The consequences of failure from wire failures are easy to assess when considering only the failure within a wire bundle. But, when the physical damage to nearby components or protection sleeves needs to be considered, the complexity dramatically increases.

The FAA’s guidance and needs for arc damage assessment focus on assessing the physical consequences of failure. Their advisory circular (AC) that provides guidance on EWIS risk assessment, AC 25.1701-1, states “…. when deciding on the choice of the barrier, factors such as dielectric strength, maximum and minimum operating temperatures, chemical resistivity, and mechanical strength should be taken into account.“ [Source: FAA AC 25.1701-1]

Additionally, considerations should be given, “… when wire bundle sleeving (or tubing) is used to provide separation, designers should consider that the sleeving itself is susceptible to the same types of damage as wire insulation. The appropriate type of sleeving must be selected for each specific application and design consideration must be given to ensuring that the sleeving is not subjected to damage that would reduce the separation it provides.”

Lastly, the selection on a safe separation distance is defined to one such that, “… an EWIS component failure will not create hazardous effects on the airplane or its systems.” “Other means of ensuring equivalent minimum physical separation may be acceptable, if testing or analysis demonstrates that safe operation of the airplane is not jeopardized. The testing or analysis program should be conservative and consider the worst possible condition not shown to be extremely improbable.”

Lectromec has developed the lab capabilities and software simulation tools for handing the needs of OEMs and aftermarket modifiers showing the physical damage profile necessary for showing compliance to FAA 25.1709. Contact Lectromec to find out how we can help with your certification effort.

The SAE is holding their 8D committee meeting this week in Louisville, KY. For those unfamiliar with the SAE 8A/D, this group develops, reviews, and updates modern aerospace wire standards. It is a committee filled with aircraft OEMS, military personnel engaged in maintenance, certification, and testing of wiring, chemical manufacturers, and wire constructors.

Among the tasks that have been taken up by this group include developing new arc damage assessment methods. The goal of these methods is to provide a standard method for assessing the damage from electrical arcing events. The reason for this is that, as of today, there has not been industry guidance on how to determine what an acceptable (safe) separation distance between wiring harnesses and other components (e.g. other wiring, structure, hydraulic lines, fuel tanks).

The work is still in the initial development phases, but the working group seems to be directing the effort toward guidance to those seeking FAA certification of their Electrical Wire Interconnection System (EWIS) with FAA regulation 25.1709. In particular, provide guidance and recommendation to those seeking the physical damage assessment described in AC 25.1701.

It might be years until the final test methods are released, but progress is being made. We will provide updates here as the method comes closer to final release.

Lectromec has been performing arc damage assessments for more than 20 years and has the facilities to perform arc damage assessments simulating most aerospace configuration. If you are looking to gather data on the electrical arc damage possible of your platform, whether for design validation or certification, contact Lectromec.

Recently when going through our file servers, we came across this old video of tests performed by Lectromec for the NTSB showing the properties of arc track testing of aromatic polyimide wire (often called Kapton®). It provides a good overview of arc track testing, the dangers of damaged wires.

Two things should be noted when viewing this video:

1) Although polyimide wires were used in these tests, it is possible to do damage to wires in the harness with more modern wire insulations.

2) In these tests, a 10kVA generator was used. Modern aircraft typically have larger generators making it possible to create significantly more damage than seen in this video.

To find out more about arc track testing and damage assessment, visit www.lectromec.com

While not directly applicable to new wire technology, engineers at Canterbury University of New Zealand recently created a method for generating extremely long arcs [link].  They did this by suspending a tiny gauge wire and then applying a high voltage.  It is almost worth taking the time to see the photos they have of their experiment – an almost straight glowing arcing.

This process should come as no surprise to those who have shorted with a small gauge wire.