Sunday, April 22, 2012

Wire Processing and Wire Integrity Testing

Wires and cables have lifetime limitations due to normal material degradation, vibrations and stresses that they come under, and the collateral damage that occurs during normal maintenance. Aging wiring can result in degraded performance due to accumulated damage from long-term exposure to chemical, thermal, electrical and mechanical stress.

These failures often appear as broken conductors, corrosion and damaged insulation which can disrupt electrical signals and /or lead to arcing. Arcing is a form of short circuit that arises when wiring with ruptured insulation come in contact with metallic structures or other exposed wiring in moisture. When this occurs, there is usually a high energy flashover which could lead to a fire even before the circuit breaker is triggered. Arcing also increases signal loss and distortion on live signal through the wire.

Electric arcing occurs not only as an electrical failure but as a contributor to all other failure modes. Maintenance and assurance of the integrity of the wiring is very challenging due to the risk involved in accessibility e.g nuclear plants, as well as due to the high density of wiring network e.g onboard aircraft and ships. For instance, modern jet transport aircraft contains hundreds of kilometres of wires.

Technicians, engineers, and managers must painstakingly track, test and maintain the wiring architecture. In some cases, some wiring defects show up during flight but remain undetectable in routine maintenance. Visual inspection has been one of the popular ways of tracking the wiring defects. However, beside the selectiveness of this method, there is the problem of high cost and the fact that much of the wire is visually inaccessible; furthermore, a major concern with visual inspections is the risk of introducing additional damage in the course of gaining access to inspect the wire.

Other methods generally used for wire integrity testing include Ohmmeter (dc), Time Domain reflectometer (TDR), Frequency Domain reflectometer FDR), among others. Although they have been used for many years, each of these techniques has its attendant limitations. For instance, when using ohmmeter, the main bus has to be cut at several points in order to find faults; this is time consuming and also increases the possibility of introducing new faults. TDR and FDR, although suitable for testing specific unconnected cabling, have been found not suitable for measuring branched harnesses. Another important factor with FDR and TDR is that they cannot be used directly on aircraft wiring when there is a live signal present because they could distort the signal, particularly in that case. New and better methods of testing and assessing wire are essential to ensure safety, and to maintain the functionality of wiring network.

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