WPAFB Wire Study

Introduction

There are two ways to compare data between products:

• Review the product specification sheets and try to line up the test information between the products.
• Run head-to-head tests and compare the results.

Option one is often an acceptable starting point for part selection, but there are often critical parameters with stricter requirements than “in compliance with standard XYZ”.

Option two can require a fair bit of testing and analysis. Also, performing a wide comparison is not done frequently as the cost can be rather high. The last product wide testing (that has been published) was performed by the US Air Force back in the early 1990’s. Although 30 years old, this data remains relevant today.

Background

The wire comparison study was undertaken at a time before the idea of the Electrical Wiring Interconnection System (EWIS) had been formalized. Back before the FAA acted on the catastrophic failure events of Swiss Air 111 and TWA 800, the US Air Force, US Navy, and US military organizations were starting to capture the threat of degrading wire systems. To support these efforts, and part selection, the USAF undertook a large effort: capture the performance of wiring components in the market and not just the samples that were sent in for qualification.

The introduction to the report states that,

"This test program evaluated existing and new wire insulation constructions in a round wire configuration for aerospace applications. The goal of the program was to identify insulation candidates with balanced electrical, thermal, and mechanical properties."

This 821-page report includes a significant amount of data. By no means can all of the contents be covered in a single article, but some of the items that are still pertinent to the examinations today are highlighted. For those looking to get an understanding of the variability of wire performance and the base parameters for many of the wires that are still in use today, this report is a treasure trove of information.

Wire Types

The wire types that were included in the evaluation are the insulation types that have dominated the aerospace market for the last three decades. These include:

• Polyimide insulated wire constructions MIL-W-81381/11, /7, and /9
• Cross-linked ETFE MIL-W-22759/33, /43, and /44 (now under AS22759/33, /43, and /44)
• Composite construction similar to AS22759/80 - /92

While there have been material improvements since this testing, the relative performance for each of these is still applicable to the wires constructed today.

The specific wire constructions (and sources) are listed in the following table.

Partial Discharge

The information in the following figure is frequently forgotten when dealing with aircraft wiring. While the corona inception voltage and extinction voltage may far exceed the expected operational voltage of the system when tested at sea level, the voltage can drop dramatically when considering high altitude applications. Each of the constructions considered in the USAF project display a noticeable decrease in the performance, with many of them having inception voltages drop to about 450V RMS when at 60,000 feet altitude.

For those considering high-voltage applications for aerospace systems, this result is a significant factor in the shift of focus in research to high-voltage wires and cables over the last several years. For the systems that are being considered today (that may range in the 600V to 2-3kV), simply selecting a wire that looks good at lower altitude applications is unacceptable.

Dynamic Cut-Through

The next data set reviewed here is the dynamic cut-through information. The dynamic cut-through tests performed in this effort examined unconditioned normal and lightweight constructions at ambient and elevated temperatures. Some areas to be highlighted from here include:

- The MIL22759 construction experienced a dramatic cut-through performance decrease between 70°C and 150°C. No other construction type tested showed such a dramatic drop in performance in this temperature range. For those using this type of wire on their vehicles or in any application, it is important to remember that while this wire type is rated for 150°C, the mechanical performance of this wire type drops dramatically above 70°C.

- The MIL-W-81381 construction made with polyimide performed the best. While many criticize the use of polyimide wire on vehicles, this is one of the strongest examples of why polyimide was a desirable wire insulation type for many years and still remains to be a very desirable insulation type as part of new wire construction (albeit with PTFE layer to the top).

Looking at High Voltage Systems

Perhaps one of the most interesting elements of the report is contained in the executive summary. Even as far back as 1991, the use of 270 voltage DC power systems was being considered. To quote from the executive summary:

“Tests examining arc propagation and 270 voltage DC power systems were also conducted. Results showed that none of the 10 original candidates, to baselines, or three additional in organic constructions tested were able to inhibit arc propagation in a 270 voltage DC power system with no additional circuit protection added. However, the addition of selected power controllers to the circuit provided the required protection.“

This is a fact that the aerospace industry is stuck with today. The current generation of wires that are part of the AS22759 family are incapable of stopping 270 voltage DC electrical arcing events by themselves. It does require the consideration of the entire system to include advanced circuit protection.

Conclusion

Comparing wire specification sheets can be difficult as they often provide information on the minimum performance requirements and not the actual performance. The data from the USAF effort, while nearly 30 years old, remains relevant for understanding the relative performance and how to approach comparative performance studies in the future.