Key Takeaways
- With the gradual industry implementation of high voltage EWIS systems in aviation, the demand for HV-specific safety standards is rapidly growing.
- High voltage OEMs currently use available industry standards, as applicable, but are still left with the task of developing and justifying their own system requirements.
- The SAE’s AS50881 standard, often considered the ‘go-to’ standard for OEMs, could benefit from proposed updates made to address the gaps for high voltage systems.
The pace of high voltage systems integration on aircraft makes it impossible for standards to support these emerging markets. This means that every high voltage OEM must create their own set of requirements using whatever background knowledge their team has or the ad hoc research that can be brought to the table. Admittedly, the aerospace industry is process-heavy and could benefit from a fresh start, and this fresh start can be supported with information gathered from the existing standards.
The automotive sector has been using high-voltage components for more than a decade and has had the benefit of several iterations of components and standards. While there are benefits to be gained, the automotive sector’s standards do not address the particular safety requirements of aerospace vehicles, and the aerospace sector does not have the historical framework for these high voltage systems.
In a recent article, Lectromec went over an automotive high voltage wiring standard and how that might be used to enhance an aerospace standard. Here, we review sections of the aerospace wiring system installation standard AS50881 and if there are gaps in how to support the high voltage aircraft sector. Beyond just identifying the gaps, means to address these gaps are also proposed.
Why AS50881?
Those unfamiliar with AS50881 should start with these articles Lectromec has written on this topic: AS50881 Introduction, AS50881 Testing, and AS50881-EN3197 Harmonization. In short, AS50881 is the industry-accepted standard for wiring system installations. This standard is used as the basis for many OEM internal wiring design guides and applies to several vehicle types including UAVs and eVTOLs.
A future article may address the gaps in the European standard EN3197; but for the time being, Lectromec shall focus on the SAE standard.
The remainder of this article calls out specific sections of AS50881 that are impacted by high voltage systems.
Wiring Selection
Section 3.8 titled “wiring selection”, already includes wiring selection requirements that should include, “typical factors to be considered in the selection [of wiring components] are voltage, arcing, arc protection”. To that end, the component’s voltage rating should already be part of the part selection process. For many, this voltage requirement has been taken for granted as low voltage systems (less than 300V) have been deployed for decades. At least for the next couple of decades, high voltage EWIS components will struggle with a lack of historical data and a lack of field experience. As discussed in Lectromec’s last article, this may mean the integration of life limits on EWIS components. The life limits will be impacted by how novel the technology is and the system voltage level.
Partial Discharge Prevention
Section 3.8.7 on “corona prevention” (referred to here as ‘partial discharge’) identifies that electrical wiring should be selected such that it prevents corona discharge and refers the reader to AS50881 section 6.6 for further details. In section 6.6, some of the key details of partial discharge prevention are provided and it specifically identifies concerns for systems with AC voltages exceeding 240Vrms. Further, AS50881 provides guidance to increase the wire insulation thickness to achieve the necessary material thickness to prevent partial discharge.
This section could be augmented to address the partial discharge performance impact of additional protection materials, such as heat shrink tubing or secondary wire harness protection, to achieve the equivalent thickness necessary to mitigate partial discharge.
It should be noted that there are wire insulation systems on the market today designed for higher voltage aerospace applications without the need for secondary protection materials. The expected performance properties of these wires/cables will be addressed in a future Lectromec article.
This figure shows the corona (partial discharge) voltage for different voltage, separation distances, and temperatures. Figure from MIL-W-5088L.
Figure one of AS50881 provides a good reference for identifying the minimum voltage according to Paschen’s curve for voltage breakdown at different altitudes. This is one part of avoiding partial discharge but not the entirety of the solution.
Wire Terminations
Section 3.8.8.1.2 discusses wire terminations. Lectromec recommends that this section also consider the proper termination of high voltage components to reduce the potential for partial discharge at the terminations.
As discussed in an earlier Lectromec article, SWAMP areas of aircraft should have additional protection for high voltage components and certainly, there should be additional consideration of any terminations in these areas. The automotive standard for high voltage (J1673 ) recommends that there should be no termination or splicing of high voltage cables in SWAMP areas; this should be integrated into section 3.8.8.1.2 of AS50881. If a splice is required, then it must withstand all of the fabrication, installation, and vehicle environment abuse; this is a non-trivial task for EWIS installed in aircraft SWAMP zones.
EMC
Section 3.10.8 of AS50881 discusses the electromagnetic compatibility and recommendations on ensuring that wiring is routed to minimize EMI. It would be advantageous to specifically state in this section that the wiring associated with high voltage systems should also be given particular attention. Of particular focus should be any high voltage wiring related to high frequency switching as this can be a source for High-Intensity Radiated Fields (HIRF) generation.
Power Systems
Section 3.10.13 on power systems aligns with the requirements of FAA regulation 25.1707(d) which dictates that wiring from different power sources must be independently routed. Specifically, this paragraph calls out “power wires size 10 or larger shall be separated from signal wiring and or communication cables per MIL-STD-464”. The rationale behind selecting size 10 wires here is due to the power carrying capability. As voltages increase, the power carrying capability of a single wire also increases. Because of this, for higher voltage systems, the power wire gauge considered may need to be reduced from 10AWG based on the voltage. This separation distance would need to be substantiated for both the arcing damage as well as electromagnetic interference.
Connectors
Section 3.14 discusses connectors and has several subparagraphs addressing the environmental factors, contacts, fireproof and firewall connectors, etc. An element that should be added as a subparagraph are statements regarding high voltage connectors. This could come in the form of multiple parts but would be recommended to include the following items:
- The separation of contacts from each other and the connector shell should consider the voltage and altitude requirements of the vehicle.
- High voltage connectors should include a second set of contacts shorter than the primary power carrying contacts that will disengage the circuit if the connector becomes unmated. This will address the risk of accidentally demating a live circuit and reduce the potential for injury to personnel.
In a similar vein, Section 3.14.15 discusses the contact voltage rating and could be adjusted to add in the items discussed above.
Splice Restrictions
Lastly, Section 3.19.4 discusses splice restrictions. It would be advantageous to include a restriction on splices for high voltage systems. Or, at the very least, the splices must not impact the voltage rating of the cable, i.e., the splice must be rated for the same voltage requirements as the cable itself and must be environmentally sealed.
Work to be Done
While there are gaps in the AS50881 for dealing with high voltage systems, it has not been until recent technological advances that these gaps needed to be addressed for more than just novel aircraft designs. If the user community sees fit, the AS50881 could be updated to address the high voltage gaps or the guidance could be integrated into stand-alone documents. Either way, the age of high voltage aircraft (those with more voltages greater than 300V) is upon us, and the challenge is to ensure the voltage stays on the conductor and does not find its way out.
For those looking for testing or engineering support on your high voltage aircraft systems, Lectromec has the lab capabilities and engineering experience to help you with your platform. Contact us today to find out how we can help you with your project.
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 is an FAA DER with a delegated authority covering EWIS certification and the chairman of the SAE AE-8A EWIS installation committee.