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RE: Preventive Maintenance For Lamps

Title: Message
At some point in time, Judith Kennedy (jken461@ECY.WA.GOV) is alleged to have said (in part):
> Can anyone provide me with some information on preventive maintenance programs for commercial/industrial lighting, 
where all lamps are periodically changed at the same time, 
whether they need it or not? 
To which I replied:
Judy --
I initially hesitated to post my reply to P2TECH because I don't have time today to cite enough references.  Then I remembered -- "hey, this is the Internet!  It doesn't matter!"  <grin>
Nonetheless, my understanding (which comes from preventative maintenance literature in the process industries -- where similar decisions are made about when/how often to replace valve packings, pump seals, etc) is that over time, an industry (or a company if it is large enough to have a good sample size) can collect sufficient data regarding mean time between failure (MTBF) to permit accurate estimation of the lifetime of a replaceable item.  These statistics can also tell you how tightly clustered the MTBF is around some mean value -- for some types of parts, the clustering may be very tight (nearly every part fails close to the mean time); for others, the clustering may be very loose (there is wide variation in the MTBF).
some example MTBF for items in the defense industry can be found at:  http://rac.alionscience.com/pdf/TypicalEquipmentMTBFValues.pdf
It is sobering to note that this table lists the MTBF for the hard disk of the 4-year old computer which I am writing this on as 10,000 hours.  At somewhat over 2,000 hours of compter use per year (you can do the math), perhaps that accounts for the feeling of impending doom I sometimes feel when I log in for the day?
Or maybe it's just the evening news that's to blame.
Anyway, the replacement cycle is then pegged to some percentage (assumedly a number less than 100%) of the MTBF value.  What percentage of this MTBF is used as the trigger for replacement depends on a number of factors including the standard deviation around that value (how loose or tight the clustering around that value is), the criticality of the part (i.e.., the safety issue that Bob refers to), the value of the part relative to replacement cost, and adjustments for type of service* the part sees.  Obviously, all other things being equal (which they hardly ever are), a high valued part will be pushed to a greater percentage of it's MTBF than a low-valued part;  a part with a high consequence of failure will be replaced at a lower percentage of it's MTBF than one which doesn't matter so much. 
A real life example some of us may be (painfully, in my case) familiar with is the timing belt on your car (or more precisely, my wife's car).  Although the replacement cost of the belt is relatively small (typically under $100 for the part),  the cost of labor is high, so a timing belt will likely cost $500-700 to replace.  Most manufacturers recommend replacement every 60,000 - 70,000 miles.
I can tell you from personal experience that timing belts will often last 100,000 miles or more.
However, if you push one past it's limit, the result can be severe engine damage.  Really costly severe engine damage.  "Does our insurance cover towing?" kind of engine damage.  So it makes economic sense to replace the timing belt well before its MTBF. 
Trust me on this one.  :(
But I'm drifting off topic again, aren't I?  In any event, in the case of lamp replacement, I suspect that the consequences of single lamp failures are relatively small; the cost of labor for replacement is high relative to the cost of the lamp (in most cases).  This would especially be true if lamp replacement were outsourced to a vendor/contractor, so that overheads associated with the maintenance staff are reduced.  In "light" of these factors (sorry), I would assume that lamps are pushed to a relatively high % of MTBF for scheduled replacement. 
Just thinking out loud here, but from a p2 perspective, this is a good example of how our community's focus on p2 as a cost-saving measure (as opposed to an environmental strategy) is sometimes a bit of a deal with the devil** -- there are times when p2 objectives (i.e.., extending the lamp utilization to the maximum possible amount of time) conflict with the economic objectives (minimizing the total lifecycle cost of providing lighting).  Of course, sometimes this is simply a good indicator that the problem is ripe for alternative solutions -- daylighting, or perhaps use of LED-based lighting (since LED's, aside from being more energy efficient, typically have MTBF of more than 100,000 hours, rather than just a couple thousand). 
Those options notwithstanding, if you're concerned about "premature" lamp replacement, it might be worth looking at whether there are other locations in the facility where "replace as needed" lighting can be justified -- and demote the lamps that are replaced on scheduled replacement to that service when they are replaced.
Hope this helps you think constructively about the problem, even if it doesn't answer all of your questions.
Scott Butner (scott.butner@pnl.gov)
Director, ChemAlliance 
*  for example, on traffic lights, the yellow lamps may be replaced less often since they are on for less time than green or red due to the nature of the traffic light cycle.  So, a logical policy might be to replace them every OTHER replacement cycle.  Besides, no one pays attention to yellow lights, anyway.
** and before I am taken to task for this statement, I am NOT suggesting that emphasizing the economic benefits of p2 is not important -- indeed, I suspect that the realization by the p2 community that this is largely an economic issue has probably accounted for the bulk of the environmental benefit brought about via p2.  But don't you sometimes find yourself wishing that protecting the environment was as strong a motivator as saving money.