The invisible fray: a critical analysis of the use of reflectometry for fray location

Significant international research and development efforts have been devoted to methods and equipment for locating wiring faults, particularly those on aging aircraft. Several reflectometry methods that send high frequency signals down the line and analyze the returned reflections have risen to the forefront of these technologies. While these methods are proving to be accurate for location of "hard" faults (open and short circuits), the location of "soft" faults such as frays and chafes remains elusive. This paper analyzes the impedance of several types of soft faults and their resultant reflectometry returns, which are shown to be smaller than returns from other sources of physical and electrical noise in the system. Through numerical simulations verified by measurement, it is shown that soft faults are virtually impossible to locate using today's reflectometry methods including time domain reflectometry, frequency domain reflectometry, and spread spectrum time domain reflectometry. The methods used in this analysis can be extended to other types of reflectometry as they emerge.