erik malund wrote:
-------------------------------


I just got a quote for some cheap switching power supplies and the quote stated "high reliability with an MTBF of 2.6 Million hours which correspond to about 300 years.

That indicates that competetive equipment can be made reliable.


Above is statistics-based, and some caution is needed here. An MTBF of 2.6 million hours does not imply that they will all last 2.6 million hours. (see Steve's remark).


If you can not make a reliable piece of equipment at the accepted cost or below, PLEASE say no


It's not as black and white as you state. I think that one has to make the equipment as reliable as possible within the limits laid out by the customer. If the customer has expectations that are set too high, for a given design/cost then it's up to the designer/engineer to warn the customer about his unrealistic expectations. One should always make sure that the customers perception of design and quality is realistic, given the design specifications and allowed cost. On the other hand, it's the designer/engineer's task to provide the highest possible quality (read: reliability) for the agreed specifications/cost.

Besides this, there is the definition of reliable. When is something reliable, and when is it not? Even if something is only used within it's design specifications, even then things can break. Problem is that the availiblity (or reliability) vs cost curve is (in my opinion) asymptotic (ie cost approaches infinity as reliability approaches 100%). Therefore (absolute) reliability will be heavily influenced by cost, and the latter is usually dictated by customer (or call it "market-driven").



Comments solicited

You asked for it, you got it.. :-)


Regards
Patrick