one sheath, the insulation within the sheath may deteriorate so that a cross type of short occurs. This contact almost always creates enough heat and pressure to rupture the sheath and put the conductors in contact with ground.

Moisture is one of the most common causes of an underground system breakdown. Impurities in the water help set up corrosion cells, break down neoprene, and rot rubber. Only a trace of moisture, when superheated by the electrical power of the circuit and converted to steam, can cause an explosion that will rip the cable to shreds. Groundwater contains enough minerals to provide an excellent conductor to all other parts of the system. Some underground cables are bonded together. The usual way to find out that an underground power cable has a problem is to check when the circuit opens.

In ducted systems, the maximum runs between manholes are 500 feet. The normal method of repair is to replace the cable. In direct burial cable systems, the cable runs may be quite long, and it would be impractical to replace the entire run. In this case, cable fault locators are used to locate the fault. Before starting to work, make sure that all power is off on the circuits in the trench before you start digging or repairing the cable.

This chapter does not discuss detailed circuit troubleshooting because each system is different. When you troubleshoot complex problems that involve airfield lighting, you should refer to the following publications: Definitive Designs for Naval Shore Facilities, NAVFAC P-272; General Requirements for Shorebased Airfield Marking and Lighting, NAVAIR 51-5OAAA-2; and Lighting and Marking Systems for Expeditionary Airfields. NAVAIR 51-4OABA-7. Problems, such as improper power connections, component connections, safety grounding, cable splices, cable terminations, and cable installations, are discussed in detail in these publications.