Lectrogram - Issue 16

Lectromec LECTROGRAM©

August 2007   Newsletter

A Resource for your Wire Maintenance Program

What is the effective distance that electrical arcing can cause damage?  Both lab tests and in-service incidents have shown that arcing is hot enough to cause significant damage when in direct contact with an object, but there is far less information on the damage that can be caused at a distance.  Expanding on previous research conducted by the Federal Aviation Administration, Lectromec is working with the FAA on the development of a computational model for arcing damage based on both analytical and empirical results.

An example of the damage at a distance can be seen in the photo below.  Multiple 7 mil (0.18 mm) copper wire strands were placed at different distances above a multi-phase wire arcing event.  Because arcing events saturate infrared cameras, these strands were used as a means of probing the temperature and energy of the arc plume.  One thing that immediately can be seen is that the arc plume has a definite direction and has sufficient energy to melt metal nearly half an inch away from the initiation site (the melting temperature of the copper alloy is about 1085^oC).

Though the energy from the arc plume is sufficient to damage single strands of copper, the amount of damage that could be expected to wire insulation is dependent upon type.  Some wire insulations, such as Polyalkene-PVDF (found in wire specs such as MIL-W-81044) are rated at a comparatively lower temperature than other insulation types, and nearby arcing may register a higher level of damage.

In addition to the hot gas, when an arc occurs it is common to see glowing detritus ejected from the arcing area.  These are melted bits of metal emanating from either the target or the source wire conductor.  It is important to consider the effective radius of the spew; the molten metal may damage nearby objects.

Further analysis and testing is being performed to determine the effective range for damage of both the spew and the arcing plume.  It is the goal of this project to devise a methodology to predict damage due to arcing, to set separation distances and to provide a proven means of evaluating mitigation techniques.  These factors should be taken into consideration for those building, operating, or maintaining an aircraft electrical wire system.

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