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Aerospace High Voltage Systems: Addressing the Gaps in Current Standards

Standard & Regulation

Key Takeaways
  • Automotive standards for high voltage systems can be used for guidance in developing standards for high voltage systems in aerospace.
  • Many similarities and several gaps exist between J1673 and AS50881; analysis of these differences (and those of other relevant standards) can highlight and guide the work needed to address high voltage system in aircraft.
  • Though much of the high voltage requirements outlined in SAE J1673 for automotive high voltage wiring systems are already inherently part of the aerospace standard AS50881, the requirements defined in AS50881 alone are insufficient to address these specific needs of high voltage systems.

Introduction

The aerospace segment has been dealing with questions of high voltage systems for the last decade. Many of these questions revolve around the generation, storage, and use of this power, but what has also been a hindrance for progress is defining how the standards for the wiring system need to be updated to address these high voltage requirements. As should be done when any new technology is being applied to a field, we should first look at other fields which have undergone the implementation of similar technologies. For example, the automotive market has been using high voltage systems for quite some time and the standards committees have produced documents supporting these technologies.

In this article, we review one of these documents on high voltage directed to the automotive market and see how it lines up with the existing aerospace standards.

HV in Cars

SAE document J1673, “High Voltage Automotive Wiring Assembly Design” provides recommended practices on the design and application of primary onboard wiring distribution system harnesses for road vehicles. The J1673 is a six-page document and by no means could it cover all of the same areas that the aerospace wiring installation standard AS50881 address with it more than 120 pages and several decades of updates, but J1673 does address a particular area of wiring system requirements.

Wiring Selection and Installation Locations

One of the first areas covered in this standard is the requirements around insulated cables. The automotive standard identifies that any wire or cable carrying a voltage between 50 and 600 volts in the same harness shall conform with a specific SAE standard J1654. This automotive standard, J1654, defines a set of performance requirements for high voltage automotive wiring. Many of the performance requirements in J1654 line up with existing test methods called out for aerospace wires such as those in the AS22759 family. The J1654 and its requirements will be covered in another article.

As should be expected, section 3.2.2 of J1673 identifies that any cable selected for installation on the vehicle should be able to handle the working environment of the vehicle. Specifically, this calls out the thermal, mechanical, electrical, and environmental requirements that should be considered when selecting the cable for installation. This lines up wonderfully with section 3.3 in the AS50881 (and FAA EWIS regulation 25.1703) identified as “selection of parts and materials”.

High Voltage Common Ground
There is a lot of common ground between AS50881 and J1673. The areas of J1673 that address specific HV challenges should be considered for those in the aerospace considering HV power systems. The total scope of changes needed to address HV is still uncertain.

Cable Sizing

Section 3.2.5 of J1673 discusses the cable size determination factors. This includes elements such as the temperature rise based upon the operational load, whether it is a steady-state or has a duty cycle, fault conditions, appropriate size for mating connections, and the mechanical strength. This lines up with the AS50881 selection of cables based on their operational requirements as defined in section 3.8.8.1.1 and further described in section 6.7 of AS50881.

Splices

Within the J1673 an entire section (3.3) is devoted to considerations for conductor splicing. In particular, the conductor splicing requirements aligned with the recommendations for power cables as identified in AS50881 (3.19.4). The J1673 does specifically state that splices for power cables should be avoided where possible; there is no such guidance in the AS50881. If a splice is required, then it must withstand all of the fabrication installation and vehicle environment abuse. Sections 3.19 in AS50881 align with this and there are many shared requirements between the two standards (save the power cable guidance of J1673).

When looking at the connector selection section of J1673 (3.4.3), the requirements are very similar to those of the connector requirements in AS50881 (3.14). The J1673 includes requirements that connectors in closed proximity have unique keying to avoid incorrectly mating or cross mating of connectors (AS50881 Section 3.14.6). Further, the requirement in J1673 is that all cavities of the connector should have some level of identification to properly identify the contacts.

HV Identification

One area in which the two standards deviate, but was discussed at the last SAE AE-8A meeting, is the proper means of identifying high voltage components. In the J1673 Section 3.5.3, high voltage wire harnesses should be visually identified with a permanent orange harness covering material. In the case where a cable is routed separately and not protected with the additional covering, the primary color of that wire insulation should be orange. This requirement deviates from the requirements of AS50881 where orange-colored wires are, as of the latest revision, allocated to only flight test wires. How the high voltage wiring identification will be integrated into AS50881 remains to be determined.

Wire Assembly Installation and Protection

Within the J1673 standard, the wire assembly and installation protection section (3.6) covers several areas that are good standard practice for any wiring but are of particular importance for these high voltage high power wiring systems. Many of these requirements align directly with the AS50881 such as the need to protect wire harnesses from

  • Heat sources (AS50881 Section 3.10.16)
  • Moving equipment (AS50881 Section 3.11)
  • Vibration and abrasion (AS50881 Section 3.11)
  • Small bend radius (AS50881 Section 3.11.7), and
  • Ensure adequate hand clearance for connection and disconnection (AS50881 Section 3.14.5.1).

Splash and SWAMP

An area that is not specifically covered in AS50881 is the identification that the high voltage connections should not be installed in splash areas where water or road abrasives can damage the wire harness. While the AS50881 does have specific requirements for wheel well harnesses and those in SWAMP areas (several sections of AS50881), this is an area that could be updated to address the particular needs of the high voltage wiring.

Wire Overload Protection

The last section of the J1673 standard discusses wiring overload protection devices (section 3.7). Specifically, that every high voltage circuit should go through overload protection. While the J1673 leaves the specific circuit protection device to the designer, the protection should conform with expected performance thresholds and should be packaged in the vehicle in a manner that prevents inadvertent access by untrained personnel. While the AS50881 does not define a requirement for high voltage circuit protection, section 3.8.8.1 specifically discusses the factors that should be considered when selecting circuit protection for wiring.

What Needs to be Done?

In conclusion, much of the high voltage requirements outlined in SAE J1673 for automotive high voltage wiring systems are already inherently part of the aerospace standard AS50881. This can be encouraging to those in the aerospace industry seeking to implement high voltage systems for their vehicles. However, the requirements defined in AS50881 alone are insufficient to address these specific needs of high voltage systems. Some changes are necessary, such as those specifically identified in this article. The work that other industries have done to address the specific challenges of high voltage systems should be considered when the high voltage elements are being added to aerospace documents. While it is unlikely that the guidance and component standards will address these challenges in the short term, we should keep in mind that there are existing references to be used as the industry moves forward.

If you have questions about how to address the testing or certification of the high voltage wiring system for your vehicle, contact Lectromec. Our engineering staff and our ISO 17025 accredited lab are ready to help move your project forward.

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.