- ASTM B470 and ASTM B355 are aerospace standards that identify performance characteristics of copper conductors.
- Adequate evaluation of conductors is crucial in wired construction because once the conductor is encased in the insulation, only electrical evaluations may be performed.
- Properties such as flexibility, resistance, fraying, and conductor plating are part of a comprehensive evaluation of the copper conductor.
The reality of wire conductors is that, except for being stripped and terminated, the conductors are visually obfuscated. After the termination, only through electrical assessment (and perhaps x-ray analysis) can the conductors be examined, and because of this, ensuring that the conductors are well built is important. Standards like the ASTM B470 “Standard Specification for Bonded Copper Conductors for Use in Hookup Wires for Electronic Equipment” or the ASTM B355 “Standard Specification for Nickel-Coated Soft or Annealed Copper Wire” provide performance characteristics of conductors.
No Conductor Sizes
One of the first items that stands out when reviewing the ASTM B470 standard is that it does not place specific tolerances on the conductor diameter, just the number of samples and frequency of examination in a production environment. The conductor diameter should then be based on the user’s particular application. This leaves room for wire/cable standards to adjust this property to fit the need; for example: in practice, aerospace wire conductors tend to be on the lower end of the diameter range to lower the per foot weight.
Only a couple of tests are described in the ASTM B470 standard. The first method described is a flex test. Those familiar with the AS4373 flex test will see similar properties with the flex test method called out in the B470 method. In this test, the conductor is terminated at both ends. One end is attached to a specified weight, with the other end routed through a pair of perpendicular mandrels to an arm that rotates around the enter point of the mandrels in a 120o arc (±60o from the vertical). The sample flexes back and forth across these mandrels until failure (loss of continuity – typically this occurs very rapidly after the first strand breaks).
While flex testing seems straight forward, there are parameters that must be tightly controlled to ensure consistent results. One such item is the weight on sample displacement. The position of the mandrels should be such that when the arm is at the +60o or -60o position, the weight on the sample is at the same position as when the sample is vertical. If this is not done, then:
- The sample is dragged across the surface of the mandrel adding chaffing stress that can accelerate the conductor degradation,
- Each time that the weight is raised or lowered, this is additional stress on the conductor.
The idea with the flex test is to cause stress on the conductors. In particular, one of the more common stresses seen with wires and cables is during connector mate/demate operations during maintenance activities. Frequently, connector mating/demating is done without placing an additional load on the wires, however, often there is a clamp or other means to secure the wiring harness and thus when the harness is then unmated, it may rest at a point that uses the clamp as a flex point.
Flex testing is designed to weaken the conductors and cause it to ultimately break, but test methods will vary on how the stress is applied. There are some flex test methods where the entire conductor is inserted into the terminal and crimped, and others where only the center conductor is crimped upon. When this is done, the idea is to stress only the center conductor yielding a more rapid degradation versus crimping to the entire conductor.
The next test that is covered in the ASTM B470 standard is the fraying test. In this test, the conductor is inserted into a small hole and the remainder of the conductor outside of the hole is then bent to the surface, creating a sharp 90° bend in the conductor. The conductor is then removed from the hole and bent back to the straight orientation; the separation of the conductor strands is observed and recorded. While the standard does not specifically state what is an acceptable condition, one would expect that the conductor strand does not splay. The test performance requirement would exist in the individual standards that call out this standard and test method.
Certainly, one of the most common tests across all the test methods and all the product standards for conductors is conductor resistance. While the conductor resistance test is a straightforward concept, there are several factors that impact the results. It is important to remember that conductor resistance is conductor plating dependent. Furthermore, the conductor resistance varies based on the conductor alloy.
When measuring the conductor resistance of short sample lengths, one of the most important elements that needs to be controlled is the connection to the conductor of the measurement equipment. If the test probes connected to the sample are not properly attached, it is possible that a high resistance point will exist and will skew the results showing a higher conductor resistance.
Handheld multimeters are great tools, but their accuracy for low resistance tests is often poor. The use of a standard handheld meter for conductor resistance assessments, unless the sample is of a significant length or high resistance, will likely yield a poor result. For example, if the sample under test is a 3 ft long 20 AWG conductor, the resistance of the sample will be about 0.03 ohms. Given that most handheld meters have a measurement accuracy of plus or minus 0.1 ohm, the sample length is unacceptable, or the test equipment is unacceptable. However, the handheld equipment is acceptable if the length of that 20 AWG conductor were much greater. If a measurement accuracy rating of better than 1% is desired, then a 1,000 ft sample length of 20AWG would be needed to be able to achieve that goal (1,000 ft of 20 AWG conductor is roughly 10 ohms of resistance).
The adhesion and uniformity of the conductor plating from ASTM B355 is the last evaluation discussed here and is a test that is frequently called out in conductor standards. There are several ways that the conductor plating is assessed:
- Polysulfide: This chemical test may be done to determine if there are any gaps or holes in the plating. See our article on the topic of polysulfide testing. In short, the conductor is submerged in a solution and interaction with the substrate (copper) is sought.
- Alternative Chemical Testing: An alternative chemical test to the polysulfide test is one where the plating is chemically stripped from the conductor and the weight of a conductor is measured to determine how much plating has been removed. This test method, identified in the AS4373, will only provide an average thickness of material across the full specimen.
- Visual: The last method that is commonly used is cross-sectional analysis. In this, a cross-section of the conductor is taken and placed into a mold, polished, and photographed. This photo is examined to determine the thickness of the plating both for the minimum and maximum plating thickness. When this test method is employed, as with many other test methods, it is recommended that multiple cross sections are taken to ensure that conclusions are not made based on a single data point.
New Alloy Qualification
One area that should be considered when proposing a new conductor is the coefficient of resistance and the impact it has on other tests. In short, to calculate the conductor coefficient of resistance requires the conductor to be submerged in a liquid bath, reach equilibrium temperature, and then measure the resistance across the sample; this is done at several temperatures to gather the relationship between temperature and sample resistance. What is not clear in many test standards is how this value has a direct impact on many tests. For example, the AS4373 method 511 Wire Fusing Time test requires the conductor to survive 5 minutes with a current that is 2.5x its free air current. The test guidance suggests using the derating charts from AS50881 to determine this current. The gap is that those charts are designed for copper conductors – not high strength copper conductors, not copper clad aluminum conductors, just copper conductors. As such, it may be necessary to gather basic test data on the conductor’s performance before executing some tests.
Conductor testing is often viewed as basic and requiring very little skill or experience to assess the conductor. Given the wide range of conductive alloys and other types of conductive materials that are used for wires and cables, the conductor complexity does not end at the metallurgical assessment but continues through production and quality assurance. Standards like ASTM B470, ASTM B355 AS4373, and AS29606 provide a framework for much of the conductor assessment in use today.
Contact Lectromec to find out how we can help with your conductor (and cable) testing needs.