1. CAUSES AND RESULTS OF ENVIRONMENTAL DAMAGE TO AIRPLANE SYSTEMS
Critical and essential airplane systems are vulnerable to damage from environmental factors. These include lightning strikes, HIRF, moisture, extreme change in pressure, extreme range of temperature, vibration, and shock.
2. MITIGATION OF DAMAGE TO AIRPLANE SYSTEMS
If electronic equipment needs to be operated in a region subject to changing electromagnetic fields, and if the currents generated by these fields are considered harmful, the recommended approach to mitigating the harmful effects is to shield and ground the electronic equipment and the interconnecting wiring. As a result, electrical currents generated by lightning or HIRF then circulate through the equipment enclosure to ground without affecting internal circuitry. This enclosure practice extends to interconnecting wiring through the use of cable shielding; that is, the shield is the enclosure that is grounded. Other damage mitigation considerations include the location of the equipment and wiring, use of effective wiring, use of good grounding practices, and building equipment to withstand transients. All these tactics are incorporated into the design of Boeing airplanes and the installed equipment.
3. SHIELDING SYSTEM CONCEPTS AND TESTING
Shields perform other functions beyond providing lightning and HIRF protection. A well-known example is low-frequency hum on airplane audio circuits usually traceable to the 400-Hz power system. The traditional solution is to install a shield and ground it at one end, which has proved to be very effective against this type of low-frequency interference. Grounding the shield at both ends is typical for lightning protection, but this tactic was found to be ineffective against the low-frequency hum. One of the effects of a lightning strike is the generation of changing electromagnetic fields inside the airplane hull. These fields are at much higher frequencies than the 400-Hz power interference. Under such conditions, shields grounded at only one end might not be effective. In some cases this can also result in the shield acting as an antenna, thus making the surge voltages even larger than they would be if the conductors were unshielded.
4. LOOP RESISTANCE TESTER DESCRIPTION AND OPERATION
To avoid encountering these circumstances during resistance testing, Boeing developed the LRT to provide a nonintrusive testing technique that will not disturb connectors, is faster, and will ensure the soundness of the overall circuit.
To properly use the LRT and achieve the best results possible, maintenance personnel should understand the following about the device: LRT elements, LRT technique, loop troubleshooting, joint mode, sensing of coupler closure and joint probe connection, maintenance requirements, safety features, and calibration and certification.
There's a lot more at this link!
http://www.boeing.com/commercial/aer...oop_story.html