Building an aircraft electrical wire testing program: Selecting tests, Part I

This article, which focuses on test selection, is a continuation of a discussion on wire selection that was started in the Determining the Right Wire for your Application article. The series could be used as a basis for understanding electrical wire testing. A quick scan of the wire tests on WireFacts.com demonstrates that there are dozens of tests developed for assessing wires. These tests evaluate the physical, chemical, and electrical properties of the manufactured end product as well as its usability under different stresses and environmental conditions. This article will focus on Physical and Electrical tests.

Physical Tests

These tests typically focus on the insulation’s ability to withstand physical stresses to prevent a breach to the conductor. Often these tests were designed with a particular failure mode (e.g. Scrape abrasion), which represents the conditions in a swaying or vibrational environment. These tests pull, push, flex, twist, compress, and cut the wire insulation to determine wire performance.

In the case of the aerospace industry, there is a drive towards design and requirement improvement. As such, you can typically assume that the newest generation wire will perform better in many aspects than the last.

A few key tests that should be considered when selecting physical tests:

1. Scrape Abrasion

The scrape abrasion test is a good test to see how well the insulation can withstand rubbing against an edge. While this test sometimes will yield inconsistent results (particularly with wrapped insulations), the results do provide good data to make relative comparisons between sample types. Tests should be performed at ambient and elevated temperatures.

2. Notch Propagation

The notch propagation test evaluates the progression of a small notch in the insulation and the longevity of the insulation integrity. If the small notch is able to quickly breach, the conductor will typically indicate a more rigid insulation. Because this test wraps the wire on a mandrel for hundreds of cycles, the conductor is more likely to fail on small gauge wires than high quality insulations.

3. Elongation to Break

This test is performed on the insulation after the conductor has been removed. Pulled until the insulation snaps, this provides good comparative data on the insulation strength. This test should also be considered when multiple accelerated aging cycles are performed.

Electrical Tests

Electrical tests analyze how a completed wire handles electrical loads. This may focus on the conductor, such as resistance measurement, on the insulation, such as insulation resistance, or a combination of the two with a ‘time to smoke’ test.

A few key tests that should be considered when selecting electrical tests:

1. Surface resistance test

In conditions with high humidity, the insulation surface conduction can be a source for leakage current. The surface resistance test evaluates the leakage current in such an environment.

2. Insulation Resistance

The insulation resistance test measures the insulation dielectric strength. If accelerated aging cycles are being performed, this is a good test to evaluate the electrical degradation of the insulation.

3. Wet or Dry Arc Track Resistance

Electrical failure can cause more damage that resetting a circuit breaker. In systems with a high cost of failure, an arc track event can damage systems, conduct electricity to other circuits, and cause significant physical damage to nearby components. This test evaluates the propensity for arc tracking. Read Lectromec’s Wet and Dry Arc Track Resistance Testing article for more information on this test.

4. Current to Smoke

The current to smoke test evaluates the electrical current level necessary to cause the wire to smoke. Most wire insulation products include chemicals that should not be inhaled; assessment of this characteristic should be considered when the application is in enclosed areas with personnel. Read Lectromec’s Electrical Tests for Aerospace Wires and Cables article for more information on electrical tests.

Conclusion

There is no “one size fits all” wire and there is not a single test program that is applicable to all wire types or environmental conditions. Just like any other component, selecting the right wire for the right application takes time and effort. For mission critical systems, wire system integrity is often as important as component or device integrity. Making an educated and informed decision upfront can significantly reduce maintenance issues down the line.

The next article will review the chemical and usability tests that can be performed on wires to assess their quality and properties for a particular application.