On 08/03/05 22:48, Jesper Louis Andersen wrote:
..
So you forget the last term by saying ''twice as much''. You have to
deduct the probability that both events occur (or it would have been
''counted'' twice).
Mathematically correct, but not realistic if one power supply fails in the
serial case it might be the other fails during the failure or repair but that's
insignificant.
As a manufacturer we always look carefully at failures, if it's a "time-bomb"
capacitor we look at other equipment with the same age and see if we have to
replace the whole series of products at all customers with maintenance
contracts. If there is =any= sign of a "series failure" we replace all
capacitors (or whole units) everywhere. We have done so a few times. Scheduled
downtime is always better and far les costly for all parties than unscheduled
downtime. Because of these kind of policies we experience very few breakdowns in
the field and have a quite peaceful life...
Two equal power supplies in parallel: Half the risk of a brakedown of
the system but still two times as much failures of power supplies and
twice the support effort for the power supplies.
Now in this case, we still have independence, but now both has to fail.
In other words
P(intersection(X = 1, Y = 1)) = P(X = 1) * P(Y = 1)
This is theory. In practice a failing power supply will be changed as
soon as it shows an error. Especially in the serial case. This means
that in practice, one has to do a more heavyweight probability analysis.
One needs the probabilities after one month, after 2, 3, 4, etc to do
the discrete case. I can assure you the probabilities are not as easy as
you are taking them to be.
Mathematically you are correct, in practice it doensn't matter. For the parallel
case you seem to forget that the system might still be up but the broken power
supply still needs maintenance. 2 times the maintenance (replacing, traveling,
etc) because of 2 times as much power supplies is not unrealistic.
The same applies for hard disks. And like with power supplies, if they are of
the same manufacturer and manufacturing date there is a big chance all power
supplies/drives will fail unexpectedly within a short timeframe because of "time
bombs" like capacitors or unexpected faster diffusion processes in integrated
circuits.
+++chefren
