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OCR for page 461
A.4.4 Spares The rule of Numb for spares inventory has traditionally been 10% (but at least one) of installed quantities of items deployed in a system. Currently, with more reliable modern electronic equipment, many manufacturers recommend sparing at levels below 10%. Factors Mat influence sparing levels include: MTBF (Mean Time Between Failure) and MOOR (Mean Time To Repair) after on site, of components, boards, assemblies, or boxes to be repaired/spared; Method of repair: box, assembly, board, component; Locations of depot supply and maintenance facilities; Geographic dispersion of system elements to be repaired; Inventory and logistics of spares including maintenance personnel needs and transport to repair site; Locations) and number of maintenance personnel and travel time to effect repairs; Manufacturers recommended spanug levels. Time to repair/replace failed items (i.e., return to spares inventory); System design including network management, bit-in-test, and redundancy (e.g., hot standby, fault tolerant, rings, etch; Time to troubleshoot and detente failure, cause, and repair requirements (network management and Built-in Test [Bill can significantly reduce this time); and System availability, as designed goal and as actually achieved. MTBF of modern communication equipment is evolving from an average of 1,000 to 10,000 hours (.! to I.! years) to 50,000 to 100,000 hours (5.7 to ~ I.4 years). Many current communication equipment suppliers provide warranties up to 5 years and watt add additional years, typically for 1% (or less) per year of the list price. Additionally, win increasing integration due prunanly to advancing semiconductor technology, commun~cabon equipment repair is evolving to Me standard practice of complete box, assembly, or board replacement via return to manufacturer for replacement/repa~r. The best sources for information on recommended levels of spares inventory, are Me equipment manufacturers who should have knowledge of Weir customers' field expenence. L:~h~c2.~\ NCHRP 3-51 Phil 2 Few Ream A=25