One of the question that we frequently receive at Lectromec is regarding the derating of wire harnesses. For those unfamiliar with the topic, we have covered this extensively in past articles. The quick summary of this is that due to the temperature rating of a wire, there is only a certain amount of electrical current that can be transmitted down the wire before the wire heats up beyond its temperature rating. This becomes a more complicated question when there are multiple wires in a single wire harness, supporting equipment (such as clamps), and other parts of the EWIS (connectors, secondary harness protection). The clear objective here is to avoid using too large a wire gauge because of the unnecessary added weight to an aircraft.
Recognizing that this is a complicated issue does not help engineers who need to come up with an implementation solution. This article proposes a process that can be used to determine the proper derating of a wire harness.
Harness Derating
Given a wire harness configuration, what can be done to determine the derating? The conservative approach would be to use the SAE aerospace standard AS50881 and the guidance that is available therein. However, if additional secondary protection is placed onto the wire harness, such as Nomex braiding, chafe protection, etc., the rate of thermal energy loss from the wire harness is impacted. The existing guidance does not provide any feedback on how to address this.
As such there are two ways that this can be addressed: either laboratory testing or numeric simulation (there are models for thermal derating, but validation of these models still remains a question). This article will review the considerations for laboratory testing.
Thermal derating of wire harnesses is not a simple task and only becomes more complicated when considering installed systems and the impact of connectors. Lectromec’s lab can help to identify the ideal derating for your configuration.
Design
The first step in the overall process is to understand the harness configuration. Does the wire harness contain only a few wires or is it a complicated harness set with tens or hundreds?
The next step requires understanding of the environmental conditions in which the harness is placed. Is there airflow in this location? What is the ambient operational temperature? Is the environment temperature and pressure controlled? Each of these factors require consideration and has an impact on the energy loss during operation.
System Level
Once the harness physical layout is understood and the environmental conditions are identified, the next step is to understand the circuit configuration. The first consideration is the number of power carrying wires and identify the current carried by these wires. As with a load analysis for an aircraft, it is important to know if these systems will function simultaneously. If it is unlikely or impossible for all of the circuits to be active simultaneously, then this needs to be a consideration with the derating and test harness set up. Otherwise, the derating factor will be very conservative and require the use of larger gauge wires than is necessary.
Connector Derating
Connector derating is one area that remains ambiguous and is difficult to identify a good set of guidance. Assume you have a 10-pin connector with contacts sized for 16-gauge wire. Now each of the 16-gauge wires independently would be able to handle 10 to 15 A. The harness derating would reduce the overall power carrying capacity of a 10 wire 16-gauge wire harness, but the harness derating does not specify what derating factors are necessary for the connector derating. The difficulty for defining a degrading factor for connectors is that there is such a variety in connector design, contacts, inserts, back shell accessories, etc. that the thermal dissipation from these is hard to generalize (e.g., if the connector is mounted on structure, the thermal conductivity of aluminum and composite structures are significantly different).
A white paper from TE connectivity starts off the article with the phrase, “can a contact rated at 10 A carry 10 A? Maybe yes, probably not.” (Article here). This is not something that is very reassuring to those that rely upon the performance of electoral components at their maximum operating capacity. However, like harness derating, this can be evaluated in a laboratory environment in such a way that data can be provided to support certification efforts. To do the testing and to do it well is important to ensure the reliability of the installed system. This testing should consider all the factors that are mentioned in this article, as well as some system specific needs that may be identified during the discovery process.
Conclusion
To get the most out of the EWIS, requires more than a simple lookup table and applying generalized conservative estimates. More detailed analysis can provide dividends in terms of reduced weight, better design, and improve reliability of the system components.
If you like to find out more about how Lectromec’s testing capabilities can support your certification effort for EWIS, contact us here.
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 has an FAA DER appointment with a delegated authority covering EWIS certification.