equipment (and even with all of this work, there may still be inherent variability of the test or the samples).

For many, the interaction with the AS4373 comes from the use of the AS22759 family of wires. This family of wire uses the AS4373 standard almost exclusively for the test methods for product verification and performance assessment.

So, while the AS4373 standard provides guidance to labs and QA facilities on the performance of tests, the AS4373 helps with the wire definition. The AS4372 comes in to set the base performance for many of the tests. As is stated in the forward for the standard:

"Wire will be assigned a level of performance for many requirements set forth in this standard. These performance levels, in addition to numerical test results, shall be listed in the applicable performance sheet"

When should AS4372 be Used?

When starting with a new wire construction, the AS4372 should be reviewed to identify the range of tests that should then become the specification performance and acceptance tests. The tests that are identified in this standard include tests that measure and assess:

  • The conductor’s mechanical and electrical properties.
  • The insulation’s thermal, mechanical, and electrical properties.
  • The completed product’s performance

(there are 46 tests identified in the AS4372, to see a list of the tests in the AS4373, see this article)

Further, the performance requirements should be compared against a recent wire specification sheet. For example, the latest revision of the AS4372 (Rev C) states, "there is not a test method for abrasion in AS4373", yet there are requirements in nearly all new AS22759 wire specifications for the performance of scrape abrasion testing both at ambient and elevated temperatures.

The reason for this step is to ensure that the performance requirements are at least matching the existing state of the industry and technology. Because the wires and cable standards published by the SAE AE-8D group are directed for aerospace applications, they should be aligned with aerospace objectives.

Building a Better Wire

Will a wire/cable construction change improve performance? At the component level, testing helps to determine basic performance, but at the system level, the system’s design will make all of the difference.

In particular, an FAA objective is for constant improvement in aircraft safety. Because of this, new wires/cables (or for that matter, any EWIS component) should be better than the previous generation. Of course, to say "better than the previous generation" is ambiguous. A wire could have a thinner insulation making it more susceptible to physical damage but could allow for multiple wire runs creating additional EWIS redundancy. In this example, the component might be a performance step back, but at the system level it is a net safety increase.

And that is a struggle with all components for aerospace. To build a component that might be better for the system but creates a need to tie a restriction to its use. That is something that has been a part of EWIS for decades; the best example of this is the light weight versus the normal weight wire constructions.

Conclusion

The AS4372 standard is a good starting point for defining the tests that should be included for product definition, evaluation, and qualification.

The companion standard AS4373 is best for the QA facilities and labs that need methods and descriptions on how to perform these tests.

If you are looking for a test lab to help you with R&D, evaluation, and/or qualification, Lectromec’s ISO 17025 accredited lab can help. We have been testing EWIS components for 36 years.

Michael Traskos

Michael Traskos

President, Lectromec

Michael 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.