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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
Representative terms from entire chapter:
maintenance personnel
